Modelling estuarine eutrophication in the context of hypoxia, nitrogen loadings, stratification and nutrient ratios

Estuarine eutrophication is a serious ecological and economic problem. Hypoxic waters resulting from decomposing phytoplankton in stratified bottom-waters are often associated with estuarine eutrophication, with the degree of the phytoplankton accumulation and subsequent decomposition corresponding to the phytoplanktonʹs density in the overlying water. In general, high phytoplankton densities occur when nutrient availability is high enough to maintain the phytoplankton population. High nutrient availabilities generally result from high nutrient loads entering estuaries from their watersheds. Therefore, the co-occurrence of stratified bottom-waters and high nutrient loadings predisposes estuaries to developing hypoxic bottom-waters. This paper presents a multinominal logistic regression model that predicts the status of estuaries as normoxic, borderline hypoxic, hypoxic and severely hypoxic based on nitrogen loadings, salinity stratifications and «phosphorus to nitrogen» load ratios. The model was applied to 10 US Gulf of Mexico estuaries and it correctly assigned eight of the 10 estuaries to their correct eutrophication status. Summaries of each estuaryʹs probabilities for being normoxic, borderline hypoxic, hypoxic or severely hypoxic are presented along with their modelled responses to changes in nutrient loadings. Other model results include nutrient loading reductions associated with moving hypoxic and severely hypoxic estuaries into the normoxic and borderline status. This estuarine eutrophication model follows previously developed eutrophication models for lakes which were based on similar premises and have been successfully used to develop management policies. The estuarine eutrophication modelling approach provides a platform for evaluating nutrient reduction management strategies in the context of an estuaryʹs modelled response to proposed nutrient reductions. Therefore, the modelling approach presented here is offered in support of developing management policies concerning estuarine eutrophication.