Phytoplankton dynamics in a recovering ecosysyem: effects of water quality improvements in the highly eutrophied Schelde estuary

The river Scheldt has an estuarine zone that extends 160 km inland. Due to high anthropogenic pressures and pollution during the second half of the twentieth century, water quality in the freshwater tidal reach was very poor, characterized by severe hypoxia and high ammonium conditions. Wastewater treatment since the 1990’s has led to an improved water quality and caused a shift from a heterotrophic to an autotrophic system. From 2003 onwards, the gradual increase in oxygen and decrease in ammonium concentration were associated with the formation of intense summer diatom blooms. Moreover, this biomass increase was characterized by a shift from small centric diatoms, including Cyclotella meneghiniana to the larger Actinocyclus normanni. Another remarkable change involved in the development of a Thalassiosira nodulineata bloom in the brackish part of the estuary since 2008. The present study aims to investigate how abiotic parameters, for instance nutrients, light and salinity concentrations, influence the species composition and dominance of the diatom community in the Scheldt estuary. Therefore, three experiments were performed with naturally abundant diatom species of the Scheldt estuary, including A. normannii, C. meneghiniana, Stephanodiscus hantzschii, T. nodulolineata and several marine Thalasiosira spp. These species were cultured and grown on different ammonia concentrations, under different light conditions, and salinity levels to investigate their optimum growth conditions. During each experiment, biomass was measured daily as initial fluorescence (F0) by Pulse Amplitude Modulate (PAM) fluorometer and the growth rate of each strain for each treatment was calculated. The results suggest that ammonium concentrations may have been an important driver for the shift in dominance from the smaller diatom C. meneghiniana to the larger A. normannii in 2003, as the latter species showed low tolerance for ammonium. Diatom species originating from the Scheldt estuary, with a lower light climate compared to the North Sea, were adapted to this low light climate and had a lower growth rate under high light conditions. The results of the salinity experiment revealed that T. nodulolineata can be considered as a typical brackish species, as this species is able to grow in salinities ranging from 0 to 34 PSU and has maximal growth rates at an intermediate salinity. As a conclusion, the spatial-temporal changes in phytoplankton biomass and composition is highly regulated by bottom-up (hydrology and abiotic parameters) mechanisms and the study of phytoplankton dynamics can be used as an indicator for water quality.

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