Wind and phytoplankton dynamics drive seasonal and short-term variability of suspended matter in a tidal basin

Abstract. Suspended particulate matter (SPM) is a key component of coastal ecosystems, modulating light availability, nutrient transport, and food web dynamics. Its variability is driven by a combination of physical and biological processes that interact across temporal and spatial scales. Using the Sylt-Rømø Bight as a natural laboratory and focusing on the period 2000–2019, in this study, we integrate statistical analysis of observational data from the Sylt Roads monitoring program and local meteorological stations, neural network modelling and Lagrangian transport simulations. This multi-method approach enables us to disentangle and quantify the relative roles of tidal and wind forcing, as well as biological processes in shaping SPM concentrations across various time scales, based on near-surface measurements at two monitoring stations. The findings show that wind intensity dominates short-term SPM variability, particularly at the shallow station, where SPM responds rapidly to local wind-induced resuspension. At the deep station, the wind effects appear with a delay of ∼5 d, aligning with tidally induced transport timescales (∼133 h) from shallower resuspension zones, as revealed by Lagrangian simulations. Seasonal patterns are further modulated by both reduced wind intensities and the onset of biological processes, such as phytoplankton blooms, which promote flocculation and subsequent settling in spring and summer. Neural network experiments highlight the shifting seasonal balance between physical and biological controls. The median concentration of SPM decreased by up to 80 % from winter to summer. Approximately 40 % of this seasonal difference can be attributed to weaker wind conditions, while the remaining ∼40 % is likely driven by biologically mediated sinking processes.