《森林生态学》课程教学资源（生态学热点）Issues in ecology - 07 海湾海岸污染 Nutrient Pollution of Coastal Rivers, Bays, and Seas

Nutrient Pollution of Coastal Rivers,Bays,and Seas Abojoo u!sanssI

Published by the Ecological Society of America Number 7, Fall 2000 Nutrient Pollution of Coastal Rivers, Bays, and Seas Issues in Ecology

Issues in Ecology Number 7 Fall 2000 Nutrient Pollution of Coastal Rivers,Bays,and Seas SUMMARY Over the past 40 years,antipollution laws have greatly reduced discharges of toxic substances into our coastal waters.This effort,however,has focused largely on point-source pollution of industrial and municipal effluent.No comparable effort has been made to restrict the input of nitrogen (N)from municipal efluent.nor to control the flows of N and phosphorus (P)that enter waterways from dispersed or nonpoint sources such as agricultural and urban runoffor as airborne pollutants.As a result,inputs of nonpoint pollutants,particularly N.have increased dramatically.Nonpoint pollution from N and P now represents the largest pollution problem facing the vital coastal waters of the United States. Nupolli is the cm thread that links an array of problems along the nation'sincuding eutrophication.harmful algal blooms."dead zones."fish kills.some shellfish poisonings,loss of seagrass and kelp beds I state rately to severely de the National R coastal systems generally changes that decrease the biologi While moderate N enr nt of some castal waters may increase fish production over-erichment generally valuable fish The marked increase has be ompanied by an increase in harmful algal blooms,and in at least some High nutrient levels and the cha ality and the makeup of the algal community are detrimental to the health of coral reefs and the diversity of animal life supported by seagrass and kelp communi- ties Research during the past decade confirms that N is the chief culprit in eutrophication and other impacts of nutrient over-enrichment in temperate coastal waters,while P is most problematic in eutrophication of freshwa- ter lakes. Human conversion of atmospheric N into biologically useable forms.principally synthetic inorganic fertilizers. now matches the natural rate of biological N fixation from all the land surfaces of the earth. Both agriculture and the burning of fossil fuels contribute significantly to nonpoint flows of N to coastal waters. either as direct runoff or airbome pollutants. N from animal wastes that leaks directly to surface waters or is volatilized to the atmosphere as ammonia may be the largest single source of N that moves from agricultural operations into coastal waters. nshgentneNrthat cr pipon then an urgent need som N comple 孕 ll as inc e over-te Peter Franks,courtesy Scripps Institute:couresy Florida Department of Environmental Protection

1 Issues in Ecology Number 7 Fall 2000 Over the past 40 years, antipollution laws have greatly reduced discharges of toxic substances into our coastal waters. This effort, however, has focused largely on point-source pollution of industrial and municipal effluent. No comparable effort has been made to restrict the input of nitrogen (N) from municipal effluent, nor to control the flows of N and phosphorus (P) that enter waterways from dispersed or nonpoint sources such as agricultural and urban runoff or as airborne pollutants. As a result, inputs of nonpoint pollutants, particularly N, have increased dramatically. Nonpoint pollution from N and P now represents the largest pollution problem facing the vital coastal waters of the United States. Nutrient pollution is the common thread that links an array of problems along the nation’s coastline, including eutrophication, harmful algal blooms, dead zones, fish kills, some shellfish poisonings, loss of seagrass and kelp beds, some coral reef destruction, and even some marine mammal and seabird deaths. More than 60 percent of our coastal rivers and bays in every coastal state of the continental United States are moderately to severely degraded by nutrient pollution. This degradation is particularly severe in the mid Atlantic states, in the southeast, and in the Gulf of Mexico. A recent report from the National Research Council entitled Clean Coastal Waters: Understanding and Reduc￾ing the Effects of Nutrient Pollution concludes that: l Nutrient over-enrichment of coastal ecosystems generally triggers ecological changes that decrease the biologi￾cal diversity of bays and estuaries. l While moderate N enrichment of some coastal waters may increase fish production, over-enrichment generally degrades the marine food web that supports commercially valuable fish. l The marked increase in nutrient pollution of coastal waters has been accompanied by an increase in harmful algal blooms, and in at least some cases, pollution has triggered these blooms. l High nutrient levels and the changes they cause in water quality and the makeup of the algal community are detrimental to the health of coral reefs and the diversity of animal life supported by seagrass and kelp communi￾ties. l Research during the past decade confirms that N is the chief culprit in eutrophication and other impacts of nutrient over-enrichment in temperate coastal waters, while P is most problematic in eutrophication of freshwa￾ter lakes. l Human conversion of atmospheric N into biologically useable forms, principally synthetic inorganic fertilizers, now matches the natural rate of biological N fixation from all the land surfaces of the earth. l Both agriculture and the burning of fossil fuels contribute significantly to nonpoint flows of N to coastal waters, either as direct runoff or airborne pollutants. l N from animal wastes that leaks directly to surface waters or is volatilized to the atmosphere as ammonia may be the largest single source of N that moves from agricultural operations into coastal waters. The National Research Council report recommended that, as a minimum goal, the nation should work to reverse nutrient pollution in 10 percent of its degraded coastal systems by 2010 and 25 percent of them by 2020. Also, action should be taken to assure that the 40 percent of coastal areas now ranked as healthy do not develop symptoms of nutrient pollution. Meeting these goals will require an array of strategies and approaches tailored to specific regions and coastal ecosystems. There is an urgent need for development and testing of techniques that can reliably pinpoint the sources of N pollutants to an estuary. For some coastal systems, N removal during treatment of human sewage may be sufficient to reverse nutrient pollution. For most coastal systems, however, the solutions will be more complex and may involve controls on N compounds emitted during fossil fuel combustion as well as incentives to reduce over-fertilization of agricul￾tural fields and nutrient pollution from animal wastes in livestock feedlot operations. Nutrient Pollution of Coastal Rivers, Bays, and Seas SUMMARY Cover photo credits, clockwise from top: Peter Franks, courtesy Scripps Institute; courtesy Florida Department of Environmental Protection; Michael Bo Rasmussen; and Nancy Rabalais, courtesy NOAA.

Issues in Ecology Number 7 al12000 Nutrient Pollution of Coastal Rivers,Bays,and Seas by Robert Howarth,Donald Anderson.James Cloem.Chris Elfring.Charles Hopkinson. Brian Lapointe,Tom Malone.Nancy Marcus,Karen McGlathery.Andrew Sharpley.and Dan Walker INTRODUCTION This article summarize es the ecological damage c b nutrient po sys why N i Antipolution laws enacted and enforced over the s have incr of the United States.While the ghting this effort has oreatly reduced point-source pollution of to materials.oxvgen-consuming organic materials (BOD).and e 0% Alte to some exter phosphorus(P)from industrial and municipa 199 effluent pipes.no comparable attempt has been made to re. and nd Nitrogen strict the input of nitrogen(N)from municipal effluent.nor to control the flows of N and p that enter waterways from ECOLOGICAL DAMAGE dispersed or nonpoint sources such as agricultural and urban FROM NUTRIENT POLLUTION runoff or windbomne deposits.As a consequence.inputs of nonpoint pollutants.particularly N.have increased dramati- Nutrient over-enrichment has a range of effects on cally.Today.pollution from the nutrients N and P represents s on ecologica the largest source of degradation in coastal waters.which sity-the variety include some of the richest and most productive habitats in in the ecosvstem the oceans.Roughly half of the global fishe es catch occurs Fertilizino lakes.rivers.or coastal waters with pre in or is dependent upon coastal waters of the world. viously scarce nutrients such as N or P usually boosts the Nutrient pollution is also called nutrient over- enrich primary productivity of these systems-that is,the produc bo P are vital to plant growth. tion of algae (phytoplankton)that forms the base of the range plag uing nea aquatic food web(Figure 2).This excessive.nutrient-induced increase in the production of organic matter is called eutrophi cation.and eutrophication is linked to a number of problems the en in aquatic ecosystems. As the mass of algae in the water L grows,the water may become murkier:and particularly as the the algae die and decompose.periods of oxygen depletion Atlantic (hypoxi oxia)occur mor ng al ate 8a can c xyge uts nd p radation of coasta night changes i utr has confirmed that N over others and on, tems. De communiti lating water quality. brown tid because of public cor n water ula ing N inputs to aquatic systems severely under-regulated. 351mp9 suhtle cha s in the pla nity and The National Academies'National Research Council othe cological factors wth and (NRC)recently reviewed the causes and consequences of this t of fish ecies and lo red fisher neglected pollution problem in a report entitled “ean Coral reefs and suh ed nlant co Coastal Waters:Understanding and Reducing the Effects of beds can be harmed by loss of light from reduced water clar Nutrient Pollution."All of the authors of this article partici ity.or from nutrient-induced gro ths of nuisance seaw eeds pated as members,staff,or invited experts- in the work Some coastal ecosystems are more susceptible to of the NRC Committee on Causes and Management of Coastal nutrient over-enrichment than others because a host of addi Eutrophication and contributed to the NRC report.This ar- tional factors can influence the extent of plant productivity ticle is intended to bring the findings and recommendations These factors include how much light is available.how exten made in that report to a broader audience of non-specialists sively algae are grazed by zooplankton and benthic suspen

2 Issues in Ecology Number 7 Fall 2000 INTRODUCTION Antipollution laws enacted and enforced over the past 40 years have increasingly restricted discharge of toxic substances into coastal waters of the United States. While this effort has greatly reduced point-source pollution of toxic materials, oxygen-consuming organic materials (BOD), and to some extent phosphorus (P) from industrial and municipal effluent pipes, no comparable attempt has been made to re￾strict the input of nitrogen (N) from municipal effluent, nor to control the flows of N and P that enter waterways from dispersed or nonpoint sources such as agricultural and urban runoff or windborne deposits. As a consequence, inputs of nonpoint pollutants, particularly N, have increased dramati￾cally. Today, pollution from the nutrients N and P represents the largest source of degradation in coastal waters, which include some of the richest and most productive habitats in the oceans. Roughly half of the global fisheries catch occurs in or is dependent upon coastal waters of the world. Nutrient pollution is also called nutrient over-enrich￾ment because both N and P are vital to plant growth. A wide range of problems plaguing near-shore waters world￾wide, from fish kills to some coral reef destruction, can be linked directly or indirectly to excessive nutrient inputs. In the United States, for example, more than 60 percent of coastal rivers and bays are moderately to severely degraded by nutrient pollution. Although such problems occur in all coastal states, the situation is particularly acute in the mid Atlantic states, southeast, and Gulf of Mexico (Figure 1). While inputs of both N and P contribute to the deg￾radation of coastal rivers, bays, and seas, recent research has confirmed that N is particularly damaging to these sys￾tems. This contrasts with findings from freshwater lakes, where P has been demonstrated to be more critical in regu￾lating water quality. Because of public concern over readily apparent fouling in lakes and rivers, water quality regula￾tions over the past 30 years have focused largely on P, leav￾ing N inputs to aquatic systems severely under-regulated. The National Academies’ National Research Council (NRC) recently reviewed the causes and consequences of this neglected pollution problem in a report entitled Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution. All of the authors of this article partici￾pated as members, staff, or invited experts in the work of the NRC Committee on Causes and Management of Coastal Eutrophication and contributed to the NRC report. This ar￾ticle is intended to bring the findings and recommendations made in that report to a broader audience of non-specialists. This article summarizes the ecological damage caused by nutrient pollution in coastal systems, discusses why N is of particular concern in these systems, and outlines the sources of N inputs to the coast. By highlighting the problem of nutrient pollution in coastal rivers, bays, and seas, this ar￾ticle builds upon two earlier volumes in the Issues in Ecology series: Human Alteration of the Global Nitrogen Cycle: Causes and Consequences (1997) and Nonpoint Pollution of Surface Waters with Phosphorus and Nitrogen (1998). ECOLOGICAL DAMAGE FROM NUTRIENT POLLUTION Nutrient over-enrichment has a range of effects on coastal systems, but in general, it brings on ecological changes that decrease the biological diversity the variety of living organisms in the ecosystem. Fertilizing lakes, rivers, or coastal waters with pre￾viously scarce nutrients such as N or P usually boosts the primary productivity of these systems that is, the produc￾tion of algae (phytoplankton) that forms the base of the aquatic food web (Figure 2). This excessive, nutrient-induced increase in the production of organic matter is called eutrophi￾cation, and eutrophication is linked to a number of problems in aquatic ecosystems. As the mass of algae in the water grows, the water may become murkier; and particularly as the algae die and decompose, periods of oxygen depletion (hypoxia and anoxia) occur more frequently. Even living al￾gae can contribute to oxygen depletion due to their oxygen consumption at night. These changes in nutrients, light, and oxygen favor some species over others and cause shifts in the structure of phytoplankton, zooplankton, and bottom-dwell￾ing (benthic) communities. For instance, blooms of harmful algae such as red and brown tide organisms become more frequent and extensive, sometimes resulting in human shell￾fish poisonings and even marine mammal deaths. Oxygen depletion can cause fish kills and create dead zones. Just as important, subtle changes in the plankton community and other ecological factors may trigger reduced growth and recruitment of fish species and lowered fishery production. Coral reefs and submerged plant communities such as seagrass beds can be harmed by loss of light from reduced water clar￾ity, or from nutrient-induced growths of nuisance seaweeds. Some coastal ecosystems are more susceptible to nutrient over-enrichment than others because a host of addi￾tional factors can influence the extent of plant productivity. These factors include how much light is available, how exten￾sively algae are grazed by zooplankton and benthic suspen￾Nutrient Pollution of Coastal Rivers, Bays, and Seas by Robert Howarth, Donald Anderson, James Cloern, Chris Elfring, Charles Hopkinson, Brian Lapointe, Tom Malone, Nancy Marcus, Karen McGlathery, Andrew Sharpley, and Dan Walker

Issues in Ecology Number Fall 2000 sion feeders.and how often a bay or estuary is flushed and eutrophication was partially reversed during the early i 99o's its nutrients diluted by open ocean water.Thus.a given in as nutrient inputs decreased following the collapse of the inputs to coastal rivers and bays will boost Soviet union and fertilizer use in fastern furon primary production more in some systems than others.Ye susceptibility to nutrient over-enrichment is not static and nutrient inputs and eutrophication in the Black Sea have can shift in response to such factors as climate change reached an all time high. In some ecosystems.moderate nutrient enrichment can occasionally lead to increased populations of economi- Effects on Ecological Communities cally valuable fishes.More severe nutrient enrichment of Eutrophication leads to changes in the structure of these same waters.however.leads to losses of catchable fish. ecological communities by at least two mechanisms:indirectl And even in systems where fish abundance is increased by through oxvgen depletion and directly by increased nutrient nutrient inputs,other valued attributes such as biological concentrations. diversity may decline.Other coastal ecosystems are highly Hypoxia and anoxia can change the makeup of a vulnerable to eutrophication so that even small increases in community by killing off more sensitive or less mobile organ- nutrient inputs can be quite damaging. Coral reefs and isms.reducing suitable habitat for others.and changing in- seagrass beds,for instance.are particularly susceptible to For instance changed conditions. o anc in the away trom large cationi to sm area o els. the an estimat q summer 1999. this hypo on th s in the Unite States include d and the dinoflagellate and that li ope.the Baltic.North.Adriatic.and nd s life st Black Seas have all experienced problems from nutrient over me dov ent if bottom enrichment,especially eutrophication. In the Black Sea Figure I-The 1999 NOAA National Es tuarine Eutrophication Assessment found 44 estuaries along the nation's coasts with high expressions of nutrient overenrichment additional 36 estuaries (not shown)display moderate effects of over enrichment(modi fied from Bricker et al.1999)

3 Issues in Ecology Number 7 Fall 2000 sion feeders, and how often a bay or estuary is flushed and its nutrients diluted by open ocean water. Thus, a given in￾crease in nutrient inputs to coastal rivers and bays will boost primary production more in some systems than others. Yet susceptibility to nutrient over-enrichment is not static and can shift in response to such factors as climate change. In some ecosystems, moderate nutrient enrichment can occasionally lead to increased populations of economi￾cally valuable fishes. More severe nutrient enrichment of these same waters, however, leads to losses of catchable fish. And even in systems where fish abundance is increased by nutrient inputs, other valued attributes such as biological diversity may decline. Other coastal ecosystems are highly vulnerable to eutrophication so that even small increases in nutrient inputs can be quite damaging. Coral reefs and seagrass beds, for instance, are particularly susceptible to changed conditions. The single largest coastal system affected by eutrophi￾cation in the United States is the so-called dead zone in the Gulf of Mexico, an extensive area of reduced oxygen lev￾els. In the early 1990s, the zone covered an estimated 9,500 square kilometers of the gulf, extending out from the mouth of the Mississippi River. By the summer of 1999, this hypoxic area had doubled to 20,000 square kilometers, an area the size of Lake Ontario or New Jersey. Other severely impacted coastal systems in the United States include Chesapeake Bay, Long Island Sound, and the Florida Keys. In Europe, the Baltic, North, Adriatic, and Black Seas have all experienced problems from nutrient over￾enrichment, especially eutrophication. In the Black Sea, eutrophication was partially reversed during the early1990’s as nutrient inputs decreased following the collapse of the Soviet Union and fertilizer use in Eastern Europe dropped sharply. This decrease was temporary, however, and both nutrient inputs and eutrophication in the Black Sea have reached an all time high. Effects on Ecological Communities Eutrophication leads to changes in the structure of ecological communities by at least two mechanisms: indirectly through oxygen depletion and directly by increased nutrient concentrations. Hypoxia and anoxia can change the makeup of a community by killing off more sensitive or less mobile organ￾isms, reducing suitable habitat for others, and changing in￾teractions between predators and their prey. For instance, recurring periods of low oxygen tend to shift the dominance in the seafloor community away from large, long-lived spe￾cies such as clams to smaller, opportunistic, and short-lived species such as polychaete worms that can colonize and com￾plete their life cycles quickly between the periods of hypoxia. Zooplankton that normally graze on algae in surface waters during the night and migrate toward the bottom in the day￾time to escape the fish that prey on them may be more vul￾nerable to predation if hypoxia in bottom waters forces them to remain near the surface. Also, planktonic organisms such as diatoms, dinoflagellates, and copepods that live in surface waters (pelagic species) yet spend some life stages resting on the bottom may be unable to resume development if bottom layers remain oxygen depleted. Figure 1 - The 1999 NOAA National Es￾tuarine Eutrophication Assessment found 44 estuaries along the nation’s coasts with high expressions of nutrient overenrichment. An additional 36 estuaries (not shown) display moderate effects of over enrichment (modi￾fied from Bricker et al. 1999).

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.