NAPSEA Safe Ecological Limits

DELIVERABLE 4.2 SAFE ECOLOGICAL LIMITS Work Package 4 Ecosystem Health 30-11-2024 www.napsea.eu Page 2 of 64 Deliverable 4.2 Grant Agreement number 101060418 Project title NAPSEA: the effectiveness of Nitrogen And Phosphorus load reduction measures from Source to sEA, considering the effects of climate change Project DOI Deliverable title Safe Ecological Limits Deliverable number D 4.2 Deliverable version concept 1 Contractual date of delivery Oktober 1, 2024 Actual date of delivery November 30, 2024 Document status Concept Document version 1.0 Online access Yes Diffusion Public Nature of deliverable Report Work Package WP 4: Ecosystem Health Partner responsible HEREON Contributing Partners UFZ, UBA, Deltares, NMI, Rijkswaterstaat Author(s) Van Beusekom, J.E.E, Schulz, G., Pein, J., Musolff, A., Rozemeijer, J. Troost, T. Editor Van Beusekom, J.E.E., van der Heijden, L. Approved by Troost, T. Project Officer Blanca Saez-Lacava Abstract This deliverable addresses the Safe Ecological Limits in the four NAPSEA case studies. For the Wadden Sea, new limits were derived from seagrass and phytoplankton composition. The reduction needed to reach these Safe Ecological Limits were put into perspective of three riverine case studies (Rhine Catchment, Elbe estuary, Hunze catchment) each with their own unique environmental setting and reduction needs. Keywords Safe ecological limits; Rhine; Elbe; Hunze; Wadden Sea Page 3 of 64 Deliverable 4.2 Contents Deliverable 4.2 ........................................................................................................................................................ 1 Safe Ecological limits .............................................................................................................................................. 1 1. ACRONYMES ..................................................................................................................................................... 5 2. EXECUTIVE SUMMARY .................................................................................................................................... 6 3. GENERAL INTRODUCTION............................................................................................................................... 7 4. SAFE ECOLOGICAL LIMITS FOR THE WADDEN SEA .................................................................................... 7 4.1 Introduction ................................................................................................................................................... 7 4.2 Area description ............................................................................................................................................ 8 4.3 Eutrophication history ................................................................................................................................... 8 4.3.1 Negative effects of eutrophication ......................................................................................................... 9 4.3.2 Increased monitoring in the international Wadden Sea: an international view on the Wadden Sea eutrophication ............................................................................................................................................... 10 4.3.3 Limiting nutrients ................................................................................................................................. 10 4.4 Safe Ecological Limits ................................................................................................................................. 12 4.4.1 Seagrass recovery as a safe ecological limit ....................................................................................... 12 4.4.2 Si limitation as a safe ecological limit .................................................................................................. 14 4.5 Discussion .................................................................................................................................................. 16 4.5.1 Loads versus concentrations in setting reduction goals ...................................................................... 16 4.5.2 Relative importance of rivers impacting the Wadden Sea ................................................................... 16 4.6 Outlook ....................................................................................................................................................... 17 5. SAFE ECOLOGICAL LIMITS FOR THE ELBE ESTUARY ............................................................................... 18 5.1 Introduction ................................................................................................................................................. 18 5.2 Area description .......................................................................................................................................... 18 5.3 Eutrophication history ................................................................................................................................. 19 5.4 Safe Ecological Limits ................................................................................................................................. 21 5.5 Modelling the O2 dynamics in the Hamburg port ......................................................................................... 21 5.5.1 Methods ............................................................................................................................................... 21 5.5.2 Response of the O2 dynamics to reduced organic matter loads ......................................................... 21 5.6 Reductions needed to stay within Safe Ecological Limits ........................................................................... 22 5.7 Discussion .................................................................................................................................................. 22 5.7.1 The role of Si ....................................................................................................................................... 22 5.7.2 The role of N and P ............................................................................................................................. 23 5.7.3 How to reduce Chlorophyll and organic matter levels in the Elbe River .............................................. 24 5.7.4 Morphological adaptions ...................................................................................................................... 24 5.8 Conclusions ................................................................................................................................................ 24 6. SAFE ECOLOGICAL LIMITS FOR THE RHINE BASIN ................................................................................... 24 6.1 Introduction and area description ................................................................................................................ 24 6.2 Eutrophication in the Rhine basin ............................................................................................................... 25 6.2.1 Nitrate status ....................................................................................................................................... 27 6.2.2 Phosphorus status ............................................................................................................................... 28 6.2.3 Chlorophyll-a status and invasive filter feeders ................................................................................... 28 6.3 Safe Ecological Limits ................................................................................................................................. 29 Page 4 of 64 Deliverable 4.2 7. SAFE ECOLOGICAL LIMITS FOR THE HUNZE CASE ................................................................................... 29 7.1 Introduction ................................................................................................................................................. 29 7.2 Area description .......................................................................................................................................... 29 7.3 Eutrophication history ................................................................................................................................. 31 7.4 Dutch WFD nutrient targets applicable to Hunze ........................................................................................ 32 7.4.1 Bioavailability ....................................................................................................................................... 32 7.4.2 Dutch WFD nutrient targets ................................................................................................................. 33 7.4.3 Nutrient targets and nutrient concentration variability .......................................................................... 35 7.4.4 Limiting nutrients ................................................................................................................................. 37 7.5 Safe Ecological Limits ................................................................................................................................. 40 7.5.1 Hunze safe ecological limits for the Zuidlaardermeer .......................................................................... 40 7.5.2 Hunze safe ecological limits for the Wadden Sea ............................................................................... 41 7.8 Discussion .................................................................................................................................................. 41 8. TOWARDS SAFE ECOLOGICAL LIMITS: A SYNTHESIS AND DISCUSSION OF THE CASE STUDIES ...... 42 8.1 Introduction ................................................................................................................................................. 42 8.2 Summary of the Case Studies .................................................................................................................... 42 8.2.1 Wadden Sea ........................................................................................................................................ 42 8.2.2 Elbe Estuary ........................................................................................................................................ 42 8.2.3 Rhine catchment case study ............................................................................................................... 43 8.2.4 Hunze case study ................................................................................................................................ 43 8.3 Reduction needs ......................................................................................................................................... 44 8.3.1 Summary of reduction needs ............................................................................................................... 44 8.3.2 The need for specific winter goals for riverine nutrients ....................................................................... 44 8.3.3 The impact of N on terrestrial ecosystems .......................................................................................... 45 8.4 Conclusion .................................................................................................................................................. 46 9. REFERENCES .................................................................................................................................................. 46 APPENDIX A......................................................................................................................................................... 54 APPENDIX B......................................................................................................................................................... 56 APPENDIX C ........................................................................................................................................................ 57 APPENDIX D ........................................................................................................................................................ 62 Page 5 of 64 Deliverable 4.2 1. ACRONYMES N Nitrogen TN Total Nitrogen P Phosphorus, mostly as PO4 Si Silicate, mostly as SiO4 WFD Water Framework Directive OGweV Oberflächenwasserverordnung Page 6 of 64 Deliverable 4.2 2. EXECUTIVE SUMMARY In this deliverable, we address Safe Ecological limits. We developed new indicators for the Wadden Sea as the ultimate receiver of nutrients from the rivers Rhine, Elbe and Hunze, and discuss the Wadden Sea reduction needs in relation to Safe Ecological Limits for the Rhine catchment, the upper Elbe Estuary and the Hunze catchment, each with their own unique ecological settings. The indicators and proposed reduction needs generally refer to 2010 – 2017 as a reference period. In the Wadden Sea case study, it was shown that N is the main limiting nutrient for phytoplankton growth. Two new indicators were developed in addition to existing indicators for nutrients and chlorophyll: seagrass recovery and Si/N ratios in winter. Seagrass recovery in the northern Wadden Sea accelerated around 2000 but no clear recovery was observed in the southern Wadden Sea. The eutrophication conditions (based on phytoplankton biomass) that prevailed during the recovery in the northern Wadden Sea were projected on the southern Wadden Sea. It was estimated that in comparison to the period 2010 – 2017 the riverine TN loads should be reduced by at least 1/3 (range: 34 – 46%). The second indicator is based on the ecological role of dissolved silicate (Si). During pre-eutrophication conditions the spring bloom was limited by N before it shifted to a Si limitation of diatoms during increased eutrophication and leading to increased blooms of algae not depending on Si. To return to a Nlimited spring diatom bloom, a reduction in riverine N loads of 30% (Elbe) – 55% (Ems) is needed. The Elbe Case Study focussed on the present O2 problems in the upper estuary caused on the one hand by the extreme large phytoplankton blooms in the riverine part of the Elbe and on the other hand by light limitation in the estuarine part due to dredging. When these riverine blooms enter the upper part of the upper estuary, grazing decimates the phytoplankton standing stock whereas light limitation precludes phytoplankton growth to compensate for the losses. This lead leads to severer O2 problems during summer. Based on models, a reduction need in organic matter loading of about 45% was estimated to reach Safe Ecological Limits of 7 mg O2 /l. Present phytoplankton levels (March – October) in the Elbe just upstream of the estuary of about 109 µg Chl a/l are well above the Safe Ecological Limits of 40.1 µg/l implying a reduction by 63%. The Rhine and Elbe are the two major rivers draining a large part of Germany and the Netherlands and impacting the Wadden Sea. The two rivers are very different. First, the Rhine is dominated by large amounts of melting water (about 48% of total discharge) from the Alps with low nutrient concentrations, whereas the Elbe is a rainfed river. Furthermore, a high number of invasive filter feeders inhabit the riverbed suppressing phytoplankton growth in the Rhine in contrast to the Elbe, where large phytoplankton blooms occur. Whereas most stations in the main stem of the Rhine indicate a good status for both N and P, most of the stations in the sub-catchments fail the good status. For N, the average reduction needed for a good status in the sub-catchments amounts to 44%, for P about 50%. In the Dutch part of the Rhine basin, nearly 50% of the water bodies had a moderate or worse status for P and about 30% had a moderate or worse status for N. The Hunze, located in the northeast of The Netherlands, south of Groningen, drains directly into the Zuidlaardermeer, that is connected to the Wadden Sea through various waterways. The ecological status improved during the last decades and several nutrient targets are nearly met (except for NH4). Still, problems remain including blue-green algae (cyanobacteria) compromising local bathing water quality and submersed vegetation growth. To change lake Zuidlaardermeer from an algae-dominated state into a clear state dominated by submersed vegetation, a critical P-load threshold of 2.75 mg P/m2/d is derived from model calculations, implying a reduction need of almost 40%. As in many other freshwater systems, the focus is on P to reach a good environmental status. To support Safe Ecological Levels for the Wadden Sea an N reduction of 34% is needed. Despite a wide range of ecological settings the reduction needs are in a similar range, between 30 and 63% for both N and P. Whereas our focus was on aquatic habitats we also discuss the reduction needs in the context of the environmental problems of N deposition on terrestrial ecosystems where reductions in N deposition in a similar range are needed. Page 7 of 64 Deliverable 4.2 3. GENERAL INTRODUCTION After a strong increase in riverine nutrient concentrations since the late 1940s, many policy-driven measures have been taken during the 1970s and 1980s to combat the adverse effects of N and P enrichment in both marine waters and in freshwater systems in Europe (e.g. de Jong, 2007). Riverine nutrient concentrations have decreased since then leading to an improvement of the environmental status (e.g. van Beusekom et al., 2019). However, in most aquatic (marine and freshwater) systems, a good environmental status (as defined by the MSRL and WFD) has not been reached yet (see Deliverable 4.1). The NAPSEA goal is to develop a more holistic view on eutrophication by looking at the Nand Pflows from the source to the sea. The aim of this deliverable is to propose new Safe Ecological Limits and discuss them in relation to the present eutrophication status based on existing indicators. We define Safe Ecologic Limits as the limits of factors that change an ecosystem in an undesirable way. This implies that indicators must be developed, adopted or modified to enable a quantification between driver and ecological response. This is a very challenging task as many natural factors (or “noise”) and human driven factors impact the status of an ecosystem. This also implies that Safe Ecological Limits have to be defined locally. This is supported by the review of the indicators (Deliverable 4.1) which observed that many local indicators and limits are defined, and that these indicators do not form a clear continuum. For example, different indicators are used for groundwater, surface water, and coastal waters and for surface water the EU member states and local water authorities also use different indicators for their WFD status assessments. Since NAPSEA focuses on the Wadden Sea as the ultimate receiver of a large part of the nutrients released in northern Europa, we will discuss the newly proposed safe ecological limits in three terrestrial aquatic systems: Rhine, Elbe and the Hunze watershed (the Netherlands) all debouching in or –especially in the case of the Rhine/Meuse and Elbestrongly impacting the Wadden Sea. Basically, N and P loads are strongly coupled to the water flow starting with rain or melting snow and ice that contain only low nutrient levels from e.g. atmospheric sources, and that are continuously enriched with nutrients on their way to the sea. During transport from the source to the sea, nutrients may be taken up by primary producers, may be released from detritus or may be removed from the biogeochemical cycle by denitrification (N only) or burial (permanent sedimentation and/or dredging) of Nand P-containing particles. Especially nutrient uptake exerts a strong impact on the local ecological conditions because of the corresponding increases in organic matter loading (e.g. Nixon, 1995). First, we focus on the Wadden Sea as the ultimate receiver of nutrients from the two major north-European rivers (Rhine and Elbe) as major nutrient sources (e.g. van Beusekom et al., 2001). Until now, phytoplankton biomass has been a major indicator to evaluate the eutrophication status of the Wadden Sea for instance in the WFD or in the MSFD as it shows a clear relation with riverine nutrient inputs (e.g. van Beusekom et al., 2019). However, defining proper chlorophyll levels is challenging: For instance: why would a level of 4 instead of 5 µg chlorophyll / liter be a good indicator for a safe ecological limit? To circumvent such arbitrary choices, we suggest two approaches that look at discontinuities in the ecosystem response to changes in riverine nutrient loads: seagrass recovery and Si limitation of the spring phytoplankton bloom. Then we evaluate ecological conditions and discuss ecological indicators for instance as used in the WFD for the two major river systems -Elbe and Rhineeach with their own specific ecological settings and for a small lowland catchment with a small lake (Hunze/Zuidlaardermeer) entering the Wadden Sea. In the last chapter of this deliverable, we summarize the results from the case studies and draw some overarching conclusions. and discuss whether current environmental goals as formulated in the WFD are able to bring both the Wadden Sea and the contributing river basins within Safe Ecological Limits. 4. SAFE ECOLOGICAL LIMITS FOR THE WADDEN SEA 4.1 Introduction The Wadden Sea is a unique intertidal coastal ecosystem stretching along the Dutch, German and Danish North Sea coast which was declared as a World Nature Heritage Site in 2004. The Wadden Sea is a relatively young ecosystem that developed about 7500 years ago after the end of the last glacial, and human impacts have changed the Wadden Sea since about 1000 years (Lotze et al., 2005). Eutrophication is one of the more recent human pressures impacting the Wadden Sea. Our understanding of the Wadden Sea eutrophication was shaped especially by the investigations since the 1950s of the Netherlands Institute for Sea Research in Den Helder and since 1969 on Texel shaped: First signs of increased eutrophication were already described in the 1970s. Van Bennekom et al. (1975) already noted the strong increase in nutrients in the Rhine. De Jonge and Postma (1974) Page 8 of 64 Deliverable 4.2 showed a 3-fold increase in phosphorus import from the North Sea into the Wadden Sea between the 1950s and 1970s. In this chapter, we will summarize our understanding of Wadden Sea eutrophication, its long-term trends and regional differences, review potential indicators and suggest Safe Ecological Limits. Phytoplankton (Chlorophyll a) is a widely used eutrophication indicator. One of the goals of the NAPSEA project is to suggest alternative indicators that may guide Safe Ecological limits for the Wadden Sea. We will focus on 1) the role of silicium (Si) as a limiting nutrient for phytoplankton and 2) on seagrass response to eutrophication. 4.2 Area description The Wadden Sea is a shallow coastal sea with a length of about 500 km and a width of 10-30 km between Den Helder in the Netherlands and the Skallingen peninsula in Denmark (Figure 1). Most of the Wadden Sea is protected from the North Sea by barrier islands. Tides play a dominant role in shaping the Wadden Sea: Tidal range is between 1.5 and >3.5 m. Highest ranges are found in the central Wadden Sea. Tidal ranges > 3 m prevent the formation of barrier islands. About 50% of the Wadden Sea are intertidal flats emerging during low tide. Sediments in most of the Wadden Sea intertidal and subtidal areas are dominated by sand, but in the more protected areas, muddy sediments prevail (e.g. Dijkema, 1991; Baptist et al., 2019). Freshwater impacts the Wadden Sea both directly and indirectly. Important direct freshwater sources into the Wadden Sea are lake IJssel (fed by the Rhine) as well as the rivers Ems, Weser, Elbe, Eider and Vårde A. The most important indirect nutrient source is the combined outflow of the rivers Rhine and Meuse. The salinity is about 30 psu but clear salinity gradients exist near river mouths and salinity can also be lower near sluices. Postma (1954) already pointed out that the Wadden Sea is a heterotrophic area importing organic matter produced by phytoplankton in the coastal North Sea (see also van Beusekom et al. 1999). This is the reason, why nutrient discharges via the river Rhine and Meuse are so important for the Wadden Sea eutrophication especially in the southern part (e.g. van Beusekom et al., 2001). Figure 1. Map of the Wadden Sea showing the most important rivers. The blue line indicates the residual currents transporting Rhine/Meuse river water towards the Wadden Sea. (an updated version is being prepared). 4.3 Eutrophication history Riverine nutrient concentrations sharply increased after WWII, peaked during the 1980s and 1990s, and decreased since then (Figure 2). At present, TN concentrations are about 50% and TP about 75% below their maximum levels, (van Beusekom et al., 2019). The increase in riverine nutrient loads is clearly reflected in the nutrient concentrations observed in the Wadden Sea (e.g. van Beusekom et al., 2001). Page 9 of 64 Deliverable 4.2 Figure 2. Historical discharges, total nitrogen (TN) concentrations and TN loads in the rivers Rhine (measured at Lobith; panels at the left) and Elbe (measured near Geesthacht; panels at the right). TN loads were either measured (green dots) or estimated based on measured DIN values and correlations between NH4 and Organic N (Elbe)or Kjeldahl-N (Rhine). Source: van Katwijk et al. (2024). Already during the 1970s, researchers from the NIOZ observed that increased riverine nutrient loads affected the Wadden Sea: De Jonge and Postma (1974) mentioned a three-fold increase in organic P concentrations between the 1950s and 1970s. Helder (1974) observed no changes in the NO3concentrations but a clear increase in NH4+-concentrations compared to the first N measurements in the Wadden Sea (Postma, 1966) indicating an increased organic matter (OM) turnover. Ecological consequences of the increased nutrient concentrations were indicated by two time series in the Western Dutch Wadden Sea on primary production and macrobenthos: Benthic primary production doubled between 1968 and 1981 from around 100 gC m-2 y-1 to more than 200 gC m-2 y-1 around 1980 (Cadée, 1984), pelagic primary production also more than doubled in the Marsdiep from about 150 gC m-2 y-1 during 1964-1976 to ~350 gC m-2 y-1 in the 1980s (Cadée & Hegeman, 1993)). Macrobenthos biomass and annual production doubled between 1970 and 1984 (Beukema & Cadée, 1986). 4.3.1 Negative effects of eutrophication In the Wadden Sea and elsewhere, seagrass ecosystems are important habitats delivering ecosystem services like nursery habitat, improved water quality, coastal protection, and carbon sequestration (Valdez et al., 2020). Negative effects of the increased nutrient loads were observed for seagrass showing a downward trend in the Dutch Wadden Sea (den Hartog and Polderman, 1975), in the Lower Saxonian Wadden Sea (Michaelis, 1987) and in the northern Wadden Sea (Dolch et al., 2013, see also van Katwijk et al., 2024). Conversely, especially since the late 1980s, massive green macroalgae blooms were observed both in the Lower Saxonian Wadden Sea and in the northern Wadden Sea (Reise, 1994; Reise & Siebert, 1994; 1997). Negative effects are among others anoxic sediments (Neira & Rackemann, 1996) and accompanying mortality of macroand meiobenthos (Reise & Siebert, 1994; Neira & Rackemann, 1996). A one-time event related to eutrophication was the occurrence of black spots covering a large part of the Lower Saxonian Wadden Sea in 1996. Large parts of the sediment were completely anoxic which turned out to be a unique phenomenon (Farke, 1997; Michaelis, 1997). Possibly, the large amounts of macroalgae played a role (compare Reise, 1994; Farke, 1997). 0 1000 2000 3000 1960 1980 2000 2020 Year Discharge (m3s) Rhine near Lobith 0 300 600 900 1960 1980 2000 2020 Year Discharge (m3s) Elbe near Geesthacht (Wehr) 0 250 500 750 1000 1250 1960 1980 2000 2020 Year Total Nitrogen (µmol/l) Kjeldahl−Nitrogen estimated from NH4 measured 0 250 500 750 1000 1250 1960 1980 2000 2020 Year Total Nitrogen (µmol/l) TN TON estimated from NH4 measured 0 200 400 1960 1980 2000 2020 Year Total N load (kTonnes/year) 0 100 200 1960 1980 2000 2020 Year Total N load (kTonnes/year) Page 16 of 64 Deliverable 4.2 happened after riverine concentrations exceeded this N/Si ratio of 1. Thus, given the maximum winter Si concentrations of about 200 µM in Elbe, Ems and Schelde, we suggest DIN concentrations of 200 µM as a Safe Ecological Limit for rivers, noting that for the Weser, no data are available. Given the lower Si levels in the Rhine, we suggest a winter DIN concentration of 135 µM as a Safe Limit for the Rhine. Table 1. Summary of reduction needs of DIN concentrations (µM) or TN loads (kT/year) for 1) transition from Si to N limited spring phytoplankton blooms and 2) seagrass recovery. Reference are the years 2010 – 2017. River Winter Si max Winter DIN max DIN Reduction to reach Si/N of 1 Reduction in annual TN loads for seagrass recovery Western Dutch Wadden Sea5 Reduction in annual TN loads for seagrass recovery Lower Saxonian Wadden Sea5 Rhine/ Meuse 135µM1 260µM 50% 38% (34-43%) 43% (39 – 46%) Ems 200µM2 370µM 55% NR 43% (39 – 46%) Weser 200µM3 330µM 40% NR NR Elbe 200µM1 290µM 30% NR NR Notes 1) Based on Pätsch (2024) 2) Pätsch (2024), Helder & de Vries (1986) 3) No data available. We assume similar values as for the Elbe 4) NR: Not relevant 5) Reduction in total nitrogen (TN) loads of the Rhine/Meuse required for seagrass recovery was based on 12 scenarios to derive thresholds: 3-, 4and 5-year averaged TN loads, and the year 2000 as start of the acceleration and 2003 as the start of the recovery setting through (van Katwijk et al., 2024). Since 2010, Elbe maximum DIN concentrations are reached in February and amount to about 290 µM. This would imply a reduction of 30% assuming winter Si concentrations of 200 µM (see also Table 1). Weser maximum DIN concentrations since 2010 are reached in January and amount to about 330 µM. This would imply a reduction of about 40% assuming winter Si concentrations of 200 µM. Ems maximum DIN concentrations since 2010 are reached in January and amount to about 370 µM. This would imply a reduction of about 55% assuming winter Si concentrations of 200 µM. In the Rhine/Meuse, maximum DIN concentrations since 2010 are reached in February and amount to about 260 µM. This would imply a reduction of almost 50% assuming winter Si concentrations of 135 µM. The winter Si concentrations suggested above are a first estimate. A more in-depth analysis of Si dynamics is needed to understand interannual differences in winter concentrations. Also, investigations are needed on particulate biogenic Si (dead frustules from diatoms) as this may be an additional source of dissolved Si. 4.5 Discussion 4.5.1 Loads versus concentrations in setting reduction goals Two approaches were used to estimate reduction needs: 1) based on TN loads from winter to summer enabling seagrass return in the southern Wadden Sea and 2) based on Si/DIN ratios in winter. At first sight, these goals may seem to be unrelated since they target quite different aspects, but from a management point of view, N input has to be reduced forboth of them in a similar amount (30-50%). After all, the difference between the two approaches is riverine discharge (loads are concentrations multiplied by discharge). If we assume that long-term, yearly discharge levels remain the same, measures to reach the suggested reduction should be similar. However, it should be noted that if long-term changes in discharge occur, this will change the TN loads. Changes in frequencies and durations of climate extremes can also change riverine loads as e.g. droughts will increase the N retention (Schulz et al., 2023). 4.5.2 Relative importance of rivers impacting the Wadden Sea The statistical analysis of the relation between eutrophication status and riverine TN loads only used the Rhine/Meuse (for the southern Wadden Sea) and the Weser/Elbe (for the northern Wadden Sea). These were chosen as they represent the largest riverine sources for the Wadden Sea. This, however, does not preclude that Page 17 of 64 Deliverable 4.2 other rivers are also impacting the Wadden Sea. From a statistical point of view, it is extremely difficult to distinguish between the contribution of the different rivers because of correlation of the TN loads among the different rivers. Also, other sources such as atmospheric deposition and the import of organic matter must be taken into account. Table 2. Relative magnitude of the TN loads of rivers impacting the Wadden Sea based on correlation with the annual TN loads (1979 -2022) from the Rhine and Meuse. River R2 Intercept (kT N/year) Slope (=Fraction of Rhine/Meuse TN Loads) Schelde 0.62 3.5 0.083 Lake IJssel 0.82 10 0.17 Ems 0.57 5.1 0.045 Weser 0.80 1.2 0.19 Elbe 0.80 -15 0.48 Table 2 summarizes the correlations between the annual TN loads from the different rivers impacting the Wadden Sea with the TN loads from the Rhine/Meuse. It clearly shows the Rhine/Meuse as the largest riverine N source for the Wadden Sea and adjacent coastal zone of a similar magnitude as Schelde, IJsselmeer, Ems, Weser and Elbe together. The close correlation suggests that EU legislation and similar management measures have impacted the European river TN loads in a similar way. We also correlated trends in N deposition into the Wadden Sea with Rhine/Meuse river loads. To correct for interannual differences in TN loads driven by discharge, we calculated flow-normalized concentrations (total annual TN load / total discharge). Again, good significant correlations were found for the Dutch (r2 = 0.58), Lower Saxonian (r2 = 0.44) and Schleswig-Holstein Wadden Sea (r2 = 0.41) suggesting that EU legislation and similar management measures impacted riverine loads and atmospheric deposition in a similar way. Model exercises show that the relative amounts of N from different sources can be traced back to their sources (e.g. Troost et al., 2013). This might open the possibility to specifically manage those sources that are the main drivers of Wadden Sea eutrophication. However, the main driver of Wadden Sea eutrophication is the import of N-containing organic matter produced in the adjacent coastal zone. This complicates the relative attribution of the different sources including atmospheric deposition as particles behave non-conservatively. For instance, particles can settle in different water masses, or nutrients released from particles end up in different water masses. As a starting point, we therefore suggest that riverine TN loads and atmospheric sources must be reduced for all river basins impacting the southern Wadden Sea by a similar magnitude as suggested in Table 1 to enable permanent seagrass recovery. As a second step we suggest taking into account the winter DIN/Si ratios. This indicates that stronger measures are needed for the river basin of the Ems as here highest DIN/Si ratios are found. 4.6 Outlook Within the NAPSEA project, two new approaches to assess Safe Ecological Limits for the Wadden Sea were developed based on seagrass dynamics and on phytoplankton limitation by dissolved Si. The focus was on reducing the N loads to the Wadden Sea as presently, N/P-ratios in winter are extremely high, high N/P ratios have a negative effect on food quality and N is ultimately limiting phytoplankton growth. In the other case studies, the role of P is discussed in more detail as P is an important nutrient potentially limiting phytoplankton growth in freshwater systems. At present, not enough data are available to test the impact of reduced nutrient loads and changed nutrient ratios (N/P ratios) on the entire food web (compare Philippart et al., 2007). Such information will be necessary to evaluate the interacting effects of reduced primary production but also changed N/P ratios on food quality availability and their impact on the carrying capacity of the Wadden Sea. Page 18 of 64 Deliverable 4.2 5. SAFE ECOLOGICAL LIMITS FOR THE ELBE ESTUARY 5.1 Introduction Both the riverine part of the Elbe and its estuary have been subject to many man-made changes including pollution and eutrophication and morphological changes like weirs, diking and dredging. In this chapter, we will focus on eutrophication and the consequences of large phytoplankton blooms in the riverine part of the Elbe for the Hamburg port area in the upper part of the Elbe estuary, where nowadays very low oxygen levels are reached down to levels which impact fish populations. We expect that mitigating this issue will also bring the Elbe loads towards the Wadden sea withing Safe Ecological Limits. In addition, N and P reductions in the headwater tributaries of the Elbe are needed to solve the oxygen problems in the estuary, although we do not consider the local safe ecological limits within these headwaters in this chapter. 5.2 Area description The Elbe is the largest river in Northern Germany (Figure 13). It originates in the Giant Mountains region in the north of the Czech Republic and has a catchment area of 148268 km2 and a total length of 1094 km (IKSE, 2005). The free-flowing part of the Elbe ends at Elbe-km 586 at the weir of Geesthacht, which was built between 1957 - 1960. The long-term mean annual discharge at Neu Darchau (1903 – 2019) is 694 m3/s (data: FGG Elbe). Figure 13. The Elbe catchment. https://en.wikipedia.org/wiki/File:Elbe_basin.png Figure 14Figure 14 Page 19 of 64 Deliverable 4.2 Figure 14. Changes in annual total nitrogen loads (upper panel), the NH4+ concentrations (middle panel) and share of organic N in the Elbe near Geesthacht (lower panel). For details on the estimation of total organic nitrogen (TON) before 1979: see van Katwijk et al., 2024. Data: data portal of the FGG Elbe (2020). After 2016, the no measurements were carried out at the Geesthacht stations and only data from the transects through the estuary (~4/year) are available. 5.3 Eutrophication history Large changes in water quality have been documented in the Elbe. Eutrophication already started in the 1950s and culminated during the 1980s (e.g. van Beusekom et al., 2019; van Katwijk et al., 2024). In the Elbe estuary, O2 concentrations during the 1980s were low due to high loads of organic matter and NH4+ (e.g. Kerner, 2000, compare Figure 14). From 1990, political and economic changes in Czechia and former Eastern Germany allowed for the quick adoption of measures leading to the elimination of the most critical sources of pollution (e.g. Adams et al. 1996; Langhammer, 2010; Rewrie et al., 2023). For instance, a rapid decline in organic matter and NH4+ loading (Figure 14) was observed. Improved water quality enabled the development of large summer phytoplankton blooms in the riverine part of the Elbe since the 1990s (e.g. Rewrie et al, 2023) with lowest levels during winter (week 45 – week 10) and highest levels in summer (week 20 – week 30) with averages of around 90 - 130 µg Chl a/l and maximum values up to 300 µg Chl a/l (Figure 15). This is reflected by organic N Page 20 of 64 Deliverable 4.2 compounds increasing to reach levels up to >75% in summer in contrast to values <5% in winter (compare the large variability in Figure 15 since about 2000 with lowest values in winter and highest in summer). During spring and summer, phytoplankton levels constantly increase in the Elbe from Schmilka, where the river enters Germany, to the end of the riverine part (Weir at Geesthacht 30 km upstream of Hamburg; see Figure 16 left). Factors that support the riverine phytoplankton blooms are low water depth in the river (better light conditions) and long residence times which both occur during low discharge events (e.g. Scharfe et al., 2009; Kamjunke et al., 2021). Figure 15. Left: Average Chlorophyll levels (March-October; 2010 - 2016). The green line shows the environmental goals for the WFD (OGewV). Right: Seasonal dynamics of phytoplankton (as Chlorophyll a) between 2010 and 2016 at the end of the river Elbe near the Weir at Geesthacht. The green line shows the environmental goals for the WFD (OGewV). Figure 16. Left: Longitudinal profile of measured (black dots) and modelled (dark grey line) seasonal means of chlorophyll a (μg/l) from Schmilka (km 0) to Cuxhaven (km 727) for May to October 2006. Number of measurements is given in brackets. Standard deviations of measurements reflected by bars and of modelled data (n = 184) by the light grey area (from Schöl et al., 2014). Page 21 of 64 Deliverable 4.2 In the upper estuary, water depth are around 4 m but increase sharply in the Hamburg port area to levels of about 18 m due to dredging. Once the Elbe phytoplankton blooms enter the deepened part of the estuary, grazing and light limitation decimate the phytoplankton standing stocks. High degradation rates lead to low oxygen concentrations in the Hamburg port area clearly below the level of 219 µmol/l (7 mg/l) as set by German law (Schöl et al., 2014; Geerts et al., 2017, see Figure 16 right). 5.4 Safe Ecological Limits As a first safe ecological limit for the Elbe estuary, we propose to use the value of 7 mg O2/l (219 µmol/l) as set by German law (OGewV). Based on model calculations we will suggest reduction needs to keep oxygen levels in the estuary within Safe Ecological Limits. As a second approach, we use the environmental goals (OGewV) for phytoplankton biomass aiming at a mean of 40 µg Chl a/l for the period March – October. The reduction needs will be discussed in relation to Safe Ecological Limits for the Wadden Sea. 5.5 Modelling the O2 dynamics in the Hamburg port 5.5.1 Methods To model the oxygen dynamics in the Elbe estuary, we use the existing SCHISM-ECOSMO model for the Elbe estuary. It uses an unstructured model grid covering the Elbe estuary with a computational mesh with a resolution between 30 m in the port of Hamburg and 500 m at the model boundary in the German Bight. As hydrodynamical core it uses the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM, Zhang et al., 2016) as hydrodynamical core, which is coupled with an ecological model, the ECOlogical System Model (ECOSMO, Yumruktepe, 2022). With the help of SCHISM, researchers have successfully modelled both idealized and realistic estuarine domains tackling research questions in the areas of hydrodynamics, sediment dynamics and ecology (Pein et al., 2021a, b; Stanev et al., 2019; Ye et al., 2018). The SCHISM solves the Reynolds-averaged Navier–Stokes equations on unstructured meshes assuming hydrostatic conditions. The model predicts water elevation, horizontal currents, vertical exchange and tracer transport. The biogeochemical model ECOSMO has successfully been applied to simulate lower trophic level biogeochemical dynamics in the North Sea (Schrum et al., 2006) and Baltic Sea (Daewel and Schrum, 2013). In the coupled physical-biogeochemical model, the local concentration of an ecological tracer changes according to 𝐶𝑡+(𝐯∇)𝐶 + (𝑤𝑑)𝐶𝑧=(𝐴𝑣𝐶𝑧)+ 𝑅𝐶, (1) where C represents an ecological state variable, v refers to the 3D velocity field, 𝑤𝑑 is a constant settling velocity and 𝐴𝜈 is the turbulent diffusion coefficient. In ECOSMO, the pelagic prognostic state variables comprise four nutrients, three functional groups of primary producers, herbivorous and omnivorous zooplankton, detritus, opal and dissolved organic matter. The term in Eq. (1) represents the biogeochemical sources and sinks modifying the respective state variable concentration C. In the Elbe estuary, the coupled modelling framework SCHISM-ECOSMO can simulate the physical-ecological dynamics on tidal to seasonal to intra-annual scales (Pein et al., 2021a). With the help of this model, it was demonstrated that tidal pumping leads to trapping of organic particulate matter in the port region that is remineralised in the same area, whereas summer heating leads to enhanced process rates and water column stratification. These processes exacerbate the oxygen depletion in the deep channels and basins in the port region (Pein et al., 2021a). 5.5.2 Response of the O2 dynamics to reduced organic matter loads Here we used the validated model configuration for the year 2012, to investigate the effect of nitrogen load reduction at the tidal weir on oxygen levels in the port of Hamburg (Figure 17). The scenario approach was as follows: Two historic scenarios adopted the ratio of contemporary to historic N loads reported by Serna et al., 2010 for the 1960s and pre-industrial times, respectively. These resulted in our historic scenarios E1960 and E1860 in which both inorganic and organic nitrogen loads were modified according to the findings of Serna et al., 2010. The historic scenarios were complemented by two idealised scenarios reducing the total nitrogen loads by 50% and 75% respectively resulting in our scenarios E-50 and E-25. Page 22 of 64 Deliverable 4.2 Figure 17. Reduction scenarios for the effect of reductions in the organic matter loading from the riverine part of the Elbe on the oxygen dynamics in the Hamburg port area. The green line marks the level of 7 mg O2/l (219 µmol O2/l) specified as Good Environmental Status in German law (OGewV). Note that the y-axis starts at 150 mmol O2/m-3. 5.6 Reductions needed to stay within Safe Ecological Limits At present, O2 levels in the Elbe reach very low levels during summer being clearly below Safe Ecological Limits of 219 µmol O2/l or 7 mg O2/l as indicated by the green line in Figure 17. We estimated the response of the O2 dynamics to organic matter loads by stepwise reducing the load into the estuary (Figure 17). As reference, we used the year 2012. A 50% reduction in organic matter loading leads to O2 levels just above 7 mg O2/l. From this we estimate a reduction need of the organic matter loading by about 40 – 45%. This range is lower than the reduction needed to bring phytoplankton biomass within Safe Ecological Limits as proposed by the WFD/ OGewV: At present (2010-2016) average phytoplankton biomass at the end of the riverine stretch of the Elbe near Geesthacht (March – October) is 109 µg Chl a /l. To reach Save Ecological Limits of 40 µg Chl a/l, a reduction of 63% is needed, which is clearly higher than the reductions of 40-45% needed to keep O2 levels in the upper estuary at levels of 7 mg O2/l (219 µmol/l). All reduction levels are relative to the year 2012. The mentioned reduction requirements however are assumed to also apply to the present conditions as since 2010 no clear trends in riverine flow-normalized concentrations have occurred (except for discharge driven changes; see previous chapter on Wadden Sea Safe Ecological Limits). 5.7 Discussion The reductions in summer organic matter loads from the river Elbe to keep O2 levels in the upper estuary above 7 mg /l O2 of 40 – 45% are lower than the reduction needs of about 60% to keep mean phytoplankton biomass (March-October) below 40 µg Chl a/l. An important question is, which factors determine phytoplankton bloom size. The major factor presently determining the phytoplankton dynamics is river discharge (Kamjunke et al., 2021) and Si availability (Scharfe et al., 2009). Apart from Si availability that limits diatom growth, P can become limiting during low discharge events (Kamjunke et al., 2021). Measures to reduce riverine N-loads have led to decreasing annual TN loads (Figure 14) and in summer to minimum concentrations decreasing from around 300 µmol DIN /l during the early 1990s to less than 50 µmol DIN /l since 2003 (Schulz et al., 2023). Recently, indications of a potential N limitation were observed during periods of extreme low discharges (Schulz et al., 2023). It remains an open question, whether future N reductions may lead to an N limited phytoplankton bloom. 5.7.1 The role of Si Scharfe et al. (2009) pointed at the role of Si in Elbe phytoplankton dynamics. Figure 18 shows the relation between DIN and Si. Both N and Si are taken up with a ratio of about 1:1. A similar ratio was observed in the Wadden Sea (see Chapter on Wadden Sea Safe Ecological Limits). Decreasing riverine N loads since the 1990s led to increasingly lower amounts of N left over after Si reached limiting concentrations. Page 23 of 64 Deliverable 4.2 Figure 18. The relation between Si and DIN during the spring blooms in the 1990s, 2000s and 2010s. The black line shows the 1:1 ratio between DIN and Si at a y-axis intercept of 100 µmol DIN/l. Data: FGG Elbe. The colours indicate the month of the year (dark = January, light blue = May) 5.7.2 The role of N and P Kamjunke et al. (2021) highlighted that mostly N and P are not limiting phytoplankton dynamics. However, during low discharge conditions, causing low water levels and long residence times, large phytoplankton blooms developed during which P can become the main limiting nutrient in the lower reaches of the Elbe during low discharge events. An important question is whether also N can become a limiting nutrient in the future. This is relevant as N is the main element limiting phytoplankton dynamics in the Wadden Sea from June/July onward (Chapter on Wadden Sea Safe Ecological Limits). Schulz et al. (2023) showed that since 2018 during low discharge conditions, extreme low NO3values were reached suggesting that N can become limiting if N levels are further reduced. First indications on the potential consequences of N limitation were observed in 2022: Already in early June 2022 NO3levels of about 3 µmol/l and DIN levels of about 5 µmol/l were observed (Lempges, 2023). During the following weeks, the phytoplankton bloom collapsed leading to low O2 levels of around 1 mg/l in the riverine part of the estuary before the water entered the deepened port area (data: Hygiene Institute, Hamburg). Figure 19. Recent seasonal cycles of DIN (left) and Chlorophyll a (right). The green bars in both graphs indicate the period when phytoplankton would have removed all DIN starting at winter DIN levels of 200 µmol/l. This level was chosen to reflect the situation at winter DIN/Si levels of 1 that were suggested to enable N-limited diatom blooms in the northern Wadden Sea. The blue line in both graphs show the smoothed seasonal dynamics. Page 24 of 64 Deliverable 4.2 5.7.3 How to reduce Chlorophyll and organic matter levels in the Elbe River In the Chapter on Safe Ecological Limits for the Wadden Sea, a maximum winter level of 200 µmol DIN/l was suggested. We have estimated how Chlorophyll levels in the Elbe would respond when the winterly DIN levels were brought down from around 300 as presently observed to the 200 µmol DIN/l mentioned above. Note that in winter DIN is dominated by NO3-. The green bar in Figure 19 (left) indicates the level of 100 µmol/l. If further management decisions would reduce the winter concentration to about 200 µmol DIN/l, limiting DIN levels could expected to be reached during the period indicated by the green bar. However, when DIN would reach these limiting levels, a Chlorophyll a level of about 140 µg /l can be expected (see the green bar in the right frame in Figure 19), which is still clearly above the goals set for the WFD (compare Figure 15). It is at present unclear how the phytoplankton blooms will further develop after N limiting conditions will be reached. In contrast to the summer situation in the North Sea and Wadden Sea nutrients are constantly entering the river system thereby potentially sustaining the summer bloom. Hence, it is questionable that a 30% reduction in N loads will actually lead to levels of 40 µg/l Chl a and O2 levels as envisioned in the framework of the WFD and demanded by the OGewV. Here, further research is clearly necessary. Given that P is at present the main limiting element in the riverine part of the Elbe, some people may draw the conclusion that riverine P loads must be further reduced. From an ecologists point of view, however, a further reduction P without a concomitant reduction in N should be avoided as this will further increase the N/P ratios with negative consequences for the aquatic food web as the plankton food quality is negatively influenced by high N/P ratios (Malzahn et al., 2007). 5.7.4 Morphological adaptions Alternative approaches to increase the resilience of the Hamburg port area to oxygen deficits include morphological adaptations as for example, the reconnection of a historic major river branch to the tidal system as proposed by Pein et al. (2024). Their model simulations demonstrate that proposed measures potentially reduce the siltation of the upper estuary and thus the need for extensive and costly maintenance dredging. Furthermore, the simulated measures also mitigate the consequences of eutrophication, such as the low oxygen content in the navigation channel. 5.8 Conclusions To reach Safe Ecological Limits regarding Elbe phytoplankton blooms, even stronger reduction measures are needed than the ~30% reduction in DIN concentrations suggested to reach Wadden Sea Safe Ecological Limits. We suggest that a reduction of both N and P is needed. But the amount of reduction needed cannot be derived yet from the available data. Dedicated models and experiments are needed to estimate the extent of N and P reductions needed to limit phytoplankton biomass build-up the Elbe. 6. SAFE ECOLOGICAL LIMITS FOR THE RHINE BASIN 6.1 Introduction and area description For the Rhine case study, we focused on the relation between the main river and the many tributaries in relation to the safe ecological limits. The Rhine basin is one of Europe's most significant and most intensively used river systems. It covers an area of approximately 185,000 km2 and flows through several countries, including Switzerland, Germany, France, and the Netherlands, before draining into the North Sea/ Wadden Sea. Its catchment area is home to 58 million people of which 30 million people depend on the Rhine and its banks as a drinking water source (Plum & Schulte-Wülwer-Leidig 2014). Half of the Rhine`s catchment area is used for agriculture. The Rhine itself is intensively used as a traffic axis contributing to the development of heavy industries and chemical industries (https://www.iksr.org/en/topics/uses/industry).The Rhine origins in the Alps and drains into the North Sea/ Wadden Sea. It´s alpine and high parts in Switzerland cover around 20% of the total catchment area and has a mean annual discharge of 1060 m3/s (gauging station Basel, Switzerland, Belz et al. 2007). It is characterized by a nival discharge regime with maximum discharge in June and July, although dampened by the large Lake Constance that the Rhine is flowing through. Lake Constance has a high surface area (539 km2), large volume (48.53 km3) and long residence time of water (4.5 years) (World Lake Database, https://wldb.ilec.or.jp/Lake/EUR-33). In contrast to the upstream part, the Rhine at Lobith, at the German-Dutch border has a mean discharge of 2220 m3/s and a mixed rain-nival regime with the dominant peak in February. Relative to the observation at Lobith, the Page 25 of 64 Deliverable 4.2 alpine and high parts of the Rhine in Switzerland deliver around 48% of the total discharge. The largest tributaries of the Rhine are the Main, Moselle, Neckar, Aare and Ruhr (Belz et al. 2007). While the main stem of the Rhine is not dammed downstream of Switzerland, its tributaries such as Main and Moselle are not free flowing and have weirs and locks to make them navigable. From the large number of inhabitants and connected wastewater inputs, the intense industry along the Rhine and from the agricultural usage of its catchment areas, the Rhine and its tributaries faced a long history of severe river pollution. While water quality greatly improved since the 1980s, the Rhine still faces challenges in chemical and biological water quality and still is a major source for nutrients and related eutrophication impacts in the Wadden Sea (van Katwijk et al. 2024). Below, we summarize ecological challenges in the Rhine and its tributaries in the frame of the NAPSEA project and discuss nutrient concentrations and loads as safe ecological limits. 6.2 Eutrophication in the Rhine basin Within the NAPSEA project, we focus on the N and P concentrations and loads in the terrestrial, estuary and marine systems. Both N and P had been a much larger problem in the main stem of the Rhine in the past, especially in the 1970s and 1980s. Since then, much has been done to improve nutrient concentrations and loads in the river (Plum & Schulte-Wülwer-Leidig 2014). This is especially true for the regulation of point source inputs, that e.g. for N was reduced by a factor of 3.6 between the mid-1980s and the 2010s (according to our data, see Figure 20). At the same time, diffuse N inputs reduced by a factor of 1.8 due to measures in the agricultural sector and reduced atmospheric inputs. For P inputs, the ban of phosphates in textile detergents in the 1980s and the enhancement of wastewater treatment plants greatly reduced point source inputs into the Rhine. For the Rhine main stem, both resulting N and resulting P concentrations consequently improved (Figure 21, Figure 22). Appendix A also gives the water quality status for N and P for the Dutch part of the Rhine. The 3-year summer average concentration target for Total-N of 2,4 mg/l is met since around 2010, except in 2019 and 2022. The target for Total-P of 0,14 mg/l has been permanently met since 2012. The trend assessment for Total-N and Total-P (Appendix B) shows significant downward trends between 1990 and 2020. Since around 2008, this trend flattens especially for Total-N. Similar to the German part of the Rhine basin, more severe eutrophication issues occur in the tributaries. In the Dutch part of the Rhine river basin district, nearly 50% of the water bodies had a Moderate or worse status for phosphorus and about 30% had Moderate or worse status for nitrogen in 2021 (IenW, 2022). Figure 20. Nitrogen inputs from diffuse (Nsurplus) and wastewater point sources to the Rhine basin as quantified in the NAPSEA project, based on Batool et al. (2022), Häußermann et al. (2019), Büttner (2020) and Sarrazin et al. (2024). Page 32 of 64 Deliverable 4.2 improving the wastewater treatment, and increasing wetlands for water and nutrient retention during high discharge. The compliance for biological WFD parameters seems to lag behind both in Hunze and Zuidlaardermeer. This can have several causes, e.g.: • Lower nutrient levels are needed for a switch to a good ecological state (alternative stable states, see shallow lakes theory (e.g. Scheffer & Nes, 2007)) • Toxicity effects of the non-complying chemicals or emerging contaminants • The legacy of nutrients (especially P) still present in the sediment of Zuidlaardermeer and, more locally, in and around the Hunze • Suboptimal stream and lake morphology, wave impact, sediment, turbidity Apart from the nutrient mitigation actions mentioned above, large investments have been made in improving the stream and lake morphology. Many parts of the Hunze have been widened, re-meandered and its riparian zones were restored. In addition, fish migration has been promoted by removing or by-passing obstacles. Around Zuidlaardermeer, the riparian zones and wetlands have also been restored and/or re-connected to the lake. Total-N concentrations in Hunze Total-N concentrations in Zuidlaardermeer Figure 27. Total-N concentrations in Hunze and Zuidlaardermeer; the 3-year summer average concentrations (black line) are compared to the WFD concentration thresholds (indicated by green tot red colors). 7.4 Dutch WFD nutrient targets applicable to Hunze 7.4.1 Bioavailability The water quality objectives in the WFD are supportive of the ecological objectives. The bioavailability of the various forms of N and P plays an important role in the effects on ecology. Inorganic forms of N and P such as ammonium, nitrite, nitrate and orthophosphate, are readily taken up by algae and are thus bioavailable. For uptake of N and P in organic form, these compounds must first be degraded. The degradability of organic N and P compounds varies greatly. Simple organic N and P compounds are usually easily absorbed. Difficult-todegrade compounds such as humic and fulvic acids may contain N and P that is not available for algal growth. Page 33 of 64 Deliverable 4.2 The same is true for inorganic particulate P compounds. Phosphate adsorbed on the surface of inorganic particles is bioavailable, while phosphate in precipitates such as iron-(hydr)oxyphosphates and calcium phosphates is unavailable or hardly available to algae. Plants, however, can better release and absorb organically and inorganically bound P with their roots, including from larger soil or sediment particles. One way to do this is to excrete acid from the roots. Little is known about to what extent rooting aquatic or riparian plants, or plants with “floating roots,” such as duckweed, can also release and absorb phosphorus is not clear. When P-rich sediments become anoxic (usually in dry and warm periods), phosphate can be released from the iron(hydr)oxides. 7.4.2 Dutch WFD nutrient targets The WFD targets for nutrient concentrations in Hunze and Zuidlaardermeer stem from the implementation process of the Water Framework Directive in the Netherlands. In the systematics of the Water Framework Directive (WFD), nutrients belong to the general physico-chemical quality elements. This means that the nutrient levels should not interfere with the achievement of the targets for biological quality elements. To come to the nutrients status classification system, summer averages of total-P and total-N are tested against biological targets (Heinis and Evers, 2007a). Nationwide nutrient standards have been derived that are differentiated by surface water type (e.g. the Hunze is water type R5; a slowly flowing headwater on sand, Zuidlaardermeer is M14; shallow buffered lake). The province of Groningen adopted these water type specific nationwide nutrient standards for Hunze (Schollema, 2020) and Zuidlaardermeer (Klomp, 2021). The general classification system distinguishes between natural water types (Van der Molen et al., 2012), heavily modified water types (Evers and Van Herpen, 2010), and artificial water types (Evers et al., 2012). The water types ponds and lakes (M types) and flowing waters (streams and rivers, R types) can be classified as either natural or heavily modified. Ditches and canals (M types) are always artificial water types. For natural water types, the standard lies at the (lower concentration limit of the) quality class “Good Ecological Status” (GES). The ecological standard for heavily modified and artificial water types is the Good Ecological Potential (GEP). This standard is derived from the most similar natural water type. In summary, the methodology used for setting the Dutch WFD targets came down to the following: 1. For ponds and lakes, a relationship was used between the summer mean nutrient concentration (total-N and total-P) and the concentration of chlorophyll-a and aquatic plants. Figure 28 shows the “example” figure used in almost all reports about the derivation of N and P WFD objectives. In these relationships the chlorophyll levels (y-axis) can be entered, belonging to a certain ecological quality ratio (EQR). The nutrient concentration (x-axis) that follows from this is the limit at which it can be stated with 90% certainty that the EQR in question is achieved. Figure 28 was derived from an inventory dataset of freshwater lakes in the 1990s (Portielje and Van der Molen, 1999). It is observed that the scatter in the relationship is large. Heinis and Evers (2007b) attribute this to the aspect that factors other than P are limiting for algal growth. This ignores the aspect that total-P is a sum parameter, in which the bioavailability of the various P components varies greatly and the concentration of the most bioavailable fraction (inorganic dissolved P) is often very low. Page 34 of 64 Deliverable 4.2 Figure 28. Derivation of the Dutch WFD objective for lakes based on the relation between P-total and chlorofyl-a concentrations; the line represents the 95 percentile of the ratio. 2. A different method was used for running waters. In this method, all phytobenthos samples from streams in the database ‘Limnodata Neerlandica’ were tested against the ecological quality ratios (EQRs) and linked to the summer average total-P and total-N concentrations. The concentration of total-P and totalN at which 90% of the biological samples meet at least the selected quality (as EQR) is then the WFD target (Figure 29). As for the lake approach, the scatter between the EQR and the summer nutrient concentrations is large. Figure 29. Derivation of the Dutch WFD objective in a running water based on the relation between the nutrient concentration (x-axis) and the EQR (Dutch: EKR) scores (y-axis). Outside The Netherlands, separate standards for the P-total, dissolved P and soluble reactive P (SRP) are common in the EU (Philips & Pitt, 2015). For Nitrogen, together with Finland, The Netherlands are the only member state with just a standard for total-N. Other member states also have WFD standards for ammonium and/or nitrate. The Dutch focus on the growing season (standards for summer average concentrations) is more Page 35 of 64 Deliverable 4.2 common for lakes; around half of the EU member states do the same, while the other half uses annual concentrations. For rivers, The Netherlands, Belgium (Flanders) and Poland are the only member states using growing season average values (Philips & Pitt, 2015). 7.4.3 Nutrient targets and nutrient concentration variability Figure 30 shows the strong seasonality in the Total-N concentrations and in the N components NO3 and NH4 in one of the upstream Hunze tributaries (Voorste Diep). The winter Total-N concentrations generally exceed the Dutch WFD targets which are only applied to the summer average concentrations. Nitrate is the dominant N species, while both nitrate and ammonium show higher concentrations in winter. The high winter concentrations coincide with high discharges, causing the winter period to be dominant in the nitrogen loads to the Zuidlaardermeer and further downstream (canals of Groningen and the Wadden sea). The relevance of winter nutrient transport to receiving water systems is not recognized in the Dutch WFD targets for nutrients based on summer concentrations. Furthermore, efforts to reduce agricultural nutrient losses will mostly affect the drainage season (winter) concentrations. The Total-P concentrations do not show a large seasonality. Still, the winter loads for Total-P can also be higher due to higher discharge. Figure 30 shows that particulate P is the dominant P fraction, especially at high concentrations. This particulate P is largely attached to iron(hydro)oxides (Particulate Inorganic Phosphorus) (Van der Grift, 2017). From high frequency monitoring in other catchments (e.g. Rozemeijer et al., 2010; Van der Grift et al., 2016, Barcala et al., 2020), it is known that very short total-P concentration peaks occur during discharge events, when P-rich sediment is remobilized. Between the events, iron and P from groundwater form iron(hydr)oxides and replenish the P-rich sediment. The typical sub-daily concentration dynamics are not captured by monthly sampling schemes (like in Figure 30) and load estimates from these monthly measurements are highly uncertain. In addition, the summer average P concentrations based on 6 measurements used for WFD compliance testing are highly uncertain (e.g. Wade et al., 2012; Halliday et al., 2015. To get better insights into the nutrient concentration dynamics and loads from the Hunze to the Zuidlaardermeer, daily flow-proportional samples were collected at station ‘4206; Oostermoersevaart duiker in weg Zuidlaren-De Groeve’ for total-N and total-P analysis since 2019 (see Figure 31). These data show that daily averaged peak concentrations over 0,3 mg/l occur several times per year, while the base level total-P concentration is between 0,1 and 0,2 mg/l. For total-N, the seasonal pattern dominates, with summer concentrations around 1 mg/l and winter concentrations around 7 mg/l. The summers of 2019, 2020, and 2022 were relatively dry. In the more average summer of 2021, the total-N concentrations in the waters were higher (1-3 mg/l) due to more leaching and shorter residence times in surface water (resulting in less biochemical processing) compared to the dryer years. The summer of 2021 also shows higher base level total-P concentrations (0,15-0,3 mg/l) compared to the dryer summers. Page 36 of 64 Deliverable 4.2 Figure 30. Dynamics in N and P species concentrations at Voorste Diep. Page 37 of 64 Deliverable 4.2 Figure 31. Daily flow proportional total-P and total-N concentrations at the outlet of the Hunze into Zuidlaardermeer. 7.4.4 Limiting nutrients Zuidlaardermeer is mainly P limited (Klomp, 2021). The N/P ratio is increasing (see Figure 32), which means that the lake becomes increasingly P-limited. The measurements of Ptot and PO4 in Zuidlaardermeer (Figure 33) show very low concentrations (below 0,01 mg/l) in summer. However, the inorganic N-species (NO3 and NH4) also show very low concentrations in midsummer (Figure 34). Figure 35 shows measured Chl-a concentrations in Zuidlaardermeer. Page 38 of 64 Deliverable 4.2 Figure 32. Summer average N/P ratio in Zuidlaardermeer (from Klomp, 2021). Figure 33. total-P and PO4-P concentrations in Zuidlaardermeer (location Zuidlaardermeer noord). Note that the PO4 detection limits were lowered from 0.05 mg/l PO4-P to 0.01 mg/l in 2006 and further down to 0.005 mg/l in 2014. Page 39 of 64 Deliverable 4.2 Figure 34. Norg, total-N, NO3 and NH4 concentrations in Zuidlaardermeer (location Zuidlaardermeer noord). Page 40 of 64 Deliverable 4.2 Figure 35. Chlorophyll-a concentrations in Zuidlaardermeer (location Zuidlaardermeer noord). 7.5 Safe Ecological Limits Within NAPSEA, the aim is to connect the safe ecological limits of receiving water systems (like the Wadden Sea and the Zuidlaardermeer) with nutrient concentration and load targets for the contributing upstream catchments. For the Hunze catchment this means that the nutrient levels should be within the ecological thresholds for: • the surface waters in the Hunze catchment itself • the receiving Zuidlaardermeer • the water bodies between downstream of Zuidlaardermeer (among which the channels around the city of Groningen and lake Lauwersmeer) • the Wadden Sea Here, we focus on the safe ecological limits for nutrients from the Hunze catchment to ensure a healthy aquatic ecosystem in the Zuidlaardermeer and in the Wadden Sea. Our focus is not on the water bodies downstream of Zuidlaardermeer. With the Zuidlaardermeer being a vulnerable water system for eutrophication, we expect that that no additional reductions are needed for the downstream water bodies. In other words: when the nutrient loads from the Hunze are within the safe ecological limits for the Zuidlaardermeer, we assume that these loads are also low enough to protect the terrestrial water bodies downstream. The same holds for the surface waters within the Hunze catchment itself, which have higher nutrient concentration targets compared to Zuidlaardermeer: when the nutrient loads from the Hunze are within the safe ecological limits for the Zuidlaardermeer, we also expect the nutrient concentrations to be within safe ecological limits for the Hunze river. The impact of nutrients from the Hunze catchment on the Zuidlaardermeer is very direct. For the Wadden Sea however, the Hunze only provides a tiny fraction of the nutrient loading. A direct link between nutrient loads from the Hunze and the ecology of the Wadden Sea is therefore hard to make. However, for a healthy Wadden Sea, the sum of nutrient loads from all contributing catchments should be within safe ecological limits. When the Hunze catchment (or another one) does not comply, this would have to be compensated for by another area. To prevent rolling off nutrient reduction needs between areas, we assume that the safe ecological limits for the Wadden Sea can be translated to all contributing catchments, including the Hunze catchment. 7.5.1 Hunze safe ecological limits for the Zuidlaardermeer Although the WFD targets (total-N and total-P) are related to the concentration of chlorophyll-a and aquatic plants, the scatter in the relationship is large, and the classification from good to bad states may appear arbitrary. Hence, here we propose to focus not on specific Chlorophyll concentrations but on whether the system is algaedominated or plant-dominated (aka in the ‘clear water state’). The critical P-concentrations at which the system switches from one state to another depend on the local properties of the system but can be derived using PCLake (Figure 36) . PCLake is an ecosystem model especially designed to study the effects of eutrophication on shallow lakes and ponds, which has been extensively applied and calibrated on Dutch lakes (Janse, 2005). The model is used to define the critical P-loadings per lake and to evaluate the effectiveness of restoration measures, while taking into account the phenomena of alternative stable states and hysteresis (e.g. Scheffer & Nes, 2007). For this purpose, also a meta-model has been developed (https://www.witteveenbos.com/nl/digitale- Page 41 of 64 Deliverable 4.2 diensten/pclake-metamodel). The meta-model can be used by water managers to derive an estimate of the critical loading values for a certain lake based on only a few important parameters, without the need of running the full dynamical model. In order to change lake Zuidlaardermeer from an algal dominated state into a clear (waterplant dominated) state, the PCLake metamodel calculates a critical P-load threshold of 2.75 mgP/m2/d. Timeseries based on measurements during the years 2003-2016 show an average P-load of 4.5 mgP/m2/d, which would mean that (relative to that period) a reduction of almost 40% would still be needed. Once the system has reached a clear state, the PClake meta model indicates that the P-load should stay below 3.5 mgP/m2/d in order not to switch back to a turbid state. Assuming that the discharge from the Hunze catchment has not changed much since 2016 (apart from some uncommonly dry years resulting in odd outliers), a 40% load reduction would still be needed present day. This can be translated into a 40% reduction of the total-P concentrations within the Hunze catchment. The annual average Ptot concentration for 2003-2016 was 0,13 mg/l, so a 40% reduction would mean an average total-P concentration target of 0,08 mg/l. Figure 36. Screenshot from the website hosting the PClake metamodel, with the values used to represent lake Zuidlaardermeer. 7.5.2 Hunze safe ecological limits for the Wadden Sea In chapter 4, two approaches were outlined to define safe ecological limits for the Wadden Sea. The first approach focusses on preventing Phaeocystis blooms in the Wadden Sea. To achieve this, a reduction of total-N loads and concentrations for the contributing rivers of 30-55% is needed, depending on the silica concentration levels (compared to 2010-2017). For the Hunze, this would mean a reduction from 3.1 mg/l (annual average totalN concentration 2010-2017) to 1,4-2,2 mg/l. As the winter dominates the total-N loads (with both higher concentrations and discharge), a winter concentration target may be more appropriate. In that case, the 20102017 average December-February concentrations of 4,5 mg/l should reduce to 2,0-3,2 mg/l. The second approach for defining safe ecological limits for the Wadden Sea focusses on restoring sea grass (see chapter 4). For this, a 34%-39% reduction of N loads is needed (relative to 2010-2017 levels). For Hunze outlet this would correspond to a total-N concentration reduction from 3.1 mg/l (annual average 2010-2017) to 1,9-2,0 mg/l. 7.8 Discussion We observe that some regional water authorities focus on P and some on N. However, to reach safe ecological limits in all water systems, both N and P are important. In general, most freshwater systems are P-limited, while receiving coastal waters are N-limited but N-limitation can also occur in freshwater systems. There also is a discrepancy in the WFD targets. 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Page 54 of 64 Deliverable 4.2 APPENDIX A Nutrient concentrations and WFD status history of the upper Rhine (Lobith and Vuren) Total N concentrations and WFD status Page 55 of 64 Deliverable 4.2 Total P concentrations and WFD status Page 56 of 64 Deliverable 4.2 APPENDIX B Trends assessment Total N and Total P for upper Rhine (Lobith and Vuren) Lobith (NL/GE border) Page 57 of 64 Deliverable 4.2 Vuren (middle NL) APPENDIX C Nutrient concentrations and WFD status history of Hunze and Zuidlaardermeer Page 64 of 64 Deliverable 4.2