The Salinity Gradient: Role of Aquatic Ecosystems in Determining Water Quality from Rivers to Estuaries
Abstract
Water quality management is a unifying force spanning the salinity gradient of aquaticecosystems from freshwater rivers to coastal oceans. The hydrologic cycle connects aquatic ecosystems through the delivery of water, nutrients, and carbon. Aquatic ecosystems regulate global elemental cycles by transporting and transforming nutrients and carbon to support ecosystem services that protect water quality. Maintaining high water quality benefits human and ecosystem health by ensuring clean drinking water and protecting coastal habitats that support fisheries and recreation. Anthropogenic- and climate-driven changes to the delivery and export of nutrients and carbon threaten the capacity of aquatic ecosystems to regulate water quality. Wastewater effluent inputs to river networks saturate their assimilative capacity, sending excess nitrogen downstream to estuaries. In coastal ecosystems, eutrophication from excess nutrient inputs accelerates seagrass meadow loss and the associated loss of ecosystem services including nitrogen cycling. Understanding the capacity of aquatic ecosystems to regulate biogeochemical cycles amid anthropogenic alteration of nutrient and carbon availability is critical for informing water quality management decisions that affect everything from wastewater treatment facility permits to seagrass conservation efforts. xviii In this dissertation, I assessed the capacity of rivers and estuaries to process excess nitrogen inputs and the impact of poor water quality on seagrass persistence. I used a combination of long-term stream chemistry data, targeted estuarine water quality sampling, field experiments, and modeling to better understand how anthropogenic alteration of biogeochemical cycles impacts freshwater and estuarine ecosystem response. This work coincided with years of climatic variability, including record-breaking precipitation totals, and seagrass habitat loss. To assess the extent of nitrogen uptake and removal at the river reach scale in a suburbanizing watershed, I used a 21+ year record of stream chemistry and new measurements of N2:Ar ratios. Downstream of a wastewater treatment facility, river reaches were net sinks of ammonium and net sources or passive transporters of nitrate. River reaches supersaturated in N2 gas occurred across net sink and source behaviors for ammonium and nitrate, indicating both anammox and denitrification potentially contributed to nitrogen removal. Next, I assessed the relative influence of hydrodynamic characteristics, water quality, and algal competition on seagrass biomass at two spatial scales during a time when seagrass acreage declined to a record low. I found hydrodynamic characteristics such as water residence time to be the strongest driver of seagrass biomass. Climate variability masked the impact of water quality and light availability on seagrasses. Finally, I quantified nitrogen removal in a eutrophic estuary through in situ denitrification measurements using a push-pull 15N-isotope pairing technique. Results suggest that high denitrification rates within patchy seagrass meadows were due to legacy nitrogen and carbon in the sediment bed. Collectively, these dissertation chapters provide new insight regarding how aquatic biogeochemical cycles respond to anthropogenic nutrient inputs. Results will inform coastal management decisions regarding the regulation of nitrogen inputs relative to the nitrogen removal capacity in both upstream watersheds and downstream, receiving estuaries.