Issues in Ecology Number al2000 Figure 2.The left panel shows the distribution of chlorc ass alona the east coas 6 of the U.S.from Boston to South Carolina as measured from the ocean color satellite SeaWIFs.Note the higher chlorophyll levels closer to shore.and the much higher lev els in enclosed bays.such as Pamlico Sound (latitude 35 and Chesapeake Bay(mouth at 37latitude).The above panel shows chlorophyll distributions within Chesapeake Bay in more detail.as measured during a phytoplankton bloom.Both images were taken in April 1998 Nutrient over-enrichment alters comm over other and alters the str ture of the phytoplank community. B nts fo ents and tra (DOM)plat n levels of N and P These water A of f unity to maintain pr oductiv affect DOM levels in est ity in the face of broad shifts in nutrient supplies general.eutrophication results in higher DOM levels and in n eases iron availability Because phytoplankton form the basis of the marine form their glasslike shells.Some silica that would otherw he flushed into estuaries is used un in nutrient-induced dia munity can have enormous consequer tom blooms upstream.As diatom production increases.silica and predators.In general.these consequences are poorly is trapped long temm in bottom sediments as diatoms die and studied.vet some outcomes are known For instance.a sink.A decline in available silica can limit growth of diatoms noted above,eutrophication can lead to a change in domi or cause a shift from heavily silicified to less silicified types of nance from diatoms towards flagellates,particularly if silica diatoms.Studies off the German coast lasting more than two is depleted from the water.Such a change can potentially decades documented a general enrichment of coastal waters degrade the food webs that support commercially valuable with N and P.along with a four-fold increase in ratios of fish species since most diatoms and other relative large forms available N and P to silica.This shift was accompanied by a of phytoplankton serve as food for the larger copepods on striking change in the composition of the phytoplankton com- which larval fish feed.The presence of small flagellates may shift the grazer community to one dominated by gelatinous organisms such as salps or jellyfish rather than finfish now as ility of logically useable orms of ir Harmful Algal Blooms Among the thousands of micr algae species cation.creating another factor that favors some algal spe in the phytoplar nkton community are a few dozen that pro 4 Issues in Ecology Number 7 Fall 2000 Nutrient over-enrichment alters community structure directly by changing competition among algal species for nutrients. Algal species have wide differences in their re￾quirements for and tolerances of nutrients and trace elements. Some species are well adapted to low-nutrient conditions while others prefer high levels of N and P. These differences allow a diverse phytoplankton community to maintain productiv￾ity in the face of broad shifts in nutrient supplies. Eutrophication alters the phytoplankton community by decreasing availability of silica, which diatoms require to form their glasslike shells. Some silica that would otherwise be flushed into estuaries is used up in nutrient-induced dia￾tom blooms upstream. As diatom production increases, silica is trapped long term in bottom sediments as diatoms die and sink. A decline in available silica can limit growth of diatoms or cause a shift from heavily silicified to less silicified types of diatoms. Studies off the German coast lasting more than two decades documented a general enrichment of coastal waters with N and P, along with a four-fold increase in ratios of available N and P to silica. This shift was accompanied by a striking change in the composition of the phytoplankton com￾munity, as diatoms decreased and flagellates increased more than ten-fold. Also, harmful blooms of colony-forming algae known as Phaeocystis became more common. The availability of biologically useable forms of iron and other essential metals also can be affected by eutrophi￾cation, creating another factor that favors some algal spe￾cies over others and alters the structure of the phytoplank￾ton community. Because iron hydroxides have extremely low solubility, organic molecules must bind with iron if it is to remain in solution in seawater. Thus, dissolved organic mat￾ter (DOM) plays a critical role in enhancing biological avail￾ability of iron in coastal waters. A variety of factors can affect DOM levels in estuaries and coastal systems, but in general, eutrophication results in higher DOM levels and in￾creases iron availability. Because phytoplankton form the basis of the marine food chain, changes in the species composition of this com￾munity can have enormous consequences for animal grazers and predators. In general, these consequences are poorly studied, yet some outcomes are known. For instance, as noted above, eutrophication can lead to a change in domi￾nance from diatoms towards flagellates, particularly if silica is depleted from the water. Such a change can potentially degrade the food webs that support commercially valuable fish species since most diatoms and other relative large forms of phytoplankton serve as food for the larger copepods on which larval fish feed. The presence of small flagellates may shift the grazer community to one dominated by gelatinous organisms such as salps or jellyfish rather than finfish. Harmful Algal Blooms Among the thousands of microscopic algae species in the phytoplankton community are a few dozen that pro￾Figure 2 - The left panel shows the distribution of chloro￾phyll an indicator of algal biomass along the east coast of the U.S. from Boston to South Carolina as measured from the ocean color satellite SeaWIFs. Note the higher chlorophyll levels closer to shore, and the much higher lev￾els in enclosed bays, such as Pamlico Sound (latitude 35o ) and Chesapeake Bay (mouth at 37 o latitude). The above panel shows chlorophyll distributions within Chesapeake Bay in more detail, as measured during a phytoplankton bloom. Both images were taken in April 1998.