Phytoplankton and water chemistry of several alluvial pools and oxbows after the flood event : a process of diversification

The process of diversification of six small floodplain pools after the flood event was investigated in the Lužnice river, floodplain, South Bohemia, Czech Republic. Water chemistry and the rapid phytoplankton development were studied in the spring for one month. Large ecosystem diversity of floodplain lentic localities is maintained by following factors: (i) different disposition of locality towards the flooding, (ii) different level of shade, caused by terrestrial vegetation and (iii) different Water basin morphology Phytoflagellate bloom (250 μg.l -1 chlorophyll a, Cryptomonas marssonii, Rhodomonas lacustris) developed in the shallow place of overflooded floodplain before its separation from the river. This bloom was horizontally heterogenous, having its maximum density in places protected from direct water current. After the separation of studied pools, their mutual differences rose: underground water from the river terrace, rich in NO 3 - , Cl - and SO 4 2- , seeped into some localities, while others were supplied by only river water, seeping through the permeable floodplain sediments. Different level of shading by surrounding vegetation resulted in different oxygen regime: oxygen concentration decreased in shaded pools, where photosynthesis was restricted. Only species-poor flagellate communities (Cryptophyceae), tolerant to low light intensities, developed there. On the other hand, high phytoplankton biomass (1200 μg.l -1 of chlorophyll a) developed in unshaded and shallow pools. Phytoplankton was dominated by Cryptophyceae, but also represented by Chrysophyceae and coccal forms (especially Chlorococales) in these pools. Hence, the water was well oxygenated there. The decrease ofwater level, typical for this spring time, enhance the role of bottom sediment processes in the whole pool ecosystem. The remarkable dominance of Cryptophyceae group in almost all localities seem to be related generally to the poor-light conditions and stable unmixed water column of pools. These characteristics are suitable for this mobile, low-light tolerant, and rapidly growing flagellates.