Growth and Loss Processes of Riverine Phytoplankton in Relation to Water Depth

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Growth of phytoplankton depends on light supply and thus on mixing depth. Rates of sedimentation and of contact with benthic filter-feeders should decline with rising water level. Therefore, discharge-dependent water depth should predispose the fate of phytoplankton transported down a river. Growth and loss of phytoplankton have been studied at different discharge levels along the lowland rivers Spree and Warnow. The mean depth of the river courses ranged from 0.3 to 3 m, the mean velocity of flow from 0.05 to 0.5 m s -1 . We compared changes in phytoplankton biomass in a water parcel flowing downstream with those in simultaneously incubated dialysis chambers. The difference between rates of biomass change along the river and in the vertical moved chambers was used as an estimate of loss rates by sedimentation and grazing by benthic filter-feeders. Caged centric diatoms realized maximum growth rates at daily light exposures of 6 to 15 E m -2 . Growth of chlorophytes was light-saturated above 15 E m -2 d -1 . Observing the seasonal mean, in all investigated river reaches light supply saturated growth of centric diatoms and limited growth of chlorophytes. Loss rates increased with decreased water depth and usually exceeded the potential growth rate of riverine phytoplankton. Longitudinal growth of phytoplankton was observed only in periods of high discharge or along deep river stretches. Sedimentation loss of diatoms was twice as high as that of chlorophytes. The light intensity optimum for growth found and the inverse relationship between water depth and plankton loss to the river bottom explain the lack of plankton in shallow rivers and the occurrence of maximum phytoplankton biomass in medium-sized rivers. Our results support the hypothesis that differences in light-adaptation and in sedimentation loss cause the dominance of chlorophytes in shallow rivers and of diatoms in deeper rivers.