Spatial and Temporal Assessment of Factors Controlling Denitrification in Coastal and Continental Shelf Sediments of the Gulf of Mexico

Denitrification in coastal sediments is a process of significant scientific interest and a major concern in coastal management. Reliable estimates of denitrification in coastal ecosystems are unavailable due in large part to a lack of understanding of the factors affecting spatial and temporal variability. The following studies are put forward as an attempt to help fill that gap in our understanding of this critical environmental process. These studies were conducted to test the hypotheses that significant spatial and temporal variability exists in the denitrification potential of coastal and continental shelf sediments of the Gulf of Mexico and that this variability is governed by environmental parameters including dissolved inorganic nitrogen, sediment organic composition, sediment porosity, bottom water dissolved oxygen concentration, sediment nitrification, and water temperature. Studies were conducted in two systems representing end member states in terms of anthropogenic nutrient loading. Apalachicola Bay, FL is relatively pristine estuary. The Louisiana Continental Shelf (LCS) is subject to the discharge of the Mississippi River and is the location of the largest area anthropogenic bottom water hypoxia in the Western Hemisphere. Sediment porosity was found to be the single most important factor relating to the spatial variability of denitrification potential. Sediment organic composition was closely correlated with denitrification potential in Apalachicola Bay but not on the Louisiana Continental Shelf. The effect of porosity on denitrification potential does not appear to be a function of sediment organic composition and may be due to the catalytic enhancement of microbial metabolism by mineral surfaces. Denitrification potential was generally higher on the Louisiana Continental Shelf than in Apalachicola Bay, FL. Seasonal variability in denitrification was observed both in Apalachicola Bay and on the LCS with peak rates occurring in the summer months. Significant inter annual variability in denitrification potential and its relationship to nitrification potential were observed on the LCS. These inter-annual differences are likely the result of the higher rate of nitrogen delivery from the Mississippi River in 2001 as compared to 2000. Higher nutrient loading in 2001 reduced the dependence of denitrifiers on nitrifiers allowing the two processes to become uncoupled. Thus, it appears that variability in the climactic factors that drive nutrient loading regulates the inter-annual variability in peak summer denitrification while seasonal factors such as temperature and primary productivity drive shorter-term seasonal changes in nitrogen cycling. Alternatively, contemporaneous spatial variability in denitrification potential seems to be driven by differences in sediment porosity that control bacterial abundance through the enhanced catalytic activity of mineral surfaces. Use of the acetylene block method for determining denitrification potential is discussed, including an assessment of the legitimate uses of this metabolic inhibitor as well as a discussion of the interpretation of such data.