Meroplanktic phytoplankton play a crucial role in responding to peak discharge events in the middle lowland section of the Loire River (France)

Meteorological and hydrological forcings influence phytoplankton at very short-time scales. The effect of turbulence, dilution, light, and nutrients are highly dynamic. Yet, our knowledge of short-term phytoplankton dynamics associated with discharge disturbances and nutrient inputs remains elusive, especially in large rivers. Based on every three-day monitoring, we studied phytoplankton in the middle Loire River (France) and related to the daily variations in water discharge and the physical and chemical parameters. We focused on summer phytoplankton (2013 and 2014), where dissolved inorganic phosphorus concentration was potentially limiting growth. We identified eight discharge events, which increased suspended matter concentration and decreased chlorophyll-a concentration. The most significant environmental drivers of phytoplankton composition were discharge and water temperature, a sensitive proxy for meteorological forcing at short-time scale. The phytoplankton composition responded to changes in hydrology along with three distinct assemblage types, where even small water discharge increase induced a community response. Meroplanktic algae being able to withstand sedimentation and resuspension could take advantage of hydrological peaks, following the benthic retention hypothesis. Our results suggest that short-term dynamics are crucial to understanding community organization and functioning in large river plankton, with meroplankton playing a decisive role in maintaining phytoplankton diversity and ecosystem functioning.