Variability in δ13C values between individual Daphnia ephippia : Implications for palaeo-studies

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY-NC-ND 4.0 https://creativecommons.org/licenses/by-nc-nd/4.0/ Variability in δ13C values between individual Daphnia ephippia : Implications for palaeo-studies © 2018 Elsevier Ltd. Accepted version (Final draft) Schilder, Johannes; van Roij, Linda; Reichart, Gert-Jan; Sluijs, Appy; Heiri, Oliver Schilder, J., van Roij, L., Reichart, G.-J., Sluijs, A., & Heiri, O. (2018). Variability in δ13C values between individual Daphnia ephippia : Implications for palaeo-studies. Quaternary Science Reviews, 189, 127-133. https://doi.org/10.1016/j.quascirev.2018.04.007 2018 1 Variability in δ13C values between individual Daphnia ephippia: Implications for 1 palaeo-studies 2 Jos Schilder1,2, Linda van Roij3, Gert-Jan Reichart3,4, Appy Sluijs3 and Oliver Heiri1 3 4 This manuscript was published in Quaternary Science Reviews 189 (2018), 127-133 5 on April 24, 2018. https://doi.org/10.1016/j.quascirev.2018.04.007 6 7 2 Variability in δ13C values between individual Daphnia ephippia: Implications for 8 palaeo-studies 9 Jos Schilder1,2, Linda van Roij3, Gert-Jan Reichart3,4, Appy Sluijs3 and Oliver Heiri1 10 1Institute of Plant Sciences and Oeschger Centre for Climate Change Research, University of 11 Bern, Altenbergrain 21, 3013 Bern, Switzerland 12 2Department of Biological and Environmental Science, University of Jyväskylä, PO Box 35, 13 40014 Jyväskylä, Finland 14 3Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Heidelberglaan 15 2, 3584 CS Utrecht, the Netherlands 16 4Royal Netherlands Institute for Sea Research (NIOZ), Landsdiep 4, 1797 SZ ´t Horntje 17 (Texel), the Netherlands 18 19 Corresponding author: Jos Schilder, [email protected] 20 21 Keywords: Daphnia ephippia, Stable carbon isotopes, Laser ablation, Lakes, Seasonality, 22 Present, Palaeolimnology, Europe, Stable isotopes 23 24 Highlights: (3-5, 85 chars) 25 *Laser ablation-based techniques can be used to measure single ephippium δ13C values 26 *Bulk ephippia δ13C values may represent strong within-sample variability 27 *This variability may be caused by seasonal or species-specific differences in diet 28 *This technique can be used to assess seasonal patterns behind shifts in bulk samples 29 *Larger numbers of ephippia in samples can increase the precision of reconstructions 30 3 ABSTRACT 31 The stable carbon isotope ratio (δ13C value) of Daphnia spp. resting egg shells (ephippia) 32 provides information on past changes in Daphnia diet. Measurements are typically performed 33 on samples of ≥20 ephippia, which obscures the range of values associated with individual 34 ephippia. Using a recently developed laser ablation-based technique, we perform multiple 35 δ13C analyses on individual ephippia, which show a high degree of reproducibility (standard 36 deviations 0.1 to 0.5 ‰). We further measured δ13C values of 13 ephippia from surface 37 sediments of three Swiss lakes. In the well-oxygenated lake with low methane concentrations, 38 δ13C values are close to values typical for algae (-31.4 ‰) and the range in values is 39 relatively small (5.8 ‰). This variability is likely driven by seasonal (or inter-annual) 40 variability in algae δ13C values. In two seasonally anoxic lakes with higher methane 41 concentrations, average values were lower (-41.4 and -43.9 ‰, respectively) and the ranges 42 much larger (10.7 and 20.0 ‰). We attribute this variability to seasonal variation in 43 incorporation of methane-derived carbon. In one lake we identify two statistically distinct 44 isotopic populations, which may reflect separate production peaks. The potentially large 45 within-sample variability should be considered when interpreting small-amplitude, short46 lived isotope excursions based on samples consisting of few ephippia. We show that 47 measurements on single ephippia can be performed using laser ablation, which allows for 48 refined assessments of past Daphnia diet and carbon cycling in lake food webs. Furthermore, 49 our study provides a basis for similar measurements on other chitinous remains (e.g. from 50 chironomids, bryozoans). 51 52 4 1. Introduction 53 The use of the stable isotopic composition of organic remains in lake sediments has been 54 emerging as a means to reconstruct changes in past ecological and climatological conditions 55 in and around lakes (Leng and Henderson, 2013). Because they are often abundantly found 56 in sediments and are resistant to degradation (Verbruggen et al., 2010), the chitinous remains 57 of aquatic invertebrates have a strong potential as a proxy for such reconstructions (Heiri et 58 al., 2012). For example, the stable carbon isotopic composition (expressed as δ13C values) of 59 the shells (ephippia) of the resting eggs of water fleas (Daphnia spp.) has been shown to 60 reflect that of the maternal Daphnia and its diet (Schilder et al., 2015b). This allows for the 61 use of δ13C values of ephippia to investigate the δ13C value of Daphnia diet in the past (see 62 e.g. Morlock et al., 2016; Rinta et al., 2016; Schilder et al., 2017). Chitinous remains of other 63 organisms, most notably chironomid head capsules, bryozoan statoblasts and Ceriodaphnia 64 ephippia have also been used to investigate past changes in the stable carbon isotopic 65 composition of aquatic invertebrates in lake food webs, often in combination with Daphnia 66 ephippia (Belle et al., 2014; Frossard et al., 2013; Morlock et al., 2016; Rinta et al., 2016; van 67 Hardenbroek et al., 2013; Wooller et al., 2012). 68 Daphnia feed on suspended organic particles, predominantly algae, in the water 69 column of lakes (Lampert, 2011). In addition, Daphnia may ingest methanotrophic bacteria 70 (MOB) or other microorganisms feeding on MOB. This leads to δ13C values in Daphnia that 71 are much lower (Kankaala et al., 2006) than typical for freshwater algae (-35 to -25 ‰, 72 (Vuorio et al., 2006)), because the δ13C values of biogenic methane in lake ecosystems (-85 73 to -50 ‰, (Jedrysek, 2005; Rinta et al., 2015; Whiticar et al., 1986)) and MOB metabolizing 74 this methane (Templeton et al., 2006) are exceptionally low. Schilder et al. (2015a) found that 75 δ13C values of ephippia from surface sediments were lower in small temperate lakes with 76 high methane concentrations in the water column than in those with low methane 77 5 concentrations in the water column. This suggests that the amount of methane-derived carbon 78 in these ephippia (and consequently their δ13C values) are systematically related to in-lake 79 methane concentrations. The δ13C values of ephippia deposited in lake sediments may thus 80 provide an indication of past availability of methane in lakes. Given the importance of lakes 81 (and other freshwater bodies) in the global carbon cycle as producers and emitters of methane 82 (Bastviken et al., 2011; Battin et al., 2009), this method can provide valuable insight into the 83 past response of lakes to environmental and climatic change in terms of methane productivity 84 and carbon cycling in general (e.g Perga et al., 2016; van Hardenbroek et al., 2014, 2013; 85 Wooller et al., 2012). This information, in turn, can be used to better predict future responses 86 of the lacustrine carbon cycle, and particularly of lacustrine methane production and 87 emission, to changes in the environment and in the climate. 88 Seasonal fluctuations in δ13C values of Daphnia exceed 10 ‰ in some lakes 89 (Morlock et al., 2016; Perga and Gerdeaux, 2006; Smyntek et al., 2012) and are partly driven 90 by changes in the δ13C value of phytoplankton. For example, the seasonal succession of 91 dominant algae species with different carbon fractionation rates can drive seasonal shifts in 92 algal δ13C values (e.g Hollander and McKenzie, 1991) and therefore the stable carbon 93 isotopic composition of food available to Daphnia. Additionally, changes in CO2 94 concentrations (due to e.g. uptake of CO2 by algae, CO2 produced as a product of 95 decomposition of organic matter or changes in pH that shift the equilibrium between the 96 various dissolved carbonate species) can also influence both the baseline δ13C value of CO2 97 and algal carbon fractionation rates (Laws et al., 1995; Smyntek et al., 2012), forming 98 another source of seasonal variability in Daphnia δ13C values. As a consequence, higher δ13C 99 values of algae are typically observed during (late) spring and summer and lower values 100 during fall and winter (Hollander and McKenzie, 1991; Morlock et al., 2016; Schilder et al., 101 2017; Smyntek et al., 2012). However, incorporation of MOB by Daphnia also likely occurs 102 6 mostly in fall when algal abundance declines and hypolimnetic methane is exposed to oxygen 103 upon lake mixing (Morlock et al., 2016; Taipale et al., 2007). This can promote large 104 seasonal fluctuations in Daphnia δ13C values in dimictic lakes with high methane production, 105 leading to Dapnia δ13C values much lower than reported for algae (-40 ‰ and lower, see e.g. 106 Kankaala et al. (2010) and Schilder et al. (2017)). In addition, there is a seasonality in the 107 production of ephippia themselves: Ephippia may be produced throughout the year, but there 108 typically are distinct production peaks in spring and in fall (Cáceres, 1998; Cáceres and 109 Tessier, 2004). 110 The extent to which these sources of (potential) seasonal variability affect 111 down-core variations in δ13C values of Daphnia ephippia is poorly constrained. In part, this is 112 because δ13C analysis of single (sub)fossil ephippia shells has to date been impossible. δ13C 113 measurements on a large number of shells from a sample (hereafter called bulk 114 measurements) represent a weighted average of δ13C values of individual ephippia which 115 may originate from populations of very uniform or highly variable δ13C values. 116 Measurements on individual ephippia would provide insights into this within-sample 117 variation. Furthermore, they could provide information on whether ephippia in a sample 118 represent one or more isotopically distinct ephippia production peaks or Daphnia populations 119 (e.g. spring and autumn production peaks of the same or different Daphnia species), and how 120 food sources different from algae (e.g. MOB) contributed to the diet of Daphnia producing 121 these ephippia. 122 The amount of Daphnia ephippia needed for down-core ephippia δ13C analysis 123 constrains the resolution of presently available records, often resulting in records with a 124 relatively low level of taxonomic and temporal resolution (e.g. Morlock et al., 2016; Schilder 125 et al., 2017). The number of ephippia needed is in turn strongly constrained by the size 126 (mass) of the ephippia found in a sediment record and the lower limit of sample mass that can 127 7 be measured with a given analytical setup. For standard total combustion isotope ratio mass 128 spectrometry (TC/IRMS), 20 or more individual ephippia are typically measured (Morlock et 129 al., 2016; Rinta et al., 2016; Schilder et al., 2017). The δ13C analysis on invertebrate remains 130 is at times performed on samples as small as 20 µg (see e.g. Belle et al., 2017) and given the 131 potentially large weight of Daphnia ephippia (0.5 to 5 µg per ephippium, with very large 132 ephippia even exceeding 5 µg; Van Hardenbroek et al., this issue) measurements could be 133 performed on samples containing only 5 to 10 individual ephippia (or even less). However, if 134 the variability in δ13C values is high in a sediment sample, measurements on bulk samples 135 containing a low number of ephippia may also lead to high apparent variability in down-core 136 records, which can complicate their interpretation. Here we apply a novel analytical setup 137 capable of analysing δ13C values of individual ephippia which we use to investigate within138 bulk sample variability. 139 Recent advances in analytical procedures (e.g. approaches based on laser140 ablation or Spooling Wire Microcombustion) now allow the measurement of the isotopic 141 composition of samples considerably smaller than those conventionally measured in isotope 142 studies of lacustrine invertebrate remains (see e.g. Eek et al., 2007; Moran et al., 2011; 143 Pearson et al., 2016), potentially allowing single or even multiple measurements on 144 individual microfossils (e.g. Nelson et al., 2007; van Roij et al., 2017; Zhao et al., 2017) . We 145 use a laser-ablation based setup for measuring the δ13C values of individual ephippia. The 146 setup has been developed for δ13C analysis of small organic particles, such as organic 147 microfossils, and has been shown to produce high accuracy and precision data for an 148 international standard, as well as for single grains of pollen and dinoflagellate cysts (Sluijs et 149 al., 2018; van Roij et al., 2017). We analyse individual Daphnia ephippia from surface 150 sediments from three Central European lakes that vary in their geographical and elevational 151 setting (subalpine versus lowland), occurrence of anoxia in deeper water layers, and the 152 8 extent to which methane enters the open water column. We aim to explore the potential of 153 this approach to investigate the variability behind δ13C values based on bulk Daphnia 154 ephippia samples such as presented by Schilder et al. (2015a). For each lake we present 155 multiple δ13C measurements on the same ephippium to constrain the reproducibility of the 156 approach and assess whether individual ephippia were homogeneous in their stable carbon 157 isotopic composition. Furthermore, for each lake we analysed another 13 individual ephippia 158 from the same surface sediment sample once to investigate the variability in Daphnia 159 ephippia δ13C values within a fossil ephippia assemblage. We expected to find larger 160 variability in ephippia δ13C values in the stratified lakes with high methane concentrations 161 than in the well-mixed lake with low methane concentrations, since it can be expected that in 162 the lakes with high methane concentrations Daphnia diet may be supplemented by strongly 163 13C-depleted, methane-derived carbon during part of the seasonal cycle (e.g. Morlock et al., 164 2016; Rinta et al., 2016; Schilder et al., 2017; Taipale et al., 2007). 165 166 2. Methods 167 2.1 Description of sites 168 For this study the surface sediments of three dimictic Swiss lakes, Burgäschisee (BUR), 169 Hinterburgsee (HIN) and Seealpsee (SEE) were investigated. HIN and SEE are mountain 170 lakes (1516 and 1141 m above sea level (asl), respectively), whereas BUR is situated in the 171 lowlands of the Swiss plateau (434 m asl). At the time of sampling (late summer 2011), all 172 three lakes were thermally stratified. BUR and HIN were characterized by anoxic bottom 173 waters, whereas the water column of SEE was fully oxygenated. Surface water methane 174 concentrations were 1.15 (BUR), 2.80 (HIN) and 1.04 (SEE) µmol l-1, and the bottom water 175 methane concentrations were 226.63 (BUR), 13.40 (HIN) and 1.26 (SEE) µmol l-1 (see Table 176 15 ephippia containing methane-derived carbon in the sample changing, or is the amount of 310 methane-derived carbon in these ephippia changing, thereby altering the mean δ13C values of 311 these ephippia? Measuring individual ephippia δ13C values may therefore allow for more 312 sensitive assessments as to whether methane-derived carbon contributed to the planktonic 313 food web of lakes. In addition, such information may potentially reveal changes in the 314 production period of Daphnia ephippia in the past. 315 Our findings also allow an assessment of the expected variability of fossil 316 ephippia δ13C values of bulk ephippia samples that originates from between-ephippium 317 variability of δ13C values. Low ephippia numbers in such bulk samples may lead to 318 substantial down-core variability in records due to an overrepresentation of ephippia with 319 relatively (compared to the sample´s average) low or high δ13C values in some samples. This 320 is especially of concern for lake systems in which strong influences of methane-derived 321 carbon in Daphnia diet may be expected (given the large variability in BUR and to a lesser 322 degree HIN as opposed to SEE). To illustrate this, we used our 14 data points per lake (under 323 the assumption that these properly reflect the variability within the lake´s sediments, and that 324 the ephippia all have equal mass) to randomly generate 40 bulk ephippia samples for each of 325 17 different sample sizes between 1 and 150 ephippia, for each lake. Figure 3 shows the 326 standard deviation from the average for those 40 samples for each sample size and lake. The 327 calculations clearly show that for SEE, with relatively low between-ephippium variability, 328 bulk samples with 20 ephippia are much more likely to return values within 1 ‰ of the true 329 average than for HIN and BUR. This implies that especially in lakes with a high variability in 330 ephippia δ13C values, short (i.e. single sample), low-amplitude isotope excursions should not 331 be over-interpreted in bulk Daphnia ephippia δ13C records, unless they are based on a very 332 large amount of ephippia or well-supported by corroborating evidence. Measuring one or 333 more secondary, independent proxies can help to separate such minor variations in bulk 334 16 Daphnia δ13C values which represent true changes in aquatic food webs and past 335 environments from those which are due to large heterogeneity of ephippia δ13C values within 336 samples. For example, earlier studies have used geochemical analyses, diatom-inferred total 337 phosphorus (Schilder et al., 2017), δ13C measurements on other invertebrate taxa (e.g. 338 chironomids, bryozoans, Ceriodaphnia) or bulk sediment organic matter δ13C analyses 339 (Morlock et al., 2016; van Hardenbroek et al., 2013), or have analysed invertebrate remains 340 from a second core from the same lake (Frossard et al., 2014) to support the interpretation of 341 down-core invertebrate δ13C records based on the analysis of bulk samples. 342 In sediments that contain two or more distinct groups or clusters of ephippia 343 δ13C values (such as is the case in BUR), down-core changes in bulk ephippia δ13C values 344 may reflect changes in one or two of these groups. For example, variations may be driven 345 only by changes in fall ephippia δ13C values. In such cases changes in bulk ephippia δ13C 346 values may be effectively dampened by stability of ephippia produced during other parts of 347 the seasonal cycle, for example ephippia from spring / early summer Daphnia that feed 348 predominantly on algae. Such mechanisms may need to be taken into account when 349 interpreting down-core bulk ephippia δ13C records. Further investigations of the main 350 ephippia production peaks in our study lakes and of their stable carbon isotopic composition 351 (e.g. with the help of sediment trap studies) would help to further constrain the effects of 352 between-year variability in Daphnia ephippia δ13C values and multiple production peaks on 353 reconstructed Daphnia δ13C values. 354 355 356 17 5. Conclusion 357 The data we present show that LA/nC/GC/IRMS analysis of Daphnia ephippia is possible 358 and a feasible approach to investigate ephippia δ13C values on individual epihippia or even 359 investigate the variability within the same ephippium with a high degree of repeatability. Our 360 results imply that the within-sample variability of epihippia δ13C values, related to e.g. 361 seasonal variability in Daphnia δ13C values and timing of ephippia production peaks, can 362 have an impact on a fossil bulk ephippia δ13C record. This has some implications for 363 preparing samples for down-core records based on bulk ephippia δ13C analysis and 364 interpreting the results, but also opens up new avenues for future research. 365 Our results indicate that bulk samples with low δ13C values may represent 366 averages of ephippium assemblages with a high between-ephippium variability in δ13C 367 values. Bulk ephippia samples therefore ideally consist of sufficiently high numbers of 368 ephippia to increase the likelihood of properly reflecting the true average δ13C value of the 369 deposited ephippia during a certain period of time. The optimal amount of ephippia depends 370 strongly on the (expected) variability within the sample and the desired accuracy of the 371 reconstruction (Figure 3). Especially when dealing with low amounts of ephippia, the use of 372 secondary, independent proxies can produce necessary constraints to interpretations of down373 core records. Measuring individual ephippia for certain parts of a record, for example for 374 sections with relatively high or low rates of incorporation of methane-derived carbon, can 375 also provide an avenue for constraining and improving interpretations. Moreover, this 376 approach can potentially also be used to investigate how the range of ephippia δ13C values 377 produced during different seasons and/or by different Daphnia poulations changed over time. 378 Finally, our results show that δ13C analyses of individual chitinous microfossils 379 in lake sediments are possible and produce reproducible results. This opens the opportunity 380 18 for applying this method to chitinous fossils of other invertebrate groups such as chironomid 381 head capsules, cladoceran (e.g. bosminid or chydorid) head shields and carapaces or 382 bryozoan statoblasts. Possible research questions which could be explored include, e.g. the 383 within assemblage variability of δ13C values for individual taxa and fossil structures, but also 384 inter taxon variability for fossil groups that produce too small amounts of fossil chitinous 385 material to measure with conventional bulk measurement techniques. 386 387 ACKNOWLEDGEMENTS 388 We thank Mat Wooller and one anonymous reviewer for their constructive comments on this 389 manuscript. This research received funding from the European Research Council under the 390 European Union's Seventh Framework Programme (FP/20072013) / ERC Grant Agreement 391 n. 239858 (RECONMET) to O.H. The Netherlands Organization for Scientific Research 392 supported this research through grant ALWOP.223 to A.S. and G.-J.R. This work benefitted 393 from European Research Council Starting Grant 259627 awarded to A.S. 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