Eutrophication rating systems

Estimation of the effects of N deposition on aquatic systems is made difficult by the large variety of forms of N found in air, deposition, watersheds, and surface waters, as well as by the myriad pathways through which N can be cycled in terrestrial and aquatic ecosystems. These complexities separate N deposition from its effects and reduce our ability to attribute known aquatic effects to known rates of N deposition. The organization of this chapter reflects this complexity. Because an understanding of the ways that N is cycled through watersheds is critical to our understanding of N effects, I begin with a brief description of the N cycle and of the transformations of N that may occur in watersheds. I then discuss the two most likely effects of N deposition (acidification and eutrophication). 

From the temporal scale of adverse effects we come to a consideration of recovery. Recovery is the rate and extent of return of a population or community to a condition that existed before the introduction of a stressor. Because ecosystems are dynamic and even under natural conditions are constantly changing in response to changes in the physical environment (weather, natural catastrophes, etc.) or other factors, it is unrealistic to expect that a system will remain static at some level or return to exactly the same state that it was before it was disturbed. Thus the attributes of a recovered system must be carefully defined. Examples might include productivity declines in an eutrophic system, re-establishment of a species at a particular density, species recolonization of a damaged habitat, or the restoration of health of diseased organisms. 

Improving the efficiency of metals removal using wetlands has proved to be difficult. Detailed assessments of long-term performance of wetland systems with a focus on metal-retention mechanisms, eutrophication, and flushing rates, and their effects on downgradient ecosystems are required if wetland treatment is to be applied in temperate regions. 

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