Organic Matter Transformations During Transit Through the San Francisco Bay Estuary

Biomarker studies in plume waters and estuaries are comparatively rare, but crucial for understanding the fate of riverine organic matter in coastal zones. We collected DOM samples during three transects (2014, 2015, 2016) in the San Francisco Bay Estuary (SFBE) during three flow regimes (winter storm, spring freshet, summer baseflow) from (near) full salinity to riverine endmembers, and POM samples from the northern SFBE at monthly intervals during the fall/winter/spring of 2011-12. Our comprehensive suite of analyses includes biomarkers (lignin, chlorophyll), optical analyses (UV-vis and fluorescence), and FT-ICR-MS. Each DOM transect had its own unique source and diagenetic signature, reflecting relative differences in wetland vs. upland sources, but also including hotspots for blooms during the spring freshet. The complex seasonality of this system is perhaps represented best by dissolved lignin acid to aldehyde ratios that increased with increasing salinity in one transect, but then decrease in another. From an FT-ICR-MS perspective, the winter storm and summer baseflow transects were surprisingly similar in several broad categories, including number of formulae, average mass, aromaticity, polyphenolics, while the spring freshet was significantly higher in all of these. In contrast, fluorescence index was most similar between the summer and spring freshet transects, while slightly higher during the winter storm. POM dynamics tell a much different story, with close ties to primary production and foodweb dynamics. Decreasing chlorophyll with increasing salinity demonstrates higher productivity in the freshwater environment combined with selective consumption of this material during transit through the SFBE, while relative contributions from vascular plant contributions as represented by lignin increase, with likely inputs from wetlands. This latter source takes on increasing significance with recent research indicating that the lower foodweb is supported by non-algal particles in ways not previously recognized.