Mississippi River diversions and phytoplankton dynamics in deltaic Gulf of Mexico estuaries: A review

River systems worldwide have become substantially influenced by human activities, including land use changes, river diversion operations, and flood control measures. Some of the unambiguous and best studied examples of effects of enhanced eutrophication on biotic resources can be found in Louisiana estuaries at the terminus of the Mississippi-Atchafalaya River system. The Mississippi River delta has experienced large losses of coastal wetlands due to a combination of human impacts and sea-level rise. State and Federal agencies are moving ahead with plans for building large-scale river sediment diversions, which will capture maximum sediment during spring flood pulses and direct a sediment subsidy into the eroding coastal basins. These large-scale river sediment diversions will also substantially increase freshwater and nutrient inputs and are likely to affect algal bloom formation, including harmful cyanobacterial blooms. There are concerns that discharge of river water containing high concentrations of N, P and Si may trigger algal blooms in the coastal receiving basins. River sediment diversions, as any other flood pulsing, will likely be disruptive to the coastal ecology and so balancing the benefits of slowing coastal land loss against potential negative effects on water quality remains a formidable management challenge. We review here the physical, chemical and biological factors affecting primary production in shallow coastal systems and provide known data on ecosystem response to freshwater diversions, large and small. We also discuss potential management approaches to mitigate the negative impacts of the diversions on the health and stability of the coastal food webs.