The biogeochemical response of shallow estuarine ecosystems to coastal hypoxia
Foster, Sarah Quinn
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Oxygen concentrations are declining in ocean waters globally. This process of deoxygenation is currently recognized as one of the most important changes occurring in marine environments increasingly affected by human-induced nutrient pollution and global climate change. Rates of deoxygenation are particularly high in shallow estuaries, where conditions of hypoxia (oxygen concentrations < 3 mg L-1) may have large effects on biogeochemical cycles by decreasing oxygen zones in sediments, altering benthic communities, and shifting diagenesis towards more anaerobic pathways. Here, I use three estuaries within Waquoit Bay (Massachusetts, USA) as models to test the impact of hypoxia on fundamental biogeochemical processes, and to evaluate decade-scale trends in oxygen dynamics and hypoxic season phenology. In Chapters 1 and 2, I investigate the effect of water-column hypoxia on ecosystem services provided by the sediments, such as nutrient regeneration, pollutant removal, and regulation of greenhouse gas emissions. In a series of experiments designed to simulate low oxygen conditions I found variable biogeochemical responses to hypoxia, including little to no change in ammonium efflux or reactive nitrogen removal through denitrification. I also found that hypoxia significantly increased sediment phosphate efflux, which lowered water-column nutrient ratios for nitrogen to phosphorus and silicon to phosphorus by 50%. In addition, hypoxia led to approximately a 60% decline in sediment uptake of nitrous oxide, representing a diminished ecosystem service. In Chapter 3, I show that oxygen dynamics and hypoxia are changing over time in Waquoit Bay. Using a 17-year (2002–2018) time series of nearly continuous water quality data I found that despite unchanged total nutrient inputs, summer oxygen concentrations declined (by 0.5–2.1 mg L-1, p<0.10), and hypoxic frequency increased (p<0.10). In addition, the hypoxic season began earlier in both the tidal river and the open basin (p<0.05). While increasing temperature accounted for 7–29% of the declines in oxygen concentrations, ecosystem gross primary production and respiration stood out as having the strongest relationship to the oxygen metrics evaluated. In the tidal river and open basin, these biological processes were most strongly related to wind speed, indicating the importance of regional meteorological change on local scale ecosystem functioning.