son 1989, Smayda 1989, Wells et al. 1991), and blooms of cyanobacteria have become more prevalent in the less saline portions of the Chesapeake Bay (D'Elia 1987) and in the Baltic Sea and related waters over the past 10 to 20 years (Smayda 1989 and references therein). Many of the new toxic phytoplankton blooms are subpopulations of previously non-toxic species, which now occur at previously unseen abundances (Smayda 1989, 1992). Brown-tide blooms of Alureococcus anophagefferens were unknown before 1985 (Sieburth et al. 1988). As discussed below, the cause(s) of increased nuisance blooms is not known, but evidence points toward the importance of increased nutrient inputs (nitrogen, phosphorus, iron) to estuaries and coastal seas.
Nutrients are elements essential for plant growth, such as nitrogen, phosphorus, silica, and sulfur. Phytoplankton production in most coastal marine ecosystems and estuaries is nutrient limited, and increased nutrient inputs lead to higher production and eutrophication (Ryther and Dunstan 1971; Graneli 1978, 1981, 1984; McComb et al. 1981; Boynton et al. 1982; Nixon and Pilson 1983; Smith 1984; Valiela 1984; D'Elia et al. 1986; Nixon et al. 1986; D'Elia 1987; Howarth 1988; Andersen et al. 1991). Unfortunately, the discussion of nutrient limitation in coastal marine waters has been surrounded by some confusion, in part because the term can have many different meanings and is often used quite loosely (Howarth 1988) and in part because of potential methodological problems in determining nutrient limitation (Hecky and Kilham 1988, Howarth 1988, Banse 1990). If one is concerned with eutrophication, then the appropriate definition of nutrient limitation is the regulation of the potential rate of net primary production by phytoplankton (Howarth 1988). Net primary production is defined as the total amount of photosynthesis minus the amount of plant respiration occurring in a given area (or volume) of water in a given amount of time. If an addition of nutrients would increase the rate of net primary production, even if this means a complete change in the species composition of the phytoplankton, then production is considered to be nutrient limited (Howarth 1988, Vitousek and Howarth 1991).
Factors other than nutrient input can also influence or partially control primary production. For instance, phytoplankton production in some estuaries (such as the Hudson River) is limited by light availability. This tends to occur in extremely turbid estuaries or in estuaries where moderate turbidity coexists with deep mixing of the water. The turbidity can result both