| Summary: | Estuaries and coastal regions are hot spots of global primary production as they account for 30% of the net marine productivity, despite representing only 7% of the total marine surface area. Nutrient patchiness is the second most influential factor (after temperature) in site seasonal variability of net primary production. This guides a consensus that nitrogen (N) loading is a primary factor in eutrophication and hypoxia. Despite this recognized connection between excess N loading and ecosystem decline, the internal N cycling and transformations in coastal areas are still not well characterized. This study quantified the sediment porewater flux of ammonium (NH4 + ) and photo-ammonification from benthic dissolved organic nitrogen (DON) as well as investigated changes in porewater DOM chemical structures of Baffin Bay - Texas during different irradiation time periods using the state of art Orbitrap Fusion Tribrid Mass Spectrometer. Baffin Bay (BB) is a semi-arid inverse estuary ecosystem that has experienced prolonged and intense brown tide blooms (Aureoumbra lagunensis species) since the 1990s, and several occurrences of hypoxia conditions that caused seagrass die-off and several occasions of large fish kill over the past decades. Pore water samples were collected from six stations in the bay during six seasons (October 2020, February 2021, June 2021, October 2021, February 2022, and May 2022) for depth profile analysis. The bay showed a large spatial variability of NH4 + benthic flux, with BB1 having the lowest average NH4 + flux at 56 ± 27 µmol.m-2.day-1 and BB3 had the highest average flux at 347 ± 211 µmol.m-2.day-1. The seasonal NH4 + average benthic flux to the entire bay water column was 124 – 244 µmol.m-2.day-1 and served as a significant source of inorganic nutrients to support the primary production. In addition, the photo-ammonification rate of benthic DON (after it entered the water column) ranged from 0.038-0.361 µmol.L-1.hour-1 and served as an additional source of inorganic nutrients to primary ...
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