from suspension of inorganic particles and from high phytoplankton biomass, and so light limitation often is a result of self-shading by the phytoplankton, as is the case in many lakes (Wetzel 1983). In estuaries where nutrient inputs are high and production is limited by light, the nutrients are simply transported further away from the source before being first assimilated by phytoplankton, as is seen in the transport of nutrients from the Hudson River and New York Harbor into the New York Bight (Malone 1982). This further transport may or may not provide sufficient dilution to avoid excessive eutrophication, which may just occur further afield from the nutrient source.
Through their grazing on phytoplankton, zooplankton and other animals can also influence the rate of primary production and the biomass of phytoplankton. This has received extensive study and discussion in freshwater ecosystems (Carpenter et al. 1985, Morin et al. 1991) and in offshore ocean ecosystems (Steele 1974, Banse 1990) although it is virtually unstudied in estuaries and coastal seas. Nonetheless, changes in grazing in estuaries may have serious effects on water quality. For instance, some researchers believe that at one time, oyster populations in the Chesapeake Bay were sufficiently high to filter the bay's entire water volume on average once every week; the currently declined oyster populations probably filter the water of the bay only once per year on average (Newell 1988). This lower grazing pressure on phytoplankton populations may be contributing to eutrophication of the Chesapeake. However, reductions in grazing pressure can result in algal blooms only where nutrient availabilities are high. Thus, nutrient supply should be viewed as the cause of eutrophication with grazing pressures being a secondary regulator.
Nitrogen is the element usually limiting to primary production by phytoplankton in most estuaries and coastal seas of the temperate zone (Ryther and Dunstan 1971; Vince and Valiela 1973; Smayda 1974; Norin 1977; Graneli 1978, 1981, 1984; Boynton et al. 1982; Nixon and Pilson 1983; Valiela 1984; D'Elia et al. 1986; Nixon et al. 1986; Frithsen et al. 1988; Howarth 1988; Rydberg et al. 1990; Vitousek and Howarth 1991; Nixon 1992), although some temperate estuaries such as the Apalachicola in the Gulf of Mexico may be phosphorus limited (Myers and Iverson 1981; Howarth 1988) and others such as parts of the Chesapeake Bay and the Baltic Sea may switch seasonally between nitrogen and phosphorus limitation (McComb et al. 1981, D'Elia et al. 1986, Graneli et al. 1990, Andersen et al. 1991). Many tropical estuarine lagoons may be phosphorus limited as well (Smith 1984, Smith and Atkinson 1984, Howarth 1988, Vitousek and Howarth 1991). Sewage often accounts for 50 percent or more of the nitrogen inputs to