Spatial variability in organic matter stocks and sources in a river-dominated estuary during wet and dry seasons

Terrestrial and phytoplankton-derived organic matter are the base of the food web in highly productive, river-dominated estuaries and drive important biogeochemical processes that control carbon, oxygen, and nutrient dynamics. In this study, we investigated the controls on the spatial patterns of organic matter quantity and quality in the water-column and sediments of Mobile Bay, Alabama. We hypothesized that spatial patterns were mainly driven by river discharge to the estuary. During the wet and dry seasons, we measured organic matter quantity and quality, as determined by concentrations, carbon to nitrogen ratios, and stable isotopic (¹³C and ¹⁵N) composition of organic matter in water and sediments. Suspended sediments were collected during June 2019 from 7 rivers that discharge to the bay. Surface and bottom water and sediments were collected from 15 sites within Mobile Bay and 4 offshore sites and will also be collected in October 2019. Heavy rainfall and runoff during spring 2019 resulted in elevated inputs of organic matter from the rivers to the bay. The near linear decrease in POC δ13C in the surface and bottom layers versus salinity suggests that salinity, is a primary determinant of the POC organic matter type. The sediment surface organic matter was > 50% freshwater origin at sites with S <10. In the sediments where the mixing model indicates that up to 40% of the organic matter at S = 0 is marine derived, salinity mixing does not explain this pattern. As river discharge subsides, we expect phytoplankton in the bay to become a dominant source of organic matter in the water-column while the sediments are expected to maintain a signature of terrestrial organic matter. In addition to detailing the spatial patterns of organic matter quality, we will present how the magnitudes and sources of organic matter contribute to the onset and maintenance of hypoxia in this system.