Planktonic food web structure and dynamic in freshwater marshes after a lock closing in early spring

We conducted a weekly survey at two stations of a French coastal marsh during the transition from winter to spring, when the sea lock gates were closed. Field measurements and laboratory experiments were combined in order to describe the structure and dynamics of planktonic food webs. Physico-chemical parameters were measured, and food web typology was described for the first time in the marsh using plankton biomass and internal flux assessment. Both stations changed from a “biological winter” to food webs identified as “multivorous” and passed through to a herbivorous food web. However, food web structure differed significantly between the two stations at the end of the study. Station A remained as a multivorous food web, while station B changed to a strong multivorous food web. After the sea lock gates closing (end of March), an increase in phosphorus concentrations in the water column at station B may have controlled bacterial and phytoplankton development and could explain, at least in part, the differences between the two stations. Our study suggests moreover that differences in water renewal between the two stations could have been responsible for the differences observed. Sea lock gates closing seems to be responsible for the rapid changes observed in food web structure, suggesting that the effects of human hydraulic management on ecosystem functioning in marshes is not yet well understood.