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 nations 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 Reducing the Effects of Nutrient Pollution concludes that: l Nutrient over-enrichment of coastal ecosystems generally triggers ecological changes that decrease the biological 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 communities. 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 freshwater 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 agricultural 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 restrict 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 dramatically. 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-enrichment because both N and P are vital to plant growth. A wide range of problems plaguing near-shore waters worldwide, 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 degradation of coastal rivers, bays, and seas, recent research has confirmed that N is particularly damaging to these systems. This contrasts with findings from freshwater lakes, where P has been demonstrated to be more critical in regulating water quality. Because of public concern over readily apparent fouling in lakes and rivers, water quality regulations over the past 30 years have focused largely on P, leaving 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 participated 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 article 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 article 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 previously scarce nutrients such as N or P usually boosts the primary productivity of these systems that is, the production 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 eutrophication, 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 algae 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-dwelling (benthic) communities. For instance, blooms of harmful algae such as red and brown tide organisms become more frequent and extensive, sometimes resulting in human shellfish 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 clarity, or from nutrient-induced growths of nuisance seaweeds. Some coastal ecosystems are more susceptible to nutrient over-enrichment than others because a host of additional factors can influence the extent of plant productivity. These factors include how much light is available, how extensively algae are grazed by zooplankton and benthic suspenNutrient 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 increase 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 economically 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 eutrophication in the United States is the so-called dead zone in the Gulf of Mexico, an extensive area of reduced oxygen levels. 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 overenrichment, especially eutrophication. In the Black Sea, eutrophication was partially reversed during the early1990s 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 organisms, reducing suitable habitat for others, and changing interactions 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 species such as clams to smaller, opportunistic, and short-lived species such as polychaete worms that can colonize and complete 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 daytime to escape the fish that prey on them may be more vulnerable 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 Estuarine Eutrophication Assessment found 44 estuaries along the nations coasts with high expressions of nutrient overenrichment. An additional 36 estuaries (not shown) display moderate effects of over enrichment (modified 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 requirements 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 productivity 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 diatom 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 community, 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 eutrophication, creating another factor that favors some algal species over others and alters the structure of the phytoplankton 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 matter (DOM) plays a critical role in enhancing biological availability 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 increases iron availability. Because phytoplankton form the basis of the marine food chain, changes in the species composition of this community 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 dominance 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 proFigure 2 - The left panel shows the distribution of chlorophyll 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 levels 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.
Issues in Ecology Number 7 al2000 duce powerful toxins or cause other harm to humans.fisher Effects on Seagrass Beds and Corals ies resources,and coastal ecosystems (Figure 3).These spe Eutrophication frequently leads to the deoradatio cies make their in thee red or brown tides-to dilute.inconspicuous concentra availability further by stimulating the growth of phytoplank tions of cells noticed only because of damage caused by their ton,epiphytes on the seagrass leaves.and nuisa potent toxins.Impacts can indude mass mortalities of wild and of ephemeral seaweeds (macroalgae)that shade out both farmed fish and shellfish,human poisonings from contaminated seagrasses and perennial seaweeds such as kelp.In addition fish or shellfish.alterations of marine food webs through dam eutrophication can lead to elevated concentrations of sulfide 智me in the sediments of seagrass beds as algae and plant material decompose on the oxygen-depleted seafloor and seagrasses Although population explosions of toxic or noxious lose their ability to oxygenate the sediments.These elevated algal species are sometimes called red tides.they are more sulfide levels can slow the growth of seagrasses.which draw As with most algal most of their nutrients from the sediments rather than the this proliferation and occasional dominance by par water column.or even poison them and lead to their decline ticular species a combination of physical,chemi Nutrient-induced changes generally lower the bio that remair logica diversity of seagrass and kelp communities.Since these ugh ham for a or year d ce and durati rleast som Pe1972 th d exa p of the link he and lution involves the recently discov noflagellate Pfiesteria.In North Carolina estuarie and in the chesaneake bay this oroanism has bee linked to fish kills and to a variety of human effects including severe learning and memory pr lems because the organism and associated fish kills have occurred in watersheds that are heavily pol luted by hog and chicken farm wastes and by mu- nicipal sewage,a strong case can be made that nu trient pollution serves as a major stimulant to out breaks of Pfiesteria or Pfiesterialike organisms.Nu Pre trient-laden wastes could stimulate the outbreaks by either of two mechanisms. First.Pfiesteria is able to take up and use some of the dissolved organic nutri- ents in the waste directly. this adaptabl organism can cons algae that have grov VE H P the Figure 3.This shows the expansion of harmful algal blooms in the United States pre-1972 to present.Ciguatera blooms accumulate toxins in fish argely in tropical waters.Brown tides destroy shellfish beds.Pliesteria kills n tid fish and harms humans.(from Ander ish poiso hat ded reg son 1995,as reprinted in NRC 2000)
5 Issues in Ecology Number 7 Fall 2000 duce powerful toxins or cause other harm to humans, fisheries resources, and coastal ecosystems (Figure 3). These species make their presence known in many ways, ranging from massive blooms of cells that discolor the water so-called red or brown tides to dilute, inconspicuous concentrations of cells noticed only because of damage caused by their potent toxins. Impacts can include mass mortalities of wild and farmed fish and shellfish, human poisonings from contaminated fish or shellfish, alterations of marine food webs through damage to larval or other life stages of commercial fisheries species, and death of marine mammals, seabirds, and other animals. Although population explosions of toxic or noxious algal species are sometimes called red tides, they are more correctly called harmful algal blooms. As with most algal blooms, this proliferation and occasional dominance by particular species results from a combination of physical, chemical, and biological mechanisms and interactions that remain poorly understood. Although harmful algal blooms have occurred for at least thousands of years, there has been an increased incidence and duration of such outbreaks worldwide over the past several decades. This increase in harmful algal blooms has coincided with marked increases in nutrient inputs to coastal waters, and in at least some cases, nutrient pollution is to blame for the outbreaks. A frequently cited example of the suspected link between harmful algal blooms and nutrient pollution involves the recently discovered phantom dinoflagellate Pfiesteria. In North Carolina estuaries and in the Chesapeake Bay, this organism has been linked to fish kills and to a variety of human health effects, including severe learning and memory problems. Because the organism and associated fish kills have occurred in watersheds that are heavily polluted by hog and chicken farm wastes and by municipal sewage, a strong case can be made that nutrient pollution serves as a major stimulant to outbreaks of Pfiesteria or Pfiesteria-like organisms. Nutrient-laden wastes could stimulate the outbreaks by either of two mechanisms. First, Pfiesteria is able to take up and use some of the dissolved organic nutrients in the waste directly. Second, this adaptable organism can consume algae that have grown more abundant because of the nutrient over-enrichment. Although the link between Pfiesteria outbreaks and nutrient pollution has not been fully proven, the evidence is sufficiently strong that legislation is already being developed and adopted to regulate waste handling at hog and chicken farms to reduce nutrient inputs to adjacent watersheds. Pfiesteria has thus prompted some agencies to address serious and long-standing pollution discharges by nonpoint sources that had previously avoided regulation. Effects on Seagrass Beds and Corals Eutrophication frequently leads to the degradation or complete loss of seagrass beds. Plant growth in these beds is often light limited, and eutrophication can lower light availability further by stimulating the growth of phytoplankton, epiphytes on the seagrass leaves, and nuisance blooms of ephemeral seaweeds (macroalgae) that shade out both seagrasses and perennial seaweeds such as kelp. In addition, eutrophication can lead to elevated concentrations of sulfide in the sediments of seagrass beds as algae and plant material decompose on the oxygen-depleted seafloor and seagrasses lose their ability to oxygenate the sediments. These elevated sulfide levels can slow the growth of seagrasses, which draw most of their nutrients from the sediments rather than the water column, or even poison them and lead to their decline. Nutrient-induced changes generally lower the biological diversity of seagrass and kelp communities. Since these Figure 3 - This shows the expansion of harmful algal blooms in the United States pre-1972 to present. Ciguatera blooms accumulate toxins in fish, largely in tropical waters. Brown tides destroy shellfish beds. Pfiesteria kills fish and harms humans. (from Anderson 1995, as reprinted in NRC 2000). Pre-1972 Present Neurotoxic shellfish poisoning Paralytic shellfish poisoning Fish kills Ciguatera Pfiesteria Brown tide Amnesic shellfish poisoning
Issues in Ecology Number 7 Fall 2000 Figure 4-Nitrogen over-enrichment can lead to nuisance blooms of ephemeral seaweeds(macroalgae,left photo).which can have severe impacts on seagrass beds and coral reefs.On the right,sponges and corals overgrown by the seaweed Codium isthmocladum in Southeast Florida. plant communities provide food and shelter for a rich and Given the high light intensities and warm tempera- diverse array of marine animals,the degradation of seagrasses tures found in coral reef waters,the growth rates of ephem- and kelp or their replacement by nuisance seaweed blooms eral seaweeds are limited largely by the availability of essen- brings marked changes in the associated animal life.These tial nutrients.Thus even slight increases in dissolved nutrient systems are particularly important as spawning and nursery concentrations can lead to expansion of these algae at the grounds for fish.Further,the roots and rhizomes of seagrasses expense of coral.Increased seaweed cover on reefs inhibits stabilize bottom sediments,and their dense leaf canopy pro- the recruitment of corals and leads to a cascade of other motes the settling out of fine particles from the water col- ecological effects.For instance,seaweed blooms can lead to umn.Loss of seagrass coverage,therefore,allows sediments oxygen depletion on reef surface as these seaweeds decom- to be stirred up.This not only reduces water clarity directly pose,and hypoxia in turn degrades habitat needed to sup- but allows nutrients trapped in the sediment to be released port high diversity of coral reef organisms and potentially into the water column,promoting additional algal blooms. important grazers. The short-lived nuisance seaweeds that result from eutrophi- There is some evidence that N availability,in addi- cation can also wash up in enormous quantities on beaches. tion to temperature,light,and other environmental factors, creating a foul smell for beachgoers and coastal residents. may influence the“coral bleaching”phenomenon一loss of Coral reefs are among the most diverse ecosystems the algal partners known as zooxanthellae that live inside in the world,and also among the most sensitive to nutrient the cells of coral animals and nourish them-that has ex- pollution.The world's major coral reef ecosystems are found panded globally in recent years. in naturally nutrient-poor surface waters in the tropics and subtropics.It was once commonly thought that coral reefs WHICH NUTRIENTS MATTER? preferred or thrived in areas of nutrient upwelling or other nutrient sources,but this idea has been shown to be incor- The major nutrients that cause eutrophication and rect.Instead,high nutrient levels are generally detrimental other adverse impacts associated with nutrient over-enrich- to reef health and lead to shifts away from corals and the ment are N and P.Nitrogen is of paramount importance both coralline algae that help build the reef structure toward domi- in causing and controlling eutrophication in coastal marine nance by algal turfs or seaweeds that overgrow or cover the ecosystems.This is in contrast to freshwater (or non-saline) reefs.For example,some offshore reefs in the Florida Keys lakes,where eutrophication is largely the result of excess P that contained more than 70 percent coral cover in the 1970s inputs.Other elements-particularly silica-may also play now have about 18 percent coral cover;mats of algal turf a role in regulating algal blooms in coastal waters and in and seaweeds now dominate these reefs,accounting for 48 determining some of the consequences of eutrophication. to 84 percent cover,and nutrient enrichment bears much of Extensive studies in the early 1970s led to consen- the blame(Figure 4).The effects of nutrient pollution,how- sus that P was the nutrient most responsible for over-enrich- ever,can often be exacerbated either by disease or overfish- ment in freshwater lakes.Since that time,tighter restric- ing,which reduce populations of sea urchins,fish,and other tions on P inputs have greatly reduced eutrophication prob- animals that graze on algae and help keep coral reefs clear. lems in these waters.However,more recent research indi- 6
6 Issues in Ecology Number 7 Fall 2000 plant communities provide food and shelter for a rich and diverse array of marine animals, the degradation of seagrasses and kelp or their replacement by nuisance seaweed blooms brings marked changes in the associated animal life. These systems are particularly important as spawning and nursery grounds for fish. Further, the roots and rhizomes of seagrasses stabilize bottom sediments, and their dense leaf canopy promotes the settling out of fine particles from the water column. Loss of seagrass coverage, therefore, allows sediments to be stirred up. This not only reduces water clarity directly but allows nutrients trapped in the sediment to be released into the water column, promoting additional algal blooms. The short-lived nuisance seaweeds that result from eutrophication can also wash up in enormous quantities on beaches, creating a foul smell for beachgoers and coastal residents. Coral reefs are among the most diverse ecosystems in the world, and also among the most sensitive to nutrient pollution. The worlds major coral reef ecosystems are found in naturally nutrient-poor surface waters in the tropics and subtropics. It was once commonly thought that coral reefs preferred or thrived in areas of nutrient upwelling or other nutrient sources, but this idea has been shown to be incorrect. Instead, high nutrient levels are generally detrimental to reef health and lead to shifts away from corals and the coralline algae that help build the reef structure toward dominance by algal turfs or seaweeds that overgrow or cover the reefs. For example, some offshore reefs in the Florida Keys that contained more than 70 percent coral cover in the 1970s now have about 18 percent coral cover; mats of algal turf and seaweeds now dominate these reefs, accounting for 48 to 84 percent cover, and nutrient enrichment bears much of the blame (Figure 4). The effects of nutrient pollution, however, can often be exacerbated either by disease or overfishing, which reduce populations of sea urchins, fish, and other animals that graze on algae and help keep coral reefs clear. Given the high light intensities and warm temperatures found in coral reef waters, the growth rates of ephemeral seaweeds are limited largely by the availability of essential nutrients. Thus even slight increases in dissolved nutrient concentrations can lead to expansion of these algae at the expense of coral. Increased seaweed cover on reefs inhibits the recruitment of corals and leads to a cascade of other ecological effects. For instance, seaweed blooms can lead to oxygen depletion on reef surface as these seaweeds decompose, and hypoxia in turn degrades habitat needed to support high diversity of coral reef organisms and potentially important grazers. There is some evidence that N availability, in addition to temperature, light, and other environmental factors, may influence the coral bleaching phenomenon loss of the algal partners known as zooxanthellae that live inside the cells of coral animals and nourish them that has expanded globally in recent years. WHICH NUTRIENTS MATTER? The major nutrients that cause eutrophication and other adverse impacts associated with nutrient over-enrichment are N and P. Nitrogen is of paramount importance both in causing and controlling eutrophication in coastal marine ecosystems. This is in contrast to freshwater (or non-saline) lakes, where eutrophication is largely the result of excess P inputs. Other elements particularly silica may also play a role in regulating algal blooms in coastal waters and in determining some of the consequences of eutrophication. Extensive studies in the early 1970s led to consensus that P was the nutrient most responsible for over-enrichment in freshwater lakes. Since that time, tighter restrictions on P inputs have greatly reduced eutrophication problems in these waters. However, more recent research indiFigure 4 - Nitrogen over-enrichment can lead to nuisance blooms of ephemeral seaweeds (macroalgae, left photo), which can have severe impacts on seagrass beds and coral reefs. On the right, sponges and corals overgrown by the seaweed Codium isthmocladum in Southeast Florida. Photo by Brian LaPointe. Photo by Michael Bo Rasmussen.
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 ecosystems 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 relative 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 temperate 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 inputs. 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 production because the sand readily adsorbs phosphate, trapping it in the sediment and leaving it largely unavailable to organisms. However, such lagoons may move toward N limitation as they become eutrophic. The primary reason is that as more nutrients enter these waters, the rate at which sediments 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 understood. 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 production 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 breakdown of N compounds (denitrification) play a more important 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 eutrophication in freshwater systems. Many coastal marine scientists 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 enrichment 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).
Issues in Ecology Number 7 Fall 2000 9.8 fold Atlantic Ocear 3.1-fold 8-fold 275 5.1-fold 199 an activity has greatly increased the flux of nitrogen to coast waters in many areas. (Based on data in for controls on N been widely accepted. periment conducted at the Marine Ecosystem Research Labo Most early studies of nutrient limitation in coastal ratory(MERL)on the shores of Narragansett Bay in Rhode waters either relied on short-term and small-scale enrichment Island.was specifically designed to see if coastal systems re 7RpS spond t nutnent additions in the same erglass lanks (mes pute because.when applied to the problem of lake eutrophi- an nsett Bay than P was limiting in lakes.Later,experiments in whol lakes clearly showed P and not N or carbon regulatec onsequently,scientist who stud ager or lak an appropriate skepticism about smal hich the p in approa dence have a ontrast.th sys thes of d either alone or with p hut not p alone caused laro this increases in both ratesof agal production and abundanceof phytoplankton. trients do not ab refle the A second whole-ecosvstem study took nlace in able for uptake by li Himmerfjarden,an estuary south of Stockholm.Sweden.on In lakes it took e stem-scale evn iments to con the Baltic Sea.Researchers traced impacts of experimental firm eutrophication can be managed most 6 changes in nutrient releases from a sewage treatment plant controlling P inputs.A decade ago.there w into the estuary from 1976 to 1993. For the first sever rable experiments testing the relative imp oortance of Nand P years.N loads gradually increased while P loads gradually in regulating eutrophication in coastal marine ecosystems. decreased.For a one-year period beginning in the fall of However.since 1990 the results of three such studies have 1983,P additions were greatly increased by halting removal shown these systems are N limited. of P during sewage treatment.In 1985.P removal was One of these experiments.a so-called mesocosm ex- resumed while N inputs jumped 40 percent as a result of ar
8 Issues in Ecology Number 7 Fall 2000 for controls on N been widely accepted. Most early studies of nutrient limitation in coastal waters either relied on short-term and small-scale enrichment experiments to infer limitation by N or made inferences from laboratory studies. These approaches had fallen into disrepute because, when applied to the problem of lake eutrophication in the 1960s and early 1970s, such approaches often led to the erroneous conclusion that N or carbon, rather than P, was limiting in lakes. Later, experiments in whole lakes clearly showed P and not N or carbon regulated eutrophication in lakes. Consequently, scientists who studied lake eutrophication and managers responsible for lake water quality developed an appropriate skepticism about smallscale or lab-based approaches. Other types of evidence have also been used to infer N limitation in coastal ecosystems, including the relatively low ratios of dissolved inorganic N to P found in many of these waters. Yet this type of bioassay result can also be criticized because concentrations of dissolved inorganic nutrients do not always accurately reflect the amounts available for uptake by living organisms. In lakes, it took ecosystem-scale experiments to confirm eutrophication can be managed most successfully by controlling P inputs. A decade ago, there were no comparable experiments testing the relative importance of N and P in regulating eutrophication in coastal marine ecosystems. However, since 1990 the results of three such studies have shown these systems are N limited. One of these experiments, a so-called mesocosm experiment conducted at the Marine Ecosystem Research Laboratory (MERL) on the shores of Narragansett Bay in Rhode Island, was specifically designed to see if coastal systems respond to nutrient additions in the same manner as lakes. A series of fiberglass tanks (mesocosms) more than 15 feet tall and six feet in diameter containing water and intact sediments from Narragansett Bay were maintained for a period of four months. Many previous studies in these mesocosms had demonstrated that these systems accurately mimic much of the ecological functioning of Narragansett Bay. In this experiment, each tank received a specific treatment: no nutrient enrichment, N alone, P alone, or both N and P. The levels of N and P enrichment paralleled levels used in an earlier whole-lake experiment in which the P inputs clearly led to eutrophication while N had no effect. In sharp contrast, the addition of the same level of N to the MERL coastal mesocosms -- either alone or with P, but not P alone caused large increases in both rates of algal production and abundance of phytoplankton. A second whole-ecosystem study took place in Himmerfjarden, an estuary south of Stockholm, Sweden, on the Baltic Sea. Researchers traced impacts of experimental changes in nutrient releases from a sewage treatment plant into the estuary from 1976 to 1993. For the first seven years, N loads gradually increased while P loads gradually decreased. For a one-year period beginning in the fall of 1983, P additions were greatly increased by halting removal of P during sewage treatment. In 1985, P removal was resumed while N inputs jumped 40 percent as a result of an Figure 6 - Human activity has greatly increased the flux of nitrogen to coastal waters in many areas. (Based on data in Howarth, 1998.)
Issues in Ecology Number 7 Fall 2000 increase in population served by the sewage treatment plant. results obtained from earlier studies.most bioassay data from Finally.N removal technology was gradually introduced to estuaries and coastal marine systems indicates that these are the se veen 1988 and 1993.even N limited.This is supported by the tallreducing theNad to the value originally seen in 1976.Throughout the 17 years of observation.both the e hehe ner.this body of clarity of the water and the abundance of phytoplankton were clearly related to the total N concentration in the estu- primary regulator of eutrophication in most coastal systems. ary.In contrast.total P was a poor predictor of phytoplank- ton abundances. Mechanisms That Lead to Nitrogen Control of Eutrophica- A third study explored long-term changes in Laholm tion in Estuaries Bay.an estuary on the southwestern coast of Sweden (Fig Whether primary production by phytoplankton is ure 5).Early signs of eutrophication appeared there in the limited by Nor P depends on the relative availability of each 1950s and 1960s and steadily increased over time.The of these nutrients in the water.Algal growth will slow when earliest reported indicator of eutrophication was a change in the concentration of the scarcest nutrient drops.Phytoplank the seaweed community,and filamentous or sheet-like sea ton require approximately 16 moles of N for every mole of F weed species typical of eutrophic conditions have gradually they take in.This N:P ratio of 16:I is called the Redfield become more prevalent.Harmful algal blooms also ratio.If the ratio of avai ble N to available P in an aquati 980s gostcmssthanl6al,aealgownhwlim he ratio is higher.primary production will tend to creasing.From the late 1960s th inp The relat eavailability of N and Ptothe phytoplank nputs ton is det cating his perio into the ecosystem from that N controls ·ho recycled,or os ett and how much N is "fixed" erted from aseous with cor clusions drawn from shor term hio studios and Nin the air directly into biologically from ratios of dissolved inorganic N to P in these ecosystems. within the ecosystem. As a result.these three ec svstem studies add credence These three factors interact in several ways to make to coasta waters, directly rough trogen
9 Issues in Ecology Number 7 Fall 2000 increase in population served by the sewage treatment plant. Finally, N removal technology was gradually introduced to the sewage treatment plant between 1988 and 1993, eventually reducing the N load to the value originally seen in 1976. Throughout the 17 years of observation, both the clarity of the water and the abundance of phytoplankton were clearly related to the total N concentration in the estuary. In contrast, total P was a poor predictor of phytoplankton abundances. A third study explored long-term changes in Laholm Bay, an estuary on the southwestern coast of Sweden (Figure 5). Early signs of eutrophication appeared there in the 1950s and 1960s and steadily increased over time. The earliest reported indicator of eutrophication was a change in the seaweed community, and filamentous or sheet-like seaweed species typical of eutrophic conditions have gradually become more prevalent. Harmful algal blooms also increased in frequency, particularly in the 1980s. During the early stages of eutrophication in Laholm Bay, inputs of both P and N were increasing. From the late 1960s through the 1980s, however, P inputs declined by a factor of almost two, while N inputs more than doubled. Plankton blooms continued and eutrophication worsened during this period, clearly indicating that N was controlling eutrophication in the bay. Although these three large-scale studies show only that N controls eutrophication in Narragansett Bay, Himmerfjarden, and Laholm Bay, the findings are consistent with conclusions drawn from short-term bioassay studies and from ratios of dissolved inorganic N to P in these ecosystems. As a result, these three ecosystem studies add credence to results obtained from earlier studies. Most bioassay data from estuaries and coastal marine systems indicates that these are N limited. This is supported by the generally low inorganic N to P ratio found in most estuaries when they are at the peak of primary production. Thus, taken together, this body of evidence leads to the conclusion that N availability is the primary regulator of eutrophication in most coastal systems. Mechanisms That Lead to Nitrogen Control of Eutrophication in Estuaries Whether primary production by phytoplankton is limited by N or P depends on the relative availability of each of these nutrients in the water. Algal growth will slow when the concentration of the scarcest nutrient drops. Phytoplankton require approximately 16 moles of N for every mole of P they take in. This N:P ratio of 16:1 is called the Redfield ratio. If the ratio of available N to available P in an aquatic ecosystem is less than 16:1, algal growth will tend to be N limited. If the ratio is higher, primary production will tend to be P limited. The relative availability of N and P to the phytoplankton is determined by three factors: l the ratio of N to P that comes into the ecosystem from both natural and human-derived sources; l how each nutrient is handled stored, recycled, or lost in the ecosystem; l and how much N is fixed converted from gaseous N in the air directly into biologically useable forms within the ecosystem. These three factors interact in several ways to make Figure 7 - Animal wastes may be the largest single source of N that moves from agricultural production to coastal waters, either directly through runoff or indirectly through volatilization and deposition of atmospheric nitrogen. Photo by Larry Rana, USDA. Photo by Gene Alexander, USDA.
