Long term nutrient trends and phytoplankton response in Delaware estuary, USA

Jonathan H. Sharp
Graduate College of Marine Studies
University of Delaware
Lewes DE 19958
USA.

The Delaware Estuary receives one of the highest nutrient loadings per water volume of any near shore aquatic environment in North America. Yet, curiously, this heavily urbanised, river-dominated estuary does not show many of the characteristics of eutrophication. Evaluation of consistent monitoring data sets shows very large changes in nutrient and dissolved oxygen concentrations and ratios over the past several decades and over the last century. Our own long-term sampling from the past two decades, supporting various research projects, allows linkage to the longer data sets and provides a better understanding of microbial (phytoplankton and bacterial) activity. Evaluation of dissolved oxygen data allows some indirect estimates of past history in microbial activity. Although, we have very little information about phytoplankton on an organism level, it is possible to infer some qualitative interpretations from the mass balance information. It appears that physical features and nutrient ratios in the estuary are more important in controlling eutrophication than are just nutrient loadings or concentrations.