Observational and Modeling Study of Driving Mechanisms Behind SPM Variability in a Tidal System

Suspended particulate matter (SPM) is a key component of coastal ecosystems, influencing light availability, primary production, and nutrient transport. This study investigates the driving mechanisms behind the seasonal and interannual variability of SPM concentrations measured at two long-term monitoring stations in the Sylt-Rømø Bight, a sandy tidally dominated basin in the Wadden Sea. Combining Sylt Roads long-term observations from 2000–2019 and numerical simulations with the coastal hydrodynamic model FESOM-C with its Lagrangian particle tracking module, we analyse the interplay of wind and tidal forcing, and biological processes in shaping SPM dynamics.

Preliminary analysis of the observational dataset reveals a pronounced seasonal cycle, with a peak in winter ~30 mg/l and a sharp decline in summer ~6.5 mg/l across both stations. These variations are associated with stronger wind events in winter and higher biological activity (reflected by chlorophyll-a concentrations) during spring and summer, indicative of phytoplankton-driven flocculation processes. The data further highlight distinct patterns: the shallower station exhibits an almost immediate response to wind events within 24 h, while at the deeper station, SPM reaches peak concentrations with a delay of ~120 h, consistent with the influence of tidally induced transport in addition to sustained wind-driven mixing. Complementary results from Lagrangian modelling effectively capture these delayed responses at deeper stations and further illustrate the tide-driven transport pathways of resuspended material within the basin.

The findings of this ongoing work provide new insights into coastal physical-biological coupling and the the relative roles of the considered processes in driving SPM variability in tidally dominated systems.