The aquatic systems of Doñana (SW Spain): watersheds and frontiers

The aquatic systems of Doñana (SW Spain): watersheds and frontiers L. Serrano1, M. Reina, G. Martín, I. Reyes, A. Arechederra, D. León & J. Toja Dep. Plant Biology and Ecology. University of Sevilla. P.O. Box. 1095, Sevilla 41080. 1corresponding author: ser[email protected] ABSTRACT Doñana includes an extraordinary variety of aquatic systems. They are broadly classified according to their location (on either aeolian sands or marshland) as their hydrology largely depends on the geomorphology of their basins. Their chemical composition is mainly influenced by rainfall, evaporative concentration, groundwater discharge, biogeochemical interactions at the sediment-water interface, and the quality composition of their watersheds. The influence of the watershed can be studied at different scales of observation. Rainfall infiltration in sandy soils is usually high so surface runoff becomes a rare event of very short duration during floods which, nonetheless, exerts a huge influence on the limnology of temporary ponds on aeolian sands. The water quality of the Doñana marshland, on the contrary, is influenced by long-term processes taking place on large-scale areas: sediment deposition, eutrophication and heavy metal pollution. The review of the main literature on the limnology of the Doñana aquatic systems during the past two decades, enable us to make a comparison in time focusing on the interactions at the frontier between terrestrial and aquatic systems within watersheds. Presently, the eastern area of the Doñana marshland is particularly affected by the low quality of the incoming flowing water compared with the more isolated southern marshes within the National Park. Water from the lower strech of the Guadiamar River (“Entremuros”), that floods the marshes of “Lucio El Cangrejo Grande”, showed a significant correlation between inorganic suspended solids and total P (r=0.807, p<0.05) during 2003-04, indicating an important contribution of inorganic particulates to the eutrophication of this area. The northern streams of the “Arroyo del Partido” watershed have not significantly improved their water quality in the last two decades despite the construction of two waste-water treatment plants, being total P correlated to dissolved phosphate concentration (r=0.995, p<0.01) during 2003-05. A general increase in NO3concentrations have been detected in all studied aquatic systems of the Doñana marshland, including those with the highest water quality (“Arroyo de la Rocina”) during the last two decades. Despite wetland management requires a watershed approach, successive hydrologic projects in Doñana have failed to address the great spatio-temporal variability of processes affecting water quality in this area. Keywords: temporary ponds, marshland, streams, water quality, long-term study, eutrophication. RESUMEN Doñana alberga una extraordinaria variedad de sistemas acuáticos que se clasifican de forma general según su localización, bien en las arenas o en la marisma, ya que su hidrología depende, fundamentalmente, de la geomorfología de sus cuencas. La composición química de sus aguas varía en función de la lluvia, la evaporación, la descarga freática, las interacciones biogeoquímicas en la interfase agua-sedimento y el estado ecológico de sus cuencas. La influencia de la cuenca se puede estudiar a escalas distintas. La lluvia se infiltra fácilmente en la arena por lo que la escorrentía se convierte en un episodio raro y breve que, sin embargo, afecta considerablemente al funcionamiento limnológico de las lagunas temporales sobre arenas. Por el contrario, la calidad del agua en la marisma de Doñana está afectada por procesos extensos y largos, como la sedimentación, la eutrofización y la contaminación por metales pesados. Una revisión de la bibliografía limnológica permite realizar un estudio comparativo de las últimas dos décadas, centrado en las interacciones que tienen lugar en las fronteras entre los ecosistemas terrestres y acuáticos que comparten las cuencas de estos cuerpos de agua. Actualmente, la zona Este de la marisma está especialmente afectada por la baja calidad de las aguas de entrada en comparación con la zona Sur del Parque Nacional que se encuentra más alejada de estos aportes. El agua que discurre por el último tramo del encauzamiento del río Guadiamar (“Entremuros”) inunda las marismas cercanas (“Lucio El Cangrejo Grande”) y mostró una correlación significativa entre la carga de materia inorgánica en suspensión y la concentración de P total (r=0.807, p < 0.05) durante el periodo 2003-04, indicando la importante contribución del material particulado inorgánico en la eutrofización de esta zona. En la zona Norte, la calidad del agua en los arroyos de la cuenca del Partido no ha mejorado significativamente en la últimas dos décadas, a pesar de la construcción y funcionamiento de dos estaciones depuradoras de aguas residuales. Las concentraciones de P total y fosfato disuelto en el agua se encontraron altamente correlacionadas (r=0.995, p < 0.01) durante el periodo 2003-05. En las dos últimas décadas, se ha detectado un incremento de la concentración de NO3en los sisteLimnetica, 25(1-2): 11-32 (2006) The ecology of the Iberian inland waters: Homage to Ramon Margalef © Asociación Española de Limnología, Madrid. Spain. ISSN: 0213-8409 Limnetica 25(1-2)01 12/6/06 13:53 Página 11 INTRODUCTION Doñana is considered the most relevant wetland area in Spain. Despite its significance for wildfowl, research into the limnological processes of this vast wetland area started merely two decades ago. Prior to this, there were some scattered information published by re-known specialists in aquatic invertebrates that visited the area following the tradition of naturalists “exploring” Doñana in the previous 19th century and providing new zoological and botanical cites to the region. This was the case of surveys for the collection of rotifers (De Ridder, 1962), crustaceans (Dussart, 1962, 1967, Estrada, 1973, Armengol, 1976) aquatic insects (Bigot & Marazanof, 1965, Marazanof, 1967) and phytoplankton (Margalef, 1976). Later, microinvertebrates continued to be studied in the ponds, particularly ciliates (PérezCabrera & Toja, 1989), rotifers (Mazuelos et al., 1993) and crustaceans (Galindo et al., 1994 a,b, Ruiz et al., 1996, Serrano & Toja 1998, Fahd et al., 2000, Serrano & Fahd, 2005). The study of macroinvertebrates was mainly focused on Odonata, Heteroptera and Coleoptera from the marshes (Montes, 1980, Montes & RamírezDíaz, 1982), and later resumed with the impact of the red swamp crayfish (Gutierrez-Yurrita et al., 1998, Alcorlo et al., 2004). An extensive survey of aquatic and semiaquatic Coleoptera has been recently performed (Millán et al. 2005). Aquatic vertebrates such as amphibians have been extensively studied by Díaz-Paniagua (1979, 1988, 1990, Díaz-Paniagua et al., in press) while fish have received some attention much later (Fernández-Delgado et al., 2000). The study of aquatic vegetation in Doñana started also with early “explorations” to be later focused on particular aspects (García-Murillo et al., this issue). A floristic revision of aquatic macrophytes is provided by García-Murillo et al. (1993) and Espinar (2000). Aquatic vegetation has also been studied with a limnological perspective (Bernués, 1990, Duarte et al., 1990, Sousa & García-Murillo, 1999; Espinar et al., 2002), being the work by Espinar (2004) the most extensive study on the ecology and distribution of aquatic macrophytes in the Doñana marshland. The water composition of the Doñana marshland and the quality of the surface waters entering the marshes were thoroughly studied during the 80’s and reviewed by Arambarri et al. (1996). The first ecological studies (aimed at relating biological populations to environmental variables) were performed by Furest & Toja (1981) and Montes et al. (1982). Later, the Doñana ponds were typified according to their hydrology and chemical composition (GarcíaNovo et al., 1991; Manzano, 2001), their hydrology and substrate (Bravo & Montes, 1993) or their hydro-chemistry and littoral vegetation (Muñoz-Reinoso, 1996). The larger size of Santa Olalla and Dulce ponds made them suitable for many limnological studies (López et al., 1991, Toja et al., 1991, Sacks et al., 1992, Serrano et al., 1994, 1999, Toja et al., 1997, López-Archilla et al., 2004) compared to the rest of ponds (Bernués 1990, López et al., 1994, Serrano & Toja, 1995). Additionally, the relevance of sediment in the functioning of these shallow aquatic systems has been brought forward in numerous publications (Grimalt et al., 1991, Jaúregui & Toja, 1993, López et al., 1997, Díaz-Espejo et al., 1999, Álvarez et al., 2001, Serrano et al., 2003). The interaction with their surrounding terrestrial ecosystems has also been studied under different perspectives, such as vegetation-groundwater interactions (García-Novo et al., 1996, Zunzunegui et al., 1998), climate change (Sousa & GarcíaMurillo, 2003) and landscape management (Muñoz-Reinoso & García-Novo, 2005). 12 Serrano et al. mas acuáticos estudiados en la marisma, incluído áquel con la mejor calidad del agua (“Arroyo de la Rocina”). A pesar de que nadie duda que la gestión de los humedales require una estrategia a nivel de cuenca hidrográfica, los sucesivos proyectos hidrológicos que se desarrollan en Doñana no llegan a abarcar la gran escala espacio-temporal de los procesos que afectan a la calidad de su aguas. Palabras clave: lagunas temporales, marisma, arroyos, calidad del agua, eutrofización. Limnetica 25(1-2)01 12/6/06 13:53 Página 12 This wealth of limnological information enable us to make a comparison of the main aquatic systems of Doñana. We will focus on processes affecting water quality that take place at the frontier between terrestrial and aquatic ecosystems at different spatio-temporal scales. STUDY AREA The Doñana region (37°N, 6°W), extends along the coastal plain of the Gulf of Cádiz from the left bank of the estuary of the Guadalquivir river to the estuary of the Tinto river, and inland from the lower Guadalquivir River valley to the uplands of “El Aljarafe” (Sevilla) and “Condado de Niebla” (Huelva). It includes several territories with a different degree of environmental protection covering over 100 000 ha: a Biological Reserve created in 1964, a National Park (designated as a Ramsar site in 1982 and a World Heritage Site by UNESCO in 1995) and a Natural Park created as a surrounding protective area in 1989 (Fig. 1). At the same time, the Doñana region constitutes a space featuring the widest variety of pressures regarding the use and assignment of water resources. In 1990, Doñana entered onto the Montreaux Record of Ramsar sites under threat because a number of disturbances related to the conservation of the marshes had the potential to change its ecological character. The Doñana region hosts a population of nearly 180 000 inhabitants whose activities are devoted to agriculture and tourism. Rice fields occupy a vast extension on the east margin (about 35 000 ha). Water for the growth of rice is mainly provided by the Guadalquivir River, while 15 000 ha of irrigation fields, scattered over the sandy soils on the west and northern areas, are watered by the aquifer which produces a groundwater withdrawal of 55-60 hm3per year (Cruz Villalón, 2005). The aquatic systems of Doñana 13 Figure 1. Location of Doñana featuring different landscapes: marshland (1), stabilized sands (2), moving dunes (3), and the ecotone between aeolian sands and marshes. Antropic activities have deeply modified the landscape into: pine/gum-trees plantations (4), irrigation fields, rice fields, dry-land plant cultivars, artificial wetlands for fish cultives, salt pans, and villages. Localización del área de Doñana y sus diferentes tipos de paisaje: marisma (1), arenas estabilizadas (2), arenas móviles (3) y ecotono entre el manto arrasado y la marisma. Se incluyen paisajes muy antropizados como plantaciones de pino/eucalipto (4), regadíos, arrozales, cultivos de secano, piscicultura extensiva, salinas y núcleos urbanos. Limnetica 25(1-2)01 12/6/06 13:53 Página 13 Additionally, two large tourist resorts lie bordering the coast (“Matalascañas”) and the marshes (“El Rocío”). The former concentrates over 63 000 people in summer, while the latter attracts over half a million people during a traditional pilgrimage held in spring. Doñana has a Mediterranean climate with Atlantic influence, generally classified as dry subhumid. Rainfall is quite variable, both within a year and over the years, with a 580 mm yearly average, about 80 % of which is distributed throughout a wet period from the end of September to the beginning of April. Summers are very dry and hot, while winters are short and mild. Water balance is generally deficient as rainfall exceeds evapotranspiration only during 3-4 months of the year (Siljeström & Clemente, 1990). Potential evapotranspiration is very high with a yearly average of about 900 mm (Ménanteau, 1982). The deviation of rainfall to the yearly average (CDYP) showed an irregular sequence of hydrologic cycles (1st October-30th September) during the past 16 years: 6 wet, 5 dry and 5 moderate cycles (Fig. 2). Doñana started to be formed in the Quaternary age when the estuary of the Guadalquivir River was enlarged and reshaped by the formation of sandy spits after the last postglacial transgression. The alluvial deposition of fine materials brought about the filling of the former estuary and progressively isolated it from the sea. In 1984, the construction of a levee on the right bank of the Guadalquivir River (“Montaña del Río”, Fig. 1) minimized the tidal influence on the marshland which eventually became a continental formation (Clemente et al., 2004). Consequently, the deposits on this ancient plain present a rather heterogeneous lithology as it is partially covered by aeolian sands, while the central plain presents a saline silty-clay layer of up to 100 m thickness with deltaic deposits of sand and gravel increasing towards the north. The permeability of the main geomorphological units is very different: the aeolian sands correspond to an unconfined aquifer (with a shallow watertable and several flow systems) while groundwater is confined below the silty-clay deposits of the floodplain. Both units composed 14 Serrano et al. Figure 2. Deviation coefficient of rainfall to a yearly average (CDYP) of 563.2 mm recorded in the past 16 hydrologic cycles (1989/90-2004/05). Confidence limits at 95 % of significance (±128 mm) are indicated by dotted lines. Each hydrologic cycle was classified as wet (CDYP> 128 mm), dry (CDYP> -128 mm) or moderate (128 mm >CDYP< -128 mm). Coeficiente de desviación a la media anual de lluvia (CDYP) de 563.2 mm durante los últimos 16 ciclos hidrológicos (1989/90-2004/05). Los límites del intervalo de confianza al 95 % de significación ( ± 128 mm) se indican con líneas punteadas. Cada ciclo hidrológico se ha clasificado como húmedo (CDYP> 128 mm), seco (CDYP> -128 mm) o moderado (128 mm >CDYP< -128 mm). Limnetica 25(1-2)01 12/6/06 13:53 Página 14 an aquifer system of about 3400 km2bottomed by impermeable marine marls known as the “Almonte-Marismas” aquifer (Llamas, 1990). The depth of the aeolian sand deposits varies from over 100 m on the coast to barely 10 m at the northern edge of the Park. Groundwater recharge is produced by rainfall infiltration in the unconfined aquifer at an estimated rate of 200 mm y-1 (Vela, 1984). Groundwater in the confined aquifer, however, flows at a rate of 0.11 cm y-1 (Konikow & Rodríguez-Arévalo, 1993). The silty floodplain and the sand mantle also create two contrasting landscapes which make contact with each other along an ecotone locally known as “La Vera-Retuerta” (Fig. 1). The aquatic ecosystems of Doñana are broadly classified according to their location (on either aeolian sands or marshland) as their hydrology largely depends on the geomorphology on their basins. Outside the protected areas, watersheds are severely altered by human activities (Fig. 1). The lower valley of the Guadalquivir River is devoted to agriculture (traditional cultivars of olive trees, irrigated crops, and rice fields) and farming of cattle and horses. The watershed of the Guadiamar River extends to the highlands on the north (Sierra de Aracena) where soils are partially covered by scrub vegetation and plantations of gum-trees. Downstream, the river receives the drainage and seepage of the spoil heaps from an open-cast polymetallic sulphide mine (Aznalcóllar) through one of its tributaries (Arambarri et al., 1996). The dumping of 5 hm3 of mud and acid water with high concentrations of heavy metals in the mining accident of April 1998 flooded an extension of 2600 ha downstream. An extensive cleaning activity took place in the river floodplain and the riverbanks were later protected as a buffer area (“Corredor Verde del Guadiamar”). The lower stretch of the Guadiamar River runs between two levees (“Entremuros”) built in 1956. Its final stretch is canalised and connected to a dead arm-river (“Brazo de la Torre”) that drains to the estuary of the Guadalquivir River (Fig. 1). The flatness of the vast floodplain occupied by the marshland (about 23 000 ha) is altered at a topographic scale of a few cm that creates depressions (locally known as “lucios”) and upper areas (“paciles”) which have the appearance of emerged islands (“vetas”) during heavy floods. On the north-western area, surface water to this plain is supplied by rainfall and the overflood of small water flows (“Arroyos de La Rocina, del Partido, Cañada Marín and Cañada Mayor”) which drain southwards into the Park through a channel called “Caño de la Madre de las Marismas del Rocío”. On the north-eastern marshland, the Guadiamar River used to drain southwards through numerous small creeks (or “caños”), but most of its water-flow is presently deviated to the estuary of the Guadalquivir river so only a minor part of it reaches the marshes through both pipes (“Caño del Guadiamar”) and a complex channel network (“Entremuros-Brazo de la Torre”) which also carries the drainage of the nearby rice-fields. Quantitavely less important, but ecologically relevant, is the ground water seepage along the ecotone (“La Vera”) that provides humidity to grass meadows and hygrophitic vegetation (“algaidas”), and feed small creeks (“caños” and “sotos”) especially during heavy rainy periods. In some spots of the marshland, groundwater seepage maintains permanent sub-surface springs (“ojos”). The Doñana marshland is flooded seasonally by freshwater, depending on hydro-meteorological conditions, as the Guadalquivir River is the only permanent river in the area and its tidal influence is currently minimal. This marked seasonality of flooding periods followed by summer drought has accentuated the endorreic character of the marshland (Clemente et al., 2004). The resulting ionic composition of the “lucios” is dominated by Cland Na+as a result of the solubilization of salts from the sediment, the concentration of salts being dependent on the frequency and duration of flooding in each area (Clemente et al., 1998). Currently operative salt pans are located on the left bank of the Guadalquivir river. On the southeastern boundary of the Natural Park, 37 artificial wetlands (total surface about 3000 ha) are devoted to extensive fish cultures (“Veta La Palma”). Tidal marshes, in contrast, have been reduced to a narrow fringe along the banks of the Guadalquivir River (Gallego & García-Novo 2003). The aquatic systems of Doñana 15 Limnetica 25(1-2)01 12/6/06 13:53 Página 15 The Holocene aeolian sand mantle is composed of several dune generations originally deposited by marine drift (Vanney & Menanteau, 1985). A system of moving dunes with several dune fronts runs parallel to the coast-line with a NW-SE direction (max. altitude: 30 m). Within the Doñana Biological Reserve, the sand mantle is mostly covered by Mediterranean scrub (stabilized dunes) with a species composition closely following water availability which, in turn, depends on groundwater flow systems of different spatial scales (Muñoz-Reinoso & García-Novo, 2005). In this undulating landscape, hundreds of small ponds appear when the water table rises above the topographical surface during heavy rains (Fig. 3). These ponds (locally known as “lagunas”) are fed by freshwater (rainfall, runoff and groundwater discharge) and have no surface or groundwater connection to the sea so they cannot be properly called lagoons though they receive salts of marine origin through airborne deposition. Their groundwater feeding is relatively complex due to changes in recharge and topographic boundaries that modify their connection to different aquifer flow systems over time (Sacks et al., 1992, Muñoz-Reinoso, 2001). They range widely in size (from rain puddles to shallow lakes) and in flooding duration (from days to decades), but they all have been reported to dry out eventually. Hence, they all are temporary water bodies exhibiting wide fluctuations of water level. Many attempts have been made to classify them into different categories (permanent, semipermanent, seasonal, ephemeral), but a short number of observations have produced contradictory results (García-Novo et al., 1991, Bravo & Montes, 1993, Manzano, 2001). As a whole, the Doñana ponds form a system of temporary water bodies of remarkable singularity in Europe with a high protection status (Serrano & Toja, 1995, Williams et al., 2001). The area protected within the Biological Reserve (Fig. 3) covers a groundwater discharge surface of about 200 km2(Allier et al., 1974). The density of ponds in this area during winter floods is 6 ponds per 100 ha (holding water for 1-6 months) and 1 pond in 100 ha (holding water for more than 6 months, García-Novo et al., 1996). A few artificial wetlands are maintained by groundwater pumping, while digging water-holes in the ground near ponds (“zacallones”) for cattle drinking during dry periods is a very common practice. 16 Serrano et al. Figure 3. Location of a variety of aquatic systems on aeolian sands within the Doñana Biological Reserve (wet meadows, temporary ponds, wet dune slacks and temporary streams). The names of the main “peridune ponds” fringing the moving dunes are indicated. Localización de diversos sistemas acuáticos sobre el manto arrasado en la Reserva Biológica de Doñana (pastizales de la Vera, lagunas temporales, corrales encharcados y caños). Se indican los nombres de las principales lagunas peridunares situadas a lo largo del frente de dunas móviles. Limnetica 25(1-2)01 12/6/06 13:53 Página 16 Aquatic systems on aeolian sands within the Doñana Natural Park are also interesting and heterogenous (Fig. 1). A cliff formed by fossils dunes (“Médano del Asperillo”) runs along the coast to the west of the National Park and reaches a maximum altitude of 100 m. Rainfall and groundwater seepage drain to the sea through small ravines crossing the cliffs, providing shelter to some hygrophytic species (Díaz-Barradas & MuñozReinoso, 1992). Ponds also appeared inland to this formation, scattered over an area covering about 25 000 ha called “El Abalario” (Sousa & García-Murilllo, 1999). Vestiges of Atlantic peatbogs can be found there and along the margins of La Rocina brook. The extensive plantations of pines and gum-trees (“Coto del Rey”) shelter numerous small ponds during floods. A total of 568 temporary wetlands on sandy soils have been recorded by the local administration in the Doñana region (Junta de Andalucía, 2002). The only permanent aquatic system over sands (the lagoon of Tarelo) fringes the pine plantation of “La Algaida” on the left bank of the Guadalquivir River. Its basin was artificially excavated for sand and gravel extraction, and it is fed by rainfall and groundwater seepage from the estuary of the Guadalquivir River (Serrano et al., 2004). MATERIAL AND METHODS Rainfall data was obtained from the meteorological station of “Palacio de Doñana” (RBDCSIC). New physico-chemical data presented here corresponded to four different locations. The experiment with limnocorrals was carried out in several temporary ponds (Doñana Biological Reserve): at November-December 1991 (Jabata pond) and November 1995 (Las Verdes and Dulce ponds). Groundwater samples were collected from shallow piezometers following the methodology used by López et al. (1994). Limnocorrals (1 m diameter, 1 m height) and runoff samplers (5 l volume) were made of translucient impervious plastic as described in Serrano et al. (1999). Samplings of the floodplain and water-flows entering the marsh area of “Lucio El Cangrejo Grande” (Doñana Natural Park) were carried out bimonthly from February 2003 to September 2004. Three sampling stations were located in the central floodplain of this area; one sampling station in the Guadiamar River at “Vuelta de la Arena” (“Entremuros”), 3 sampling stations along its canal (“Canal de Aguas Mínimas”) and one sampling station at the rice field main outlet (“Canal Principal de Desagüe”). In May 2004, five sites within the lower Guadiamar River watershed were sampled, both in the upper stretch of “Entremuros” and in several tributaries: “Arroyo de la Cigüeña”, “Arroyo de Gato”, “Arroyo Chucena” and “Arroyo Algarbe”. Samplings of the Rocina-Partido watershed were performed in November 2003, December 2004, March and June 2005. The Rocina brook was sampled in its main stream just before The aquatic systems of Doñana 17 Table 1. Chemical composition of water inside limnocorrals (rainfall + groundwater discharge), outside them (rainfall + groundwater discharge + runoff), in surface runoff and phreatic water below several ponds at the onset of their filling period. Composición química del agua dentro de los limnocrrales (lluvia + descarga subterránea) y fuera de ellos (lluvia + descarga subterránea + escorrentía), de la escorrentía superficial y el agua freática en algunas lagunas al comienzo de su llenado. La Jabata pond Las Verdes pond La Dulce pond (7/11/91) (5/12/91) (12/11/95) (12/11/95) phreatic inside outside phreatic outside runoff inside outside runoff E.C. (mS cm-1) 0.81 0.87 0.41 0.74 2.70 1.86 12.5 5.41 0.88 pH 7.0 6.4 7.3 8.2 7.2 6.3 7.3 7.5 7.1 Alkalinity (meq l-1) 0.7 0.6 0.2 1.3 0.6 0.2 2.5 0.8 0.6 i-P (µg l-1) 111 9 9 37 251 115 208 242 533 N-NO3-(µg l-1) 1.4 15.4 14.0 0.5 12.1 3.9 150.6 156.4 13.6 N-NO2-(µg l-1) 1.9 2.4 1.5 7.3 642.6 24.9 - - - NH4+(mg l-1) 0.96 0.28 0.45 0.20 0.45 0.41 0.08 0.80 1.13 Limnetica 25(1-2)01 12/6/06 13:53 Página 17 draining to the marsh. The Partido stream was sampled across the “Raya Real”, at its delta, and in a tributary (“Caño Marín”). Conductivity (compensated with temperature at 20 ºC) and pH were recorded in situ. Water samples (1-2 l) were collected, stored at 4 ºC and filtered in the laboratory through Whatman GF/C filters within the next 24 h. Suspended solids were analyzed gravimetrically, in 3-4 replicates, using previous dry filters (100 ºC). The concentration of inorganic suspended solids was estimated after ignition (450 ºC, 4 h). The rest of analyses were carried out in duplicates. COD determinations were performed using KMnO4as oxidative agent. Total alkalinity was determined by titration (Rodier, 1981). The concentration of i-P was determined following the method of Murphy & Riley (1962). Tot-P was analyzed as i-P after acid digestion of the unfiltered water sample with 0.5 M H2SO4and K2S2O8(0.5-1 g) at 120 °C for 4 h (De Groot & Golterman, 1990). The concentration of NO2-and NH4+were determined by colorimetry (Rodier, 1981). The concentration of NO3-was measured as NH4+after complete oxidation with TiCl3(Golterman, 1991). RESULTS AND DISCUSSION The chemical composition of the Doñana aquatic systems is mainly influenced by rainfall, evaporative concentration, groundwater discharge, biogeochemical interactions at the sedimentwater interface, and the quality composition of their watersheds. The influence of the watershed can be studied at different scales of observation. Rainfall usually infiltrates easily through sandy soils so surface runoff rarely reaches the ponds (only for very short time-lapses during heavy rainfall). The water quality of the Doñana marshland, on the contrary, is influenced by long-term processes taking place on large-scale areas (such as sediment deposition, eutrophication and heavy metal pollution). The chemical composition of the unconfined aquifer is considered rather uniform, dominated by Ca(HCO3)2and with a salinity usually lower than 500 µg l-1, except at discharge areas because of the influence of biochemical processes (Llamas, 1990). As a whole, the aquifer shows a vertical gradient in salinity, from brine-water near the land surface to freshwater at 80 m of depth (Konikow & Rodríguez-Arévalo, 1993). Below the confined aquifer, salinity increases in a NE-SW direction pushing the interphase between the aquifer recharge and the fossil marine groundwater to the NE boundary of the Park where extensive irrigation fields happen to be developing (Plata & Ruiz, 2003). The high proportion of NaCl in rainwater due to marine influence affects the composition of shallow groundwater, but the high ratio of Mg2+ over Na+suggests that wet and dry atmospheric deposition has not yet been properly addressed (Lozano, 2004). The successive cycles of flooding and evaporation in the discharge areas have enriched in NaCl the shallow free groundwater (or phreatic) below the pond basins as it was shown by López et al. (1994) in water samples collected in piezometers (<2 m depth) during a dry hydrologic year. Ca2+ dominated over Na+ only in a small depression within the dune tail where recharge dominated over discharge flow and water was temporary deposited. Conductivity and total alkalinity of groundwater below the ponds showed little relationship to morphometry or trophic state, suggesting the existence of local flow systems. Limnocorral experiments were used in the temporary ponds to isolated both surface and groundwater sources at the onset of their filling period. The chemical composition of water inside the limnocorral (rainfall+groundwater discharge) and outside it (rainfall+groundwater discharge+runoff) was compared to the phreatic water below the ponds and to surface runoff collected in the watershed (Table 1). The chemical composition of the water filling the ponds resulted from an interaction of both surface and groundwater sources during discharge, but general trends were difficult to predict. Firstly because water budgets have not yet been elucidated with sufficient resolution and, secondly because of the impact of sparse local events on these shallow systems. For example, heavy rainstorms (>80 mm) that only represented 1.4 % of total 18 Serrano et al. Limnetica 25(1-2)01 12/6/06 13:53 Página 18 observations in a 7-year record (Serrano et al., 1999) can disrupt the pond development and revert it to earlier successional stages (Toja et al., 1991). De Castro-Ochoa & Muñoz-Reinoso (1997) elaborated a multiple regression model for water-table fluctuations of the dune wetslacks that depended on rainfall, time lapse between two consecutive measurements and present depth. They found, however, that groundwater discharge at the ponds did not fit into their dune aquifer model because pond feeding did not only depend on rainfall. A hydrologic budget for the largest pond during the rainy season (OctoberMarch) estimated that surface sources (rainfall and runoff) accounted for 48 % of the water input during very dry years (<250mm of annual rainfall), but reached 100 % during extremely wet cycles (>1000 mm of annual rainfall). The rest of the water input was due to groundwater discharge, from both a shallow phreatic and a deep water table (Sacks 1989). A review of studies on the ionic composition in the Doñana aquatic systems reveals the influence of the different scales of observation in each study. The hydroperiod (or duration of water on the surface) and the water origin are relevant hidrologic features that influence the ionic composition of water, but their assesment is greatly affected by the duration of the study period. The first extensive survey (47 ponds) of water chemical composition was performed by García-Novo et al. (1991) during the heavy floods of winter 1990. Again, Na+and Cl-were the dominant ions in all pond waters. The ratio of Ca2+ over Na+ was not a good predictor compared to the Mg2+/Na+ ratio, probably due to the local effect of microtopography and vegetation on CaCO3precipitation. A ratio of Mg2+ over Na+ higher than 0.25 (in meq/l) corresponded to discharge areas, and a lower ratio indicated recharge areas where water runoff could be temporary deposited due to the presence of a high concentration of organic matter and/or clay in the sediment. They segregated the ponds in three discharge groups according to their location, mineralization and trophic state (Fig. 3): 1) ponds in “Las Naves”, 2) those along the ecotone between the moving dunes and the stabilized sands (“peridune pond” system), and 3) those between the stabilized sands and the marshland (“La Vera”). Later, Muñoz-Reinoso (1996) enlarged this classification to 5 groups by including the wet-slack formation of the moving dunes, and dividing the “peridune ponds” in two other groups according to pond size. Water pH is usually alkaline in the discharge areas, while acidic water (pH 4-5) has been occasionally reported when rainfall is temporary deposited on rich organic soils. The combination of alkaline waters over siliceous sand basins makes these water bodies rather singular compared to other European wetlands (Serrano & Toja, 1995). Hydrochemical classifications of ponds based on absolute limits have, so far, proved evasive as the ionic composition of the Doñana ponds change widely in time (Serrano & Toja, 1995). Table 2 shows the change in water conductivity recorded in some of the most visited ponds during the last two decades. In Taraje pond, conductivity ranged from 0.1 to 8.5 mS cm-1 during wet years, and from 1.2 to 22.0 mS cm-1 The aquatic systems of Doñana 19 Table 2. Maximum and minimum values of water conductivity (20 ºC) recorded in the water surface of the most visited ponds from 1989/90 onwards, indicating wet (w), dry (d) and moderate cycles (m). Valores máximos y mínimos de la conductividad (20 ºC) registrados en la superficie del agua de las lagunas más visitadas desde 1989/90, indicando los ciclos húmedos (w), secos (d) y moderados (m). Hydrologic Santa Olalla Dulce Taraje cycle Max Min Max Min Max Min 1989/90w4.1 0.4 5.8 0.2 7.4 0.1 1990/91 m4.3 1.3 7.1 0.8 10.6 1.6 1991/92 m16.5 2.3 12.5 1.0 22.0 4.1 1992/93 d28.4 4.4 10.6 1.2 15.4 6.9 1993/94 d6.0 2.0 1.7 13.3 1.2 1994/95 d------ 1995/96 w1.1 0.7 5.9 0.5 1.8 0.3 1996/97 w0.7 0.3 0.6 0.3 2.3 0.4 1997/98 w0.6 0.4 0.5 0.4 8.5 0.5 1998/99 d11.6* 1.7* 16.6* 1.6* 11.1 6.1 1999/00 m9.2* 1.9* 3.5* 1.0* 5.6 1.5 2000/01 w5.7* 1.7* 2.5* 0.6* 3.4 0.9 2001/02 m------ 2002/03 m8.3 2.3 3.7 0.8 2.7 1.7 2003/04 w7.7 1.1 3.3 0.4 4.4 0.4 2004/05 d19.3 1.9 1.9 1.0 14.9 8.6 * LÓPEZ-ARCHILA et al. (2004) Limnetica 25(1-2)01 12/6/06 13:53 Página 19 sands covering about 3400 km2. The Doñana marshland covers a vast extension of both deeply transformed areas (dessicated marsh, rice fields, irrigation fields) and less transformed marshes (National and Natural Parks) which are hydrologicaly connected to each other. The surface watershed draining to the marshland extends by narrow corridors to the pyritic deposits of the northern uplands, while the lower valley of the Guadalquivir River is the recipient of waters draining through a huge watershed of more than 57 500 km2. Both surface and groundwater resources ought to be extremely difficult to manage at this massive scale and so, successive hydrologic projects carried out in Doñana have failed in many aspects. The recently developed hydrologic regeneration plan for Doñana (Doñana 2005) will bring a better connectivity to the Guadalquivir River, but rice fields continue to be segregated in this estrategy despite they are not isolated from the Doñana aquatic systems, neither by water surface nor by aerial depositions. The outlet water from rice-pads is flooding the nearby marshes (on the eastern margin of both the Natural and the National Park) every year at the end of the rice growing-season. Instead of lamenting on the issue, rice fields could become a source of freshwater in the hydrologic budget of the nearby marshes which, in turn, will create more concern on the control and monitoring of fertilizers and pesticides in the area. ACKNOWLEDGEMENTS We are grateful to all the people that have participated in our limnology group during the past decades: A. Furest, T. López, C. Guisande, N. Gabellone, M. A. Casco, J. Prenda, J. C. Muñoz, J. Jaúregui, N. Mazuelos, J. A. García-Sánchez, M. D. Galindo, F. García-Bouzas, A. DíazEspejo, J. L. Moreiras, I. Calzada-Bujak, K. Fahd, M. D. Burgos, A. Plazuelos and E. Alcalá. Recent data corresponded to research projects supported by “Consejería de Medio Ambiente, Junta de Andalucía” and the Ministry of Envrionment (Doñana 2005). REFERENCES ALBAIGÉS, J., J. ALGABA, P. ARAMBARRI, F. CABRERA, G. BALUJA, L. M. HERNÁNDEZ & J. CASTROVIEJO. 1987. Budget of organic and inorganic pollutants in the Doñana National Park (Spain). Sci. 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