Coastal ecosystems are faced with increasing pressures from human activities. Perhaps one of the most profound impacts is that of excess nitrogen loading which drives a series of negative consequences. Excess nitrogen fuels primary productivity and the subsequent enhanced microbial decomposition of organic matter, consumes oxygen and releases carbon dioxide, which causes large fluctuations in pH. Changes in organic matter availability, oxygen concentrations, and pH can have significant yet unconstrained implications for sediment recycling and removal of biologically important nutrients such as nitrogen and phosphorus. Such changes can also impact the production and consumption of two powerful greenhouse gases – nitrous oxide and methane. Here I use two temperate estuaries, Long Island Sound (New York, USA) and Waquoit Bay (Massachusetts, USA) to assess the role of human impacts on coastal sediment biogeochemical fluxes. In Chapter 1, I investigate the influence of organic matter loading on sediment nutrient cycling, excess nitrogen filtering, and greenhouse gas emissions in Long Island Sound, a heavily nutrient polluted estuary. To provide a comprehensive analysis of these benthic fluxes and their environmental drivers, I incubated sediment cores from five stations along a west to east transect representing a gradient of high to low nutrient inputs and organic matter deposition. I found sediments across the estuary removed only 9% of land-based nitrogen entering the system and had a nitrogen removal efficiency of 30%. Additionally, sediments were often a source of inorganic nitrogen and phosphorus as well as nitrous oxide and methane. This study provides the first directly measured rates of sediment nitrogen removal and production in Long Island Sound. In Chapters 2 and 3, I investigate the effect of coastal acidification on benthic fluxes of greenhouse gases and nutrients across the sediment-water interface in Waquoit Bay. I collected sediment cores from two sites experiencing different rates of nutrient loading and experimentally altered the overlying water pH through a series of incubations representing moderate (pH 7.3) and extreme (pH 6.3) pH conditions. My results show low pH conditions have a strong effect on greenhouse gas and nutrient fluxes and responses vary by site. Specifically, in the high nutrient impacted site, nitrous oxide flux increased and methane flux decreased under acidification. In the low nutrient impacted site acidification drove reduced nitrous oxide flux, while methane flux decreased in the moderate treatment and increased in the extreme treatment. Acidification also affected benthic nutrient fluxes and drove the high nutrient impacted site to become phosphorus limited. Furthermore, the relationships and drivers between nutrient availability and nutrient fluxes shifted under acidification. This dissertation provides additional insight into how coastal ecosystems respond to human impacts. In Chapter 1, I present a critical missing piece of the nitrogen budget of a heavily impacted estuary. In Chapters 2 and 3, I begin to elucidate how low pH conditions can impact sediment biogeochemistry in estuarine ecosystems. Efforts to improve our understanding of human impacts on sediment biogeochemical fluxes will create better informed coastal management practices for these dynamic systems under a changing climate. / 2025-10-23T00:00:00Z