Tidal freshwater ecosystems such as rivers and estuaries are located at the interface between land and sea. These ecosystems provide important natural services including primary production, nutrient distribution and regulation, and waste management - services that are at risk due to climate change. Two aspects of climate change, sea level rise and lower precipitation in some watersheds, will shift the location of freshwater-saltwater mixing zones to upper rivers and continuously increase salinity intrusion into freshwater habitats. Sea level is predicted to rise up to 100 cm by 2100, and by itself can have multiple impacts on freshwater ecosystems, including direct toxic effects, habitat degradation and alteration of nutrient cycles. Salinity increases may also have significant effects on the nitrogen (N) dynamics at freshwater habitats. Nitrification (ammonia conversion to nitrate) and denitrification (nitrate conversion to dinitrogen) are key microbial processes controlling the intensity and duration of eutrophication caused by excess N loading. Salinity intrusion might diminish microbial N removal capacity in freshwater sediments as nitrification and denitrification are reduced due to substrate limitation and physiological inhibition. This could result in prolonged eutrophication and hypoxia at rivers and estuaries. Alternatively, anaerobic ammonium oxidation (anammox; ammonium and nitrite conversion to dinitrogen gas) may become a more important microbial N removal process and help recover N removal capacity in freshwater sediments since anammox bacteria can tolerate salinity increases. This project will study 1) the importance of anammox in tidal freshwater sediments, 2) the effects of salinity on freshwater anammox bacteria and the sedimentary N cycle, and 3) predictions on an alternative N removal pathway in sedimentary communities threatened by sea level rise. The proposed research will integrate molecular microbial techniques, chemical tracer methods and mathematical models to examine sedimentary N removal processes in the Cape Fear River and New River of North Carolina.
This proposal will result in the following benefits and outcomes; 1) revealing the structures and activities of microbial communities involved in N removal processes in tidal freshwater sediments, 2) determining differential responses of sediment communities to seawater intrusion, 3) understanding the impacts of sea level rise on sedimentary N removal processes in freshwater ecosystems, 4) predicting a key microbial process for net N removal capacity of freshwater ecosystems experiencing sea level rise. This project will also provide an in-depth understanding of microbial responses in freshwater sediment communities under threat of sea level rise. Interdisciplinary approaches will provide insight into the linkages between community structure and activity as well as salinity effects on riverine sedimentary N cycling. Broader impacts will be manifested through diverse educational components, outreach, and improved research infrastructure at the University of North Carolina Wilmington (UNCW) and the University of Connecticut. Three PIs (microbial ecologist, biogeochemist and ecosystem modeler) will provide interdisciplinary training to at least three graduate students seeking a MS or Ph.D. degree. Additional educational impact will be accomplished both in the classrooms and through individual undergraduate research projects at the two institutions. Research experience for high school students will be achieved by including the project within the UNCW "Summer Ocean Ventures" and "Ocean17" programs. The high school students will participate in DNA analysis and mesocosm experiments. The PI will provide a lecture of microbial nitrogen cycle to the students and design three days of laboratory and field courses to detect and measure denitrification during the Ocean 17 camp. The graduate students recruited for this proposed research will act as teaching assistants for the high school students. The program will culminate with a Summer Ventures Symposium where students will present their work. Outreach will be facilitated by partnering with a local citizens group (NC-Coastal Ocean Federation) to recruit volunteers to participate in sampling cruises and water monitoring program, and by using the existing UNCW "Planet Ocean" lecture series as a public platform to discuss coastal water resources issues and actions.
