Seasonality of macrophyte dominance in flood-pulsed lakes of the Danube Delta

Lakes in the Danube Delta (Romania) are influenced by the degree of connectivity with the main river branches. Lakes close to the inflow of river water show high concentrations of suspended sediments, whereas lakes isolated within the reedbed complexes have clear 'brown' water (high concentrations of humic substances). We describe the changes of phytoplankton, submerged vegetation, and plant-associated macro-invertebrates between early- and late season 2001, in inflow-, intermediate-, and isolated lakes. In September, all lakes showed high abundances of phytoplankton (cyanobacteria, diatoms) with coincident high turbidities. Potamogeton spp. disappeared from the vegetation during summer, whereas stands of Nitellopsis obtusa and Ceratophyllum demersum survived. As a consequence, lakes of intermediate connectivity lost their submerged vegetation; in contrast, isolated lakes remained dominated by vegetation. The observed seasonal development in lakes along the connectivity gradient can be explained as being the result of external processes (the flood-pulse) and internal responses (differential tolerance of the respective vegetation types).