Contrasting long-term trends and shifts in phytoplankton dynamics in two large rivers

Knowledge about long-term dynamics of phytoplankton in river ecosystems as well as the physical and chemical drivers that potentially control the plankton is essential for predicting future developments, e.g. in response to global climate change. The present study analyzes long-term trends in phytoplankton biomass and shifts in the timing of phytoplankton spring blooms observed in the large rivers Rhine and Elbe from 1990–2009 and 1994–2009, respectively, and analyzes the factors potentially regulating phytoplankton biomass. While phytoplankton biomass in the Elbe was high (seasonal mean chlorophyll-a concentration: 62 µg/L) and showed an increasing tendency, it was much lower in the Rhine (seasonal mean chlorophylla concentration: 10 µg/L) where it decreased significantly during the study period. This decrease coincided with an earlier occurrence of the phytoplankton spring maximum. In the Elbe, the timing of low discharge conditions was crucial for the occurrence of the spring bloom, i.e. an earlier end of the discharge maximum was connected with an earlier spring bloom. In the Rhine, we found a positive correlation between the timing of the spring bloom and the end of winter flood flow. The maximum chlorophylla values during the bloom correlated with the timing of maximum light availability in the Rhine. The findings indicate that climate related factors, like discharge or light conditions, have a high potential to regulate phytoplankton spring bloom dynamics in large rivers. Such dependence could be relevant for predicting phytoplankton development under climate change.