Detecting Eutrophication in Coastal North Carolina with the 𝛿15N Values of Seagrasses and Algae

Date of Award


Document Type

Restricted (Opt-Out)

Degree Name

Bachelor of Science


Environmental Science and Policy

First Advisor

David Gillikin


Nitrogen loading, nitrogen pollution, fertilizer, effluent, algae, seagrass, aquaculture


Coastal regions have been known to experience nitrogen pollution in the form of waste- water inputs or agricultural fertilizer runoff that is impacting aquatic biogeochemistry. Excess nutrients lead to eutrophication and cause a multitude of imbalances in coastal waters, including hypoxia. Here we develop an algae and seagrass isoscape in waters within the Cape Lookout National Seashore in North Carolina, from Jarrett Bay and adjoining creeks to the Atlantic Ocean. An approach for detecting impacts of sewage and agricultural sourced nitrogen is the use of stable nitrogen isotopes (d15N) in marine plants and algae, which can be bioindicators of aquatic ecosystems. Marine algae are excellent recorders of N inputs because they use N directly from the dissolved inorganic N pool with little fractionation, which is time-averaged over the growing season. Wastewater is typically enriched in 15N, while inorganic fertilizer is depleted in 15N. We expect the results to show increased d15N values in areas adjacent to densely populated locations (indicating septic tank seepage into the coastal waters). There is limited agricultural run-off in the region because of land restoration efforts headed by the North Carolina Coastal Federation, so we do not expect a large influence from fertilizers. Clear hot spots of sewage input were not detected. Across all of the species of algae, δ15N values generally ranged from +1 and +6‰ at all sites. The average δ15N values amongst all sites studied was 3.73 ±1.21‰ (n=63) for algae. A large scatter in δ15N values was found between species at each site. Given the relatively low δ15N values it is unlikely that nitrogen loading from organic sources such as sewage or animal waste is prevalent.

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