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, this study integrates a long-term biogeochemical time series from the Sylt Roads monitoring program and meteorological observations with Lagrangian transport simulations and neural network modelling to disentangle and quantify the relative roles of tidal dynamics, winds, and phytoplankton mediated biological processes in shaping SPM concentrations measured at two stations near the water surface.

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 days, aligning with tidally induced transport timescales (~133 hours) 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, with phytoplankton blooms promoting flocculation and subsequent settling in spring and summer. Neural network experiments highlight the shifting seasonal balance between physical and biological controls: models trained on winter data overestimate summer SPM levels by up to 80 %, with only ~40 % of this discrepancy explained by weaker winds and the remainder likely reflecting biologically mediated sinking processes.