improved sewage treatment in the East River in New York City with resulting increase of flows of nitrogen from the East River into the Sound (Parker and O'Reilly 1991).
In addition to anoxia and hypoxia, eutrophication can lead to the dieback of seagrass beds, which are important habitats and nursery grounds for a variety of fish and other animals. One mechanism for such dieback is a shading out of the grasses by the abundant phytoplankton in the overlying water, a process thought to have caused the dieback of macrophytes in the upper portions of the Chesapeake Bay (Kemp et al. 1983, Twilley et al. 1985, D'Elia 1987), in the Dutch Wadden Sea (Gieson et al. 1990), and of both tropical and temperate seagrasses in Australia (Kirkman 1976, Cambridge and McComb 1984, Cambridge et al. 1986). Dieback caused by such shading usually manifests itself in a rather gradual loss of the seagrasses (Robblee et al. 1991), although the occurrence of unusual nuisance algal blooms in 1985 and 1986 greatly reduced the abundance of seagrass beds near Long Island (Dennison et al. 1989). Shading by enhanced epiphytic (Twilley et al. 1985) and macroalgal (Valiela et al. 1990) growth may be an additional cause of seagrass dieback. Nutrient enrichment may also have a direct physiological response on seagrasses, with internal nutrient imbalances appearing to lead to reduced survival (Burkholder et al. 1992a).
Beds of attached macro-algae on bottom sediments or rocks can also be adversely affected by eutrophication. Nutrient enrichment of rocky intertidal areas typically leads to a reduction in the overall diversity of both attached algae (Borowitzka 1972, Littler and Murray 1978) and associated animals (Gappa et al. 1990). These nutrient-enriched areas tend to be dominated by opportunistic algae with rapid growth rates, such as Cladophora sp. and Enteromorpha sp., which can take advantage of the elevated nutrient levels and shade out other species (Littler and Murray 1975, 1978). This phenomenon is seen clearly along the Swedish coast of the Baltic Sea, where since the mid-1970s nuisance forms of filamentous algae (Cladophora and Enteromorpha species) have become more dominant, coinciding with a decline of the former dominant bladderwrack algae, Fucus sp. (Baden et al. 1990, Rosenberg et al. 1990). The bladderwrack is used by herring in spawning, and the change to dominance by filamentous macroalgae has led to decreased hatching of herring eggs (Rosenberg et al. 1990). Subtidal forests of giant kelp (Macrocystis pyrifera) died back and failed to reproduce during the 1960s and early 1970s along the Palos Verdes Peninsula near the Los Angeles County outfalls (Wilson et al. 1980). This may have been due to light limitation, perhaps a result of eutrophication, but was more likely due directly to solids discharged in poorly treated sewage. Toxic