R/E-24C

Impact/Outcome:

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.