Benthic nitrogen cycling in Weeks Bay, Alabama

Background/Question/Methods

We examined benthic nitrogen fluxes monthly over a one year period at two locations, Mid Bay and Magnolia River, in Weeks Bay, AL NERR site. Weeks Bay is a shallow sub-estuary of Mobile Bay, with an average depth of 1.5 m, and high inputs of nitrogen (N) supported by a predominately agricultural watershed. The objectives of the study were (1) to determine seasonal patterns in benthic nitrogen cycling and (2) determine if there were differences between the two locations. Intact sediment cores were collected monthly by divers and incubated over a 24-hour period in a temperature controlled chamber at ambient water column temperature. Benthic nitrogen flux rates were determined in stirred sediment cores by measuring changes in overlying water column nitrate (NO₃^-), ammonium (NH₄⁺), and dinitrogen gas (N₂) concentrations. N₂ production was measured by membrane inlet mass spectrometry (MIMS).

Results/Conclusions

The highest NH₄⁺ fluxes, 300 µM m^-2 hr^-1 and 239.6 µM m^-2 hr^-1, Magnolia River and Mid Bay respectively, occurred in the summer corresponding to the highest water column temperatures (30-31 ⁰C). The Mid Bay site exhibited higher denitrification rates although there were no detectable differences in benthic NO₃^- fluxes.  The Magnolia River site exhibited higher benthic NH₄⁺ fluxes than the Mid Bay site. N₂ production was assumed to occur primarily via denitrification and exhibited a seasonal pattern with highest rates occurring in the spring, 68.4 µM m^-2 hr^-1 and 20.5 µM m^-2 hr^-1, and fall 96.9 µM m^-2 hr^-1 and 176.9 µM m^-2 hr^-1, Magnolia River and Mid Bay respectively, although there was no detectable pattern in NO₃^- fluxes. In order to further elucidate factors regulating denitrification, potential denitrification rates were measured in slurries at two depths (0-3 and 3-6 cm) by the acetylene inhibition technique concurrent with benthic flux measurements. Both sites exhibited significant increases in denitrification with the addition of NO₃^- although neither site exhibited a clear seasonal pattern. In contrast to denitrification rates measured in intact cores, the Magnolia River site generally exhibited higher rates than the Mid Bay site. Our study suggests NO₃^- is a regulating factor of denitrification rates in Weeks Bay, Alabama. The study of denitrification in Weeks Bay may serve as a proxy for patterns in Mobile Bay, which has the fourth largest watershed in the U.S. and high inputs of N.