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The northern Gulf of Mexico (GOM) is well known for hypoxic water conditions (O2≤2 mg L−1), and is often referred to as the “Dead Zone”. The area of the Dead Zone has increased remarkably during the recent decades due to the increased coastal eutrophication. Under such conditions, denitrification process that removes “available nitrogen” from the system would likely become more active, and it needs to be better quantified to understand the nature of biogeochemical nitrogen cycles in the northern GOM. Despite its significance, few denitrification studies have been conducted for this area. We estimate the temporal variation of denitrification rates of the bottom waters at the northern Gulf of Mexico encompassing the “Dead Zone” during July of the 1985–2007 period (except for 1988–1990). We use historic hydrographic data and the extended Optimum Multi-Parameter analysis. Denitrification rates of the bottom waters in the region have gradually decreased from 1985 to 1997, and then increased to 2007. The water mass mixing composition of bottom waters on the Texas–Louisiana inner shelf has changed since ∼1997. The Texas–Louisiana Coastal Water part has increased and that of the Subtropical Underwater has decreased. This change appears to have influenced the denitrification rates in the study area. We suggest that denitrification rates of bottom waters in the northern Gulf of Mexico are controlled not only by biogeochemical factors (i.e. organic matter supply and remineralization), but also by physical factor (i.e. stratification and relative contributions from different water masses). |
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