Issues in Ecology Number 7 Fall 2000 cates that in numerous estuaries and coastal marine ecosys- stood.For example,much of the Caribbean Sea away from tems-at least in the temperate zone-N is generally the immediate shorelines appears to be N limited. more limiting to phytoplankton growth than P and N inputs The identity of the nutrient that limits plant produc- are more likely to accelerate eutrophication.A "limiting" tion switches seasonally between N and P in some major nutrient is the essential plant nutrient in shortest supply rela- estuaries such as Chesapeake Bay and portions of the Gulf of tive to the needs of algae and plants,and adding it increases Mexico,including the"dead zone."Runoff in these systems the rate of primary production. is highest in the spring,and at that time the N:P ratio of the There are exceptions to the generalization that N is runoff determines which nutrient is limiting.In the summer, limiting in coastal ecosystems.For instance,certain temper- when runoff drops sharply,however,processes that occur in ate estuaries such as the Apalachicola on the Gulf coast of the sediment such as P adsorption and the bacterial break- Florida and several estuaries on the North Sea coast of the down of N compounds (denitrification)play a more impor- Netherlands appear to be P limited.In the case of the North tant role in determining which nutrient is in shortest supply. Sea estuaries,P limitation most likely results from stringent Even in these systems,N is probably responsible for controls the Dutch government has imposed on P releases, the major harmful impacts of eutrophication.When waters combined with high and largely unregulated human N in- are N limited,the algal community is dominated by diatoms, puts.In contrast,the high ratio of N to P in nutrient inflows which tend to sink to the bottom,spurring decomposition to the Apalachicola may reflect the relatively small amount processes that use up dissolved oxygen and create hypoxia. of human disturbance in the watershed and relatively low In contrast,when primary production is P limited in these nutrient inputs overall. systems,the phytoplankton are dominated by smaller or In tropical coastal systems with carbonate sands and lighter algal species and relatively little sinks to the bottom. little human activity,P is generally limiting to primary pro- duction because the sand readily adsorbs phosphate,trap- Euidence for Control of Coastal Eutrophication by Nitrogen ping it in the sediment and leaving it largely unavailable to Both researchers and policymakers have been slower organisms.However,such lagoons may move toward N limi- to accept the need for tighter restrictions on N inputs than tation as they become eutrophic.The primary reason is that to acknowledge the need for P control to manage eutrophi- as more nutrients enter these waters,the rate at which sedi- cation in freshwater systems.Many coastal marine scien- ments adsorb phosphate slows and a greater proportion of tists recognized the N problem decades ago,yet the need for the P remains biologically available. controls on N inputs was hotly debated throughout the 1980s. Even nutrient-poor tropical seas may be N limited Only since the 1990s,when results from large-scale enrich- away from shore,although the reasons are poorly under- ment studies in three estuaries were published,has the need 8000 800 7000 Nitrogen Exceptional 700 Phosphorus plankton blooms 6000 600 Filamentous green algae 5000 500 4000 400 Early indications of eutrophication 3000 300 2000 200 1000 100 0 0 1950 1960 1970 1980 1990 Year Figure 5-The chart shows inputs of nitrogen and phosphorus to Laholm Bay on the coast of Sweden from 1950 to 1988. Note that P inputs decreased after 1970 due to control efforts,while inputs of N increased.Eutrophication first became apparent in the bay in 1970 and became much worse in the subsequent two decades,clearly indicating that N caused the eutrophication(modified from Rosenberg et al.1990,as printed in NRC 2000).7 Issues in Ecology Number 7 Fall 2000 cates that in numerous estuaries and coastal marine ecosys￾tems at least in the temperate zone N is generally more limiting to phytoplankton growth than P, and N inputs are more likely to accelerate eutrophication. A limiting nutrient is the essential plant nutrient in shortest supply rela￾tive to the needs of algae and plants, and adding it increases the rate of primary production. There are exceptions to the generalization that N is limiting in coastal ecosystems. For instance, certain temper￾ate estuaries such as the Apalachicola on the Gulf coast of Florida and several estuaries on the North Sea coast of the Netherlands appear to be P limited. In the case of the North Sea estuaries, P limitation most likely results from stringent controls the Dutch government has imposed on P releases, combined with high and largely unregulated human N in￾puts. In contrast, the high ratio of N to P in nutrient inflows to the Apalachicola may reflect the relatively small amount of human disturbance in the watershed and relatively low nutrient inputs overall. In tropical coastal systems with carbonate sands and little human activity, P is generally limiting to primary pro￾duction because the sand readily adsorbs phosphate, trap￾ping it in the sediment and leaving it largely unavailable to organisms. However, such lagoons may move toward N limi￾tation as they become eutrophic. The primary reason is that as more nutrients enter these waters, the rate at which sedi￾ments adsorb phosphate slows and a greater proportion of the P remains biologically available. Even nutrient-poor tropical seas may be N limited away from shore, although the reasons are poorly under￾stood. For example, much of the Caribbean Sea away from the immediate shorelines appears to be N limited. The identity of the nutrient that limits plant produc￾tion switches seasonally between N and P in some major estuaries such as Chesapeake Bay and portions of the Gulf of Mexico, including the dead zone. Runoff in these systems is highest in the spring, and at that time the N:P ratio of the runoff determines which nutrient is limiting. In the summer, when runoff drops sharply, however, processes that occur in the sediment such as P adsorption and the bacterial break￾down of N compounds (denitrification) play a more impor￾tant role in determining which nutrient is in shortest supply. Even in these systems, N is probably responsible for the major harmful impacts of eutrophication. When waters are N limited, the algal community is dominated by diatoms, which tend to sink to the bottom, spurring decomposition processes that use up dissolved oxygen and create hypoxia. In contrast, when primary production is P limited in these systems, the phytoplankton are dominated by smaller or lighter algal species and relatively little sinks to the bottom. Evidence for Control of Coastal Eutrophication by Nitrogen Both researchers and policymakers have been slower to accept the need for tighter restrictions on N inputs than to acknowledge the need for P control to manage eutrophi￾cation in freshwater systems. Many coastal marine scien￾tists recognized the N problem decades ago, yet the need for controls on N inputs was hotly debated throughout the 1980s. Only since the 1990s, when results from large-scale enrich￾ment studies in three estuaries were published, has the need Figure 5 - The chart shows inputs of nitrogen and phosphorus to Laholm Bay on the coast of Sweden from 1950 to 1988. Note that P inputs decreased after 1970 due to control efforts, while inputs of N increased. Eutrophication first became apparent in the bay in 1970 and became much worse in the subsequent two decades, clearly indicating that N caused the eutrophication (modified from Rosenberg et al. 1990, as printed in NRC 2000).