Impacts of climate change on net productivity of coastal waters: implications for carbon budgets and hypoxia

General circulation models predict that freshwater discharge from the Mississippi River (USA) to the coastal ocean would increase 20 % if atmospheric CO2 concentration doubles. Here we use a coupled physical-biological 2-box model to investigate the potential impacts of increased freshwater and nutrient inputs on the production and decay of organic matter in the coastal waters of the northern Gulf of Mexico. Model results for a doubled CO2 climate indicate that the annual net productivity of the upper water column (NP, 0 to 10 m) is likely to increase by 65 g C m(-2) yr(-1), relative to a 1985-1992 average (122 g C m(-2) yr(-1)). Interestingly, this projected increase is of the same magnitude as the one that has occurred since the 1940s due to the introduction of anthropogenic nutrients. An increase in annual NP of 32 g C m(-2) yr(-1) was observed during the Great Mississippi River Flood of 1993, thus indicating the general validity of a doubled CO2 scenario. The total oxygen uptake in the lower water column (10 to 20 m), in contrast, is likely to remain at its present value of about 200 g O-2 m(-2) yr(-1). Thus, carbon export and burial, rather than in situ respiration, are likely to be the dominant processes balancing coastal carbon budgets, leading perhaps to an expanded extent of the hypoxic zone.