Summer drought conditions promote the dominant role of phytoplankton in riverine nutrient dynamics

Large rivers play a relevant role in the nutrient turnover from land to ocean. Here, highly dynamic planktonic processes are more important compared to streams making it necessary to link the dynamics of nutrient turnover to control mechanisms of phytoplankton. We investigated the basic conditions leading to high phytoplankton biomass and corresponding nutrient dynamics in the eutrophic River Elbe (Germany). In a first step, we performed six Lagrangian samplings in the lower river part at different hydrological conditions. While nutrient concentration remained high at low algal densities in autumn and at moderate discharge in summer, high algal concentrations occurred at low discharge in summer. Under these conditions, concentrations of silica and nitrate decreased and rates of nitrate assimilation were high. Soluble reactive phosphorus was depleted and particulate phosphorus increased inversely. Rising molar C:P ratios of seston indicated a phosphorus limitation of phytoplankton. Global radiation combined with discharge had a strong predictive power to explain maximum chlorophyll concentration. In a second step, we estimated nutrient turnover exemplarily for N during the campaign with the lowest discharge. Mass balance calculations revealed a total nitrate uptake of 455 mg N m^-2d^-1 which was clearly dominated by assimilatory phytoplankton uptake whereas denitrification and other benthic processes were only of minor importance. Phytoplankton density, which showed a sigmoidal longitudinal development, dominantly explained gross primary production, related assimilatory nutrient uptake and respiration. Chlorophyll a concentration and bacterial abundance affected the composition of dissolved organic matter and were positively related to a number of CHO and CHNO components with high H/C and low O/C ratios but negatively to several CHOS surfactants. In conclusion, nutrient uptake in the large river strongly depends on the growth conditions for phytoplankton, which are favored during summer drought conditions.