Seasonal Variations in Flocculation and Erosion Affecting the Large-Scale Suspended Sediment Distribution in the Scheldt Estuary
The Importance of Biotic Effects
Dante M.L. Horemans (Universiteit Antwerpen)
Y.M. Dijkstra (TU Delft - Mathematical Physics)
HM Schuttelaars (TU Delft - Mathematical Physics)
Koen Sabbe (Universiteit Gent)
Wim Vyverman (Universiteit Gent)
Patrick Meire (Universiteit Antwerpen)
Tom J.S. Cox (Universiteit Antwerpen, NIOZ Royal Netherlands Institute for Sea Research)
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Abstract
Many estuaries exhibit seasonality in the estuary-scale distribution of suspended particulate matter (SPM). This SPM distribution depends on various factors, including freshwater discharge, salinity intrusion, erodibility, and the ability of cohesive SPM to flocculate into larger aggregates. Various authors indicate that biotic factors, such as the presence of algae and their excretion of sticky transparent exopolymer particles (TEP), affect the flocculation and erosion processes. Consequently, seasonality in these biotic factors may play a role in the observed seasonality in SPM. Whereas the impact of abiotic factors on seasonality in SPM is well studied, the relative contribution of biotically induced seasonality is largely unknown. In this study, we employ two approaches to assess the aggregated importance of biotically induced seasonality in flocculation and erosion on seasonality in SPM in the Scheldt estuary. In the first approach, we focus on seasonality of in situ observations in the Scheldt estuary of turbidity, floc size, Chlorophyll-a, and TEP, showing that the abiotic parameters show seasonality, while seasonality in TEP is ambiguous. The second approach concerns a reverse engineering method to calibrate biotically affected parameters of a coupled sediment transport-flocculation model to turbidity observations, allowing us to compare the modeled SPM concentrations to the observations. Driven by seasonality in freshwater discharge, the model captures the observed seasonality in SPM without requiring biotically induced seasonality in flocculation and erosion, which is supported by the absence of seasonality in TEP.