Cross System Comparison of Oxygen Effects on Nitrogen Cycling in Tidal Sub-Estuaries of Patapsco River

Principal Investigator:

Jeremy Testa

Start/End Year:

2018 - 2019


Chesapeake Biological Laboratory, University of Maryland Center for Environmental Science

Co-Principal Investigator:

Fellow: Drew Hobbs

Strategic focus area:

Healthy coastal ecosystems


Rock Creek is a tidal tributary to the Patapsco River in Anne Arundel County, Maryland and has historically had poor water quality including the development of water-column anoxia. An aeration system was installed into the creek in 1988 to help alleviate the water quality problems. Several recent studies have been conducted on the creek in order to view how aeration affects sediment nitrogen and oxygen fluxes, but less emphasis was placed on the associated production of problematic compounds. For example, with the establishment of anoxia hydrogen sulfide (H2S) is released from anoxic sediments, which can inhibit nitrification and negatively impact other aquatic life. Alterations of nitrogen cycling in low-oxygen environments can also lead to elevated production of the greenhouse gas nitrous oxide (N2O) as a byproduct of the transformation of nitrate (NO3-) to N2 gas. Measuring and quantifying N2O concentrations within an aquatic system can provide insight into the rates of denitrification that may be occurring, as well as the availability of the gas in nutrient enriched waters. Thus, I propose to measure the concentrations of H2S and N2O, nitrogen concentrations, and sediment-water fluxes of oxygen and nitrogen during September 2018 and spring 2019. These measurements will be made in the aerated Rock Creek, as well as two adjacent creeks (Stoney Creek and Bodkin Creek) that have never been studied despite their immediate proximity to Rock Creek, lack of aeration, and potential for similarly oxygen-depleted conditions. These surveys will provide useful and relevant data to help (a) better understand the impacts of engineered aeration of coastal systems, (b) investigate the potential for sulfide accumulation to alter nitrogen cycling, and (c) improve our understanding of N2O availability in Chesapeake Bay.


Aerating a Polluted Tributary Helps Combat the Effects of Too Many Nutrients

Summary: Maryland Sea Grant has supported a unique whole-ecosystem study of aeration in a highly polluted Chesapeake Bay tributary. The work yielded new data to inform discussions of whether aeration should be scaled up to improve Bay water quality, prompting decision makers in Maryland and Finland to consider application of this research to management of other impaired waterways.

Relevance: Since 1988, aerators have been operating in Rock Creek, a tributary of the highly polluted Patapsco River in Baltimore. The system pumps 15,000 liters of air per minute into the water, helping to oxygenate the water column and limit nutrients and algae growth. It also provides scientists an opportunity to study oxygen's impacts in eutrophic systems. Learning how aeration affects biogeochemistry and water quality parameters, rates of photosynthesis and oxygen consumption, phytoplankton growth, and low-oxygen nutrient recycling-the cycle in which nutrients recycle within the system rather than being eliminated-could help managers assess the effectiveness of aeration as a mitigation strategy for other Chesapeake waterways. It also helps them understand and quantify the cost of low oxygen in terms of reducing the natural ability of estuaries to remove nutrients when the water column is hypoxic.

Response: Over three years, Maryland Sea Grant-supported researchers, as well as two REU students, a fellow, and an intern, engaged in a whole-ecosystem experiment using the aeration system. By manipulating oxygen content, they quantified the system's benefits related to how nitrogen, phosphorous, and oxygen were recycled in the water column and in sediments. They learned that, within 24 hours after the system was turned off, oxygen disappeared almost entirely, impacting regions beyond the aerated area. This finding suggests that the aerators can improve oxygen far beyond their locations at the tidal creek's landward end. They determined that, while aeration does help diminish low-oxygen nutrient recycling, this process began much sooner for phosphorous than for nitrogen. They also developed a nitrogen mass balance budget for the creek, which quantified the impact of hypoxic conditions. It found an approximately 23% reduction in denitrification rates as well as enhanced nitrogen recycling. Using an estimate of $280 per pound of nitrogen removal, this means that Rock Creek aeration provides a benefit of approximately $27,353,480.

Results: The ability to quantify the cost of hypoxia to lost denitrification contributes to ongoing nutrient-trading discussions related to water quality best management practices. The researchers' work helped inform the decision to replace the $1 million aeration system in Rock Creek, and data on the creek's eutrophication has spurred Anne Arundel County and the Restore Rock Creek stewardship group to implement more restoration activities in the watershed. The research also led to the principal investigator serving as an advisor on a report to the government of Finland in its efforts to explore mitigation strategies in the Baltic Sea.

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