tion is largely due to excess inputs of phosphorus. Also discussed in this chapter are other elements—particularly silicon and iron—that may be important in regulating harmful algal blooms in coastal waters and in determining some of the consequences of eutrophication.
After extensive study in the early 1970s, a consensus developed that phosphorus was the nutrient most responsible for nutrient over-enrichment in freshwater lakes (Edmondson 1970; Vollenweider 1976; Schindler 1977), and since then better control of phosphorus loadings to lakes has gone a long way toward mitigating freshwater eutrophication (Carpenter et al. 1998). In contrast, research indicates that in numerous estuaries and coastal marine ecosystems (at least in the temperate zone) nitrogen generally is more limiting to primary production by phytoplankton, and nitrogen inputs are more likely to accelerate eutrophication (Howarth 1988; Vitousek and Howarth 1991; Nixon 1995; Paerl 1997). Note that the concept of “nutrient limitation” is often poorly defined and used rather loosely; the committee follows the definition of control of the potential rate of primary production, allowing for potential changes in the composition of the ecosystem (Howarth 1988). Thus, a nutrient is limiting if its addition to the system increases the rate of net primary production.
There are exceptions to the generality that nitrogen is limiting in coastal ecosystems. For instance, certain temperate estuaries, such as the Apalachicola on the Gulf coast of Florida and several estuaries on the coast of the Netherlands in the North Sea appear to be phosphorus limited (Myers and Iverson 1981; Postma 1985; Brockman et al. 1990). In the case of the North Sea estuaries, phosphorus limitation is probably the result of extremely high nitrogen inputs combined with fairly stringent control of phosphorus inputs (Howarth et al. 1995, 1996). In the case of the Apalachicola, phosphorus limitation results from a relatively high ratio of nitrogen to phosphorus in nutrient inputs, although in this case the high ratio may reflect the relatively small amount of human disturbance in the watershed and the relatively low nutrient inputs overall (Howarth 1988; Billen et al. 1991).
For nearshore tropical marine systems, it is commonly believed that phosphorus is more limiting of primary production (Howarth et al. 1995). This is probably true for many tropical lagoons with carbonate sands that are relatively unaffected by human activity (Smith and Atkinson 1984; Short et al. 1990). However, such lagoons may move toward nitrogen limitation as they become eutrophic (McGlathery et al. 1994; Jensen et al. 1998). Also, even oligotrophic tropical seas may be nitrogen limited away