Date Awarded


Document Type


Degree Name

Master of Science (M.Sc.)


Virginia Institute of Marine Science


Nitrogen (N) has been shown to limit primary production in many estuarine systems, including the New River Estuary (NRE), NC, a moderately eutrophied system with large areas of photic sediment. The NRE receives major inputs of allochthonous nutrients from agriculture and confined animal feeding operations as well as other sources. Autochthonous sources of N in the NRE include both remineralization and Nfixation. Whereas allochthonous sources are usually most important in winter/spring and during periods of high fresh water discharge, autochthonous sources are likely to become more important in summer. N-fixation, which can be performed by both autotrophic cyanobacteria and heterotrophic bacteria, was shown to vary in response to light levels, organic matter and temperature. To assess the importance of N-fixation at a system-wide scale we sampled seasonally along the estuarine gradient at multiple water depths (with a range of light availabilities) and multiple sediment depths. Benthic N-fixation activity was determined using the acetylene reduction method. Molecular characterization of the microbial communities along with the molybdate inhibition technique were used to verify the relative importance of autotrophic to heterotrophic sulfate reducing N-fixers. The majority of benthic NFix in the NRE was performed by sulfate-reducing bacteria. Although highest rates of benthic N-fixation were in the top 0 – 1 cm, a substantial portion occurred down to 10 cm in shallow and deep water samples. N-fixation rates were highest in the mid and lower estuarine sites during summer, with estimates as high as 2407 μmol N m-2 d-1. Benthic N-fixation rates varied seasonally and were a significant source of autochthonous N to the NRE, contributing up to 19% of total new N inputs during spring.



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