activities (Karr and Chu 1997). Coastal waters are particularly complex because different types of estuaries, embayments, and shelf systems differ in their responses to nutrient enrichment. This means that varying levels of nutrient input can cause very different responses in different systems.

This chapter examines classification schemes that could enhance attempts to understand, predict, and manage eutrophication and other impacts of nutrient over-enrichment in the nation’s coastal waters. It reviews existing and developing estuarine classifications. While the emphasis is on estuaries, where impacts of nutrient loading are most acute, the approach is equally appropriate for coastal systems in general, including those in the nearshore region of the continental shelf.

The diverse physical settings (defined by a number of parameters including geology, soil type, climatic setting, and topology) of estuaries and their watersheds give rise to different types of estuarine systems. While there are numerous similarities between all estuaries, there are also some basic differences. For example, both a drowned river valley estuary, such as Chesapeake Bay, and a bar-built estuary, such as Plum Island Sound in northeastern Massachusetts, have temporal and spatial patterns of salinity that reflect seasonal variations in freshwater discharge. However, while Chesapeake Bay is a deep-water, plankton-dominated system where waters have a long residence time, Plum Island Sound is a shallow, emergent, marsh-dominated system where waters have a short residence time. The expected quantitative values for indicators of ecological health or for susceptibility to nutrient over-enrichment are not the same for these two systems, even though many of the same biological or ecological attributes may work as indicators in these disparate situations. Knowledge of the physical setting and the undisturbed ecosystem condition must underpin any monitoring and management effort to restore a coastal system impaired by nutrient over-enrichment.

MAJOR FACTORS INFLUENCING ESTUARINE SUSCEPTIBILITY TO NUTRIENT OVER-ENRICHMENT

Certain key characteristics appear to be of primary importance in determining estuarine response to nutrient enrichment. These factors range from biotic factors to physical setting to hydrodynamic regime. Twelve of the most important factors are:

  1. Physiographic setting. Characterization of the physiographic setting could include a geomorphic descriptor of an estuary (e.g., inverted continental shelf estuary like the Mississippi River plume, coastal embayment, and drowned river valley), a descriptor of the



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