Spatio-temporal variation in particulate and dissolved organic matter dynamics in the southern North Sea

Turbid coastal waters are dynamic systems where fine-grained sediments interact with organic matter, significantly influencing the fate of both the components. We investigated the seasonal dynamics of particulate (POC) and dissolved (DOC) organic carbon pools along a suspended particulate matter (SPM) gradient from nearshore to offshore waters in a mid-latitude coastal zone. To assess temporal and spatial variations in POC composition, we quantified the relative contributions of phytoplankton (POC phyto ), heterotrophs (POC het ), detritus (POC det ), and mineral-associated organic matter (POC mineral ) to the bulk POC pool. In nearshore waters, frequent tidal resuspension and high SPM concentrations led to elevated POC det and POC mineral fractions, masking the increase in POC phyto , despite the higher primary productivity during the spring bloom. In contrast, offshore waters exhibited a greater relative contribution of POC phyto , with seasonal POC increases corresponding to elevated chlorophyll a (Chl a ) levels in spring and summer, similar to the open-ocean dynamics. These trends were further reflected in particulate organic carbon to nitrogen (POC:PON) ratios and POC:Chl a ratios, commonly used to assess sources and quality of organic matter. The cross-shore gradient in organic matter partitioning, with dominance of POC nearshore and DOC offshore, highlights the role of particle resuspension and phytoplankton production in controlling organic carbon distribution between the two pools. Overall, our findings underscore the complex interplay between biological production, nutrient cycling, hydrodynamic forces, and SPM in shaping the composition and fate of organic carbon in turbid coastal systems.