Structure and dynamics of zooplankton in a semi-arid wetland, the NationalPark Las Tablas de Daimiel (Spain)

Zooplankton structure and dynamics were studied in a freshwater wetland sub ject to strong hydrologic fluctuations. This wetland underwent a six-year d rought that terminated at the end of 1996. At that time, inundation area an d water level increased, diminishing macrophyte cover in some cases and enh ancing a continuing eutrophication problem. Sampling was performed monthly from January 1996 to December 1998 in five shallow water sites. Zooplankton were a mixture of limnetic and littoral species (66 rotifer, 15 cladoceran , and 10 copepod taxa, plus unidentified oligotrichid and peritrichid cilia tes), the composition of which changed coincident with the shift of the hyd rologic conditions. The most common taxa were bdelloid rotifers and Lecane closterocerca. The microcrustaceans Ceriodaphnia dubia, Daphnia curvirostri s, and Acanthocyclops robustus were frequent. Ciliates were the most import ant component of zooplankton in terms of biomass, except in spring when cla docerans were dominant. Rotifer biomass was the lowest fraction of zooplank ton. When community structure was described by taxonomic classes, mean biom ass, or time biomass trajectories, only weak spatial patterns were found fo llowing the flux of water in the wetland. Each zoo-plankton group was disti nctly affected by flood. Ciliates and rotifers increased biomass after the flooding, possibly as a result of the enhancement of eutrophication; cladoc eran populations decreased after the flooding likely due to macrophyte loss that facilitated planktivorous fish control. Copepods increased biomass co nsequent to the stabilized water level. There was a clear seasonal trend of zooplankton biomass seemingly unaffected by flood. Ciliates and rotifers h ad fluctuating low biomass in winter-spring, with maximum biomass in summer -autumn; cladocerans appeared and peaked only in spring; copepods peaked in late winter and spring.