Silica retention in the Scheldt continuum and its impact on coastal eutrophication (SISCO): Final report EV/17

SISCO (Silica retention in the Scheldt continuum and its impact on coastal eutrophication) was an interdisciplinary consortium consisting of biologists, (bio)geochemists and hydrodynamic-biogeochemical modellers. The overall objective was to elucidate the biogeochemical cycling of silicon (Si) and its anthropogenic perturbation in the Scheldt continuum river-estuary-coastal zone. We aimed specifically at 1) identifying the sources and sinks of Si in the aquatic continuum, 2) quantifying the major processes controlling the biogeochemical behaviour of Si in the water column, 3) evaluating the early diagenesis of Si in order to determine the burial fluxes and internal recycling rates within the sediments, and 4) developing a Si module within an existing transport-reaction model to assess the Si fluxes carried by the Scheldt to the Southern Bight of the North Sea. To achieve the aims, we applied an integrated approach combining 1) analyses of historical data, 2) field surveys and laboratory investigations, and 3) model development. Analysis of historical chlorophyll a data (1995-2006) in the upper Scheldt estuary exhibited a high inter-annual variability of the phytoplankton biomass in summer, which could be attributed primarily to the flushing rates of the freshwater in the upper estuary. This points towards a strong hydrodynamic control on phytoplankton development in the upper estuary. The intensive phytoplankton monitoring survey in 2003 showed that phytoplankton biomass not only differed between the tidal Rupel and Scheldt branches but also in their tributaries. The difference was ascribed to the smaller cross-section of the rivers in the Rupel basin, resulting in a higher water velocity and lower retention time of the water, which would limit phytoplankton development. Study of the phytoplankton community composition along the river-estuarine-coastal zone continuum of the Scheldt revealed a succession from a riverine to a marine phytoplankton community. There is no single estuarine phytoplankton community but rather a succession of estuarine species along the salinity gradient. Monthly monitoring survey of phytoplankton in 2003 in the Belgian coastal zone exhibited a pronounced spatio-temporal variability in the timing and magnitude of the spring bloom. The spring bloom started earlier in the western part of the Belgian coastal zone probably due to a more favourable mixing depth to photic depth ratio. The magnitude of the spring bloom was higher in the eastern part of the coastal zone, probably because of higher nitrogen (N) and phosphorus (P) inputs from the Scheldt. Temporal evolution of dissolved and biogenic silica concentrations along the Scheldt freshwater estuary and in its tributaries was investigated during one year in 2003. In the tributaries, dissolved silica (DSi) concentrations remained high and biogenic silica (BSi) concentrations were low throughout the year. In the freshwater estuary during summer however, DSi was completely consumed and BSi concentration increased. Mass balance calculations showed that silica consumption and retention in the freshwater estuary were important at a seasonal time-scale: from May to September, one third of the total amount of riverine silica was retained. The consumption and retention were high when discharge was low (and inversely), suggesting that silica retention would be of much less significance when annual fluxes were considered. The longitudinal distribution of DSi and BSi along the salinity gradient in the Scheldt estuary was determined. The 1D-CONTRAST model was used to simulate the conservative mixing in the brackish estuary of DSi from the freshwater and the seawater end-members; existing datasets of DSi and salinity was used as inputs for boundary conditions. Comparison of the observed DSi profiles with the model outputs indicated a consumption of DSi in the brackish estuary in spring-summer and a conservative transport in winter. BSi were found to be closely link