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

Abstract

We present the first complete sections of humic substance fluorescence (HS fl)-values and total organic carbon (TOC) concentrations through the extended estuary of the Baltic Sea, Kattegat, and Skagerrak. Both HS fl-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 HS fl-values decreased from north to south, while TOC concentrations remained relatively constant, indicating an increasingly biologically-labile TOC pool from north to south. Both HS fl-values and TOC concentrations decreased strongly from the Baltic Sea to Kattegatt and Skagerrak. There were no major net sources or sinks for HS fl or TOC in Kattegat. However, the Jutland Current was an HS fl and TOC source for Skagerrak, probably derived from Danish rivers. Fraction of forested area in drainage basins of the Baltic Sea correlated positively with HS fl-values in freshwater end-members, but not with TOC concentrations. Land-runoff-volume relative to basin volume correlated positively with HS fl-values in surface waters, but not with TOC concentrations. In the Baltic Proper, deepwater oxygen concentrations correlated negatively with HS fl-values, indicating a significant source for HS fl (but not TOC) in suboxic and anoxic deepwaters. Photobleaching decreased HS fl-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.