Seasonal and Interannual Variability of the Dissolved Organic Matter Composition in the Delaware Estuary

Estuaries as connectors between the freshwater and marine aquatic systems are known as hotspots of biogeochemical element cycling. In one of the largest and best studied temperate estuaries, the urban Delaware Estuary, we investigated the seasonal and interannual variability of dissolved organic matter (DOM) over three years along the salinity gradient. The decrease in bulk DOC concentrations along the transect remained remarkably similar between sampling intervals and was driven by conservative dilution of river- with the low-DOC oceanic endmember, except in the oligohaline zone. A more detailed analysis of the DOM molecular composition via ultrahigh resolution mass spectrometry (FT-ICR-MS), however, revealed that DOM compositional differences were, besides the dominating conservative mixing of terrestrial and marine endmembers, also driven by the river discharge regimes and seasonality. High discharge conditions in the upper estuary were related to an increase of aromatic DOM compounds. Seasonal changes in DOM composition were mostly correlated to bacterial production rates in the upstream river and phytoplankton biomass in the polyhaline estuary suggesting bio-degradation by heterotrophic bacteria and addition of DOM compounds by phytoplankton, respectively. A relative increase of heteroatom (N, S)-containing DOM components was observed during periods of high phytoplankton growth. We argue that long-term studies in combination with in-depth DOM molecular analysis will provide the necessary basis to understand the processes that transform terrigenous DOM in land-ocean transition zones before its discharge into the coastal ocean.