MODELING ESTUARINE EUTROPHICATION IN THE CONTEXT OF HYPOXIA, NITROGENLOADINGS, 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, wit h the degree of the phytoplankton accumulation and subsequent decompos ition corresponding to the phytoplankton's density in the overlying wa ter: In general, high phytoplankton densities occur when nutrient avai lability is high enough to maintain the phytoplankton population. High nutrient availabilities generally result from high nutrient loads ent ering estuaries from their watersheds. Therefore, the cc-occurrence of stratified bottom-waters and high nutrient loadings predisposes estua ries to developing hypoxic bottom waters. This paper presents a multin ominal logistic regression model that predicts the status of estuaries as normoxic, borderline hypoxic, hypoxic and severely hypoxic based o n nitrogen loadings, salinity stratifications and 'phosphorus to nitro gen' load ratios. The model was applied to 10 US Gulf of Mexico estuar ies and it correctly assigned eight of the 10 estuaries to their corre ct eutrophication status. Summaries of each estuary's probabilities fo r being normoxic, borderline hypoxic, hypoxic or severely hypoxic are presented along with their modelled responses to changes in nutrient l oadings. Other model results include nutrient loading reductions assoc iated with moving hypoxic and severely hypoxic estuaries into the norm oxic and borderline status. This estuarine eutrophication model follow s previously developed eutrophication models for lakes which were base d on similar premises and have been successfully used to develop manag ement policies. The estuarine eutrophication modelling approach provid es a platform for evaluating nutrient reduction management strategies in the context of an estuary's modelled response to proposed nutrient reductions. Therefore, the modelling approach presented here is offere d in support of developing management policies concerning estuarine eu trophication. (C) 1998 Academic Press Limited.