Decoupling of total organic carbon concentrations and humic substance fluorescence in a an extended temperate estuary

We present the first complete sections of humic substance fluorescence (HSfl)-values and total organic carbon (TOC) concentrations through the extended estuary of the Baltic Sea, Kattegat, and Skagerrak. Both HSfl-values and TOC concentrations correlated strongly with salinity, indicating terrestrial runoff as the main source for both components. However, their distributions do not result from a simple two end-member mixing and the two pools have different controls. In the Baltic Sea, surface water HSfl-values decreased from north to south, while TOC concentrations remained relatively constant, indicating an increasingly biologically-labile TOC pool from north to south. Both HSfl-values and TOC concentrations decreased strongly from the Baltic Sea to Kattegatt and Skagerrak. There were no major net sources or sinks for HSfl or TOC in Kattegat. However, the Jutland Current was an HSfl and TOC source for Skagerrak, probably derived from Danish rivers. Fraction of forested area in drainage basins of the Baltic Sea correlated positively with HSfl-values in freshwater end-members, but not with TOC concentrations. Land-runoff-volume relative to basin volume correlated positively with HSfl-values in surface waters, but not with TOC concentrations. In the Baltic Proper, deepwater oxygen concentrations correlated negatively with HSfl-values, indicating a significant source for HSfl (but not TOC) in suboxic and anoxic deepwaters. Photobleaching decreased HSfl-values in surface waters of the Baltic Proper, but had no effect on TOC concentrations. Despite terrestrial material being the main source for both TOC and HS, differences in control mechanisms result in a partial decoupling of their concentrations within the estuary.

Research highlights► Terrestrial runoff was the main source of HSfl and TOC. ► Biological lability of DOM increased seaward. ► Fraction of forested basin area basin correlated with HSfl-values in freshwater end-members. ► Land-runoff-volume to basin volume correlated positively with HSfl-values in surface water. ► Suboxic and anoxic deepwaters of the Baltic Proper are a significant source of HSfl.