Phytoplankton structure and dynamics in a semiarid wetland, the National Park "Las Tablas de Daimiel" (Spain)

We have studied a hypertrophic wetland in the semiarid National Park "Las T ablas de Daimiel". Five areas were sampled monthly from March 1996 to Decem ber 1998. Following an increasing level of eutrophication, increases in bot h total species richness and Euglenophyte populations were observed, as wel l as a progressive substitution of Oscillatoriean Cyanophytes by Nostocalea n ones. The dominant species were Planktothrix agardhii (GOMONT) ANAGNOSTID IS & KOMAREK, Peridinium willei HUITFELDKAAS, Cryptomonas erosa EHRENBERG, Cyclotella meneghiniana KUTZING and Anabaena sphaerica BORNET & FLAHAULT. T he only similarities between areas were the extremely eutrophic conditions and the algal flora, but the structural features of these assemblages lacke d a common pattern among sites. The Shannon-Wiener diversity index embraced one of the widest ranges yet noted for wetlands (10(-2) and 4 bits/mm(3), average in different areas was 1.6-2.2 bits/mm(3)). Phytoplankton biomass t ended to increase in humid years (1997-1998). Biovolume average ranged from 4-14 mm(3)/L and peaked in late summer, with the lowest values in winter-s pring. No relationships were found among nutrient concentrations, zooplankt on abundance and phytoplankton biovolume. Only two areas (the more isolated , so the less disturbed) showed a clear-cut pattern: the first, a eutrophic euryhaline pond with the highest diversity in winter-spring, supporting sm all pennate diatoms and cryptophytes followed by big blue-green filaments i n summer and colonial chorophytes in autumn; the second was a perturbed pon d with the highest diversity in summer-autumn, when a miscellany of large-s ized algae occurred, followed by blooms of Cryptomonas in winter. Phytoplan kton assemblage dynamics displayed quite stochastic trajectories due to gra zing, allelopathy and alternating self-organization processes, resulting fr om interaction between the hydrology and shallow morphology of a semiarid w etland. Phytoplankton dynamics in a wetland are more complex than in lakes due to the large number of interacting factors. A conceptual ecological mod el for algal dynamics follows.