inputs will also vary depending on the location of the input. Seasonal variation in agricultural activity (such as fertilizer application, fossil fuel combustion, or precipitation) results in changes in nutrient loads that a recovery body may see during the year. Furthermore, variation in the load contributed by one of many tributaries may also vary, resulting in seasonal and geographic variability.
Allochthonous organic matter inputs. Organic matter contributes directly to eutrophication. The relative magnitude of inorganic versus organic nitrogen load influences the balance between autotrophic and heterotrophic metabolism (Hopkinson and Vallino 1995). The relative magnitude of dissolved versus particulate organic matter loads influences residence time of inputs, as particles are preferentially trapped by processes operating in the estuarine turbidity maximum and by gravity. The carbon:nitrogen stoichiometry of organic matter remineralized by the benthos and denitrification further influence the balance between autotrophic and heterotrophic processes in estuaries. Algal blooms are an example of an autotrophic process and net oxygen uptake is an example of a heterotrophic process.
Recognizing that there are basic differences in estuarine susceptibility to nutrient over-enrichment, the development of a conceptual framework or classification scheme for organizing how to think about susceptibility will enable scientists and managers to better understand and predict the effects of human activities on estuarine and coastal ecosystems, and thus to more effectively manage human activities. Variations in the 12 factors discussed above result in different responses. Hence, systematic variation in these 12 factors can result in systematic responses. Thus, coupling an understanding of how these factors vary from estuary to estuary of known load-response behavior can lead to a predictive framework or classification scheme.
It is not adequate to understand eutrophication and other processes in a few, well-studied coastal systems. Useful understanding will require a systematic means of extending the results from one coastal system to others that have not been studied extensively. There are too many estuaries in North America to carry out comprehensive ecosystem studies of all those undergoing nutrient enrichment, and scientists and managers do not presently have a broad enough understanding of estuarine and coastal processes to choose representative systems for detailed analysis. Development of a scheme for classification of estuarine and coastal systems is a prerequisite to understanding and reducing the effects of nutrient over-