ABSTRACT

The Dynamics of Dissolved and Particulate Nitrogen in the Northwest Gulf of Mexico.
(August 1998)

Diego Guillermo López-Veneroni

Texas A&M University (USA)

The concentrations and stable isotopes 15-N/14-N) of several nitrogen pools of the NW Gulf of Mexico were measured to trace the sources and cycling of this element in the region. The del 15-N of particulate nitrogen (PN) and nitrate, and dissolved organic nitrogen (DON) concentrations at the Mississippi River outflow show that terrestrially-derived nitrate is cycled into PN and released as DON, which is the principal nitrogen species advected by the plume. Higher dissolved inorganic nitrogen (DIN) concentrations in plume than in adjacent shelf waters suggest that nitrification and benthic mineralization are important sources of recycled nitrogen. A total dissolved nitrogen (DON + DIN) maximum, along with del 15-N-PN and other parameters, show that the plume-shelf front is also a nitrogen pool of the system.

The coupling of stable isotopes of PN and nitrate by a Rayleigh equation in an anticyclonic ring suggests that the 15-N-enriched residual nitrate fluxing from the pycnocline determines the average mixed-layer del 15-N-PN composition. Thus, a principal source of nitrogen for the ring is nitrate of oxidative origin.

Del 15-N-PN values of mesozooplankton in plume, shelf, slope and ring waters show a similar trophic composition in these systems. The trophic chain in plume waters is relatively 15-N-enriched as a result of terrestrially-derived nitrate incorporation. The shelf's trophic chain follows a mixing line between terrestrial and upwelled sources into the system. The del 15-N values of slope water zooplankton are lower and fall within a narrow range in agreement with the subsurface del 15-N-nitrate- source. In contrast, the del 15-N values of zooplankton from the ring suggests a longer trophic chain which increases the residence time of nitrogen in its nutrient-impoverished euphotic zone.

A mass balance model that incorporates measured 15-N of various pools shows that terrestrial fluxes are the principal sources of nitrogen to the NW Gulf of Mexico inner shelf. DON needs to be considered in the overall nitrogen balance of the region. Assuming a del 15-N of ca. 6-7 per mil for bulk DON, then less than 10 percent of the incoming nitrogen is lost to denitrification and ca. 20 percent is buried in the sediments.