Contiguous Low Oxygen Waters between the Continental Shelf Hypoxia Zone and Nearshore Coastal Waters of Louisiana, USA: Interpreting 30 Years of Profiling Data and Three-Dimensional Ecosystem Modeling.

The multidecadal expansion of northern Gulf of Mexico continental shelf hypoxia is a striking example of the adverse effects of anthropogenic nutrient enrichment on coastal oceans. Increased nutrient inputs and widespread shelf hypoxia have resulted in numerous dissolved oxygen (DO) water quality problems in nearshore coastal waters of Louisiana. A large hydrographic dataset compiled from research programs spanning 30 years and the three-dimensional hydrodynamic-biogeochemical model CGEM (Coastal Generalized Ecosystem Model) were integrated to explore the interconnections of low DO waters across the continental shelf to nearshore coastal waters of Louisiana. Cross-shelf vertical profiles showed contiguous low DO bottom waters extending from the shelf to coastal waters nearly every year in the 30+ year time series, which were concurrent with strong cross-shelf pycnoclines. A threshold Brunt-Väisälä frequency of 40 cycles h was critical to maintaining the cross-shelf subpycnocline layers and facilitating the formation of a contiguous low DO water mass. Field observations and model simulations identified periods of wind-driven bottom water upwelling lasting between several days to several weeks, resulting in both physical advection of oxygen-depleted offshore waters to the nearshore and enhanced nearshore stratification. Both the upwelling of low DO bottom waters and in situ respiration were of sufficient temporal and spatial extent to drive DO below Louisiana's DO water quality criteria. Basin-wide nutrient management strategies aimed at reducing nutrient inputs and shelf hypoxia remain essential to improving the nearshore coastal water quality across the northern Gulf of Mexico.