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Nutrient Pollution of Coastal Rivers Bays and Seas 61 Number 7 Fall 2000 Issues in Ecofo Published by the Ecological Society oi America bmin Minibar Fall 2m 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 pointsource pollution of industrial and municipal effluent No comparable effort has been made to restrict the input of nitrogen N from municipal ef uent nor to control the flows of N and phosphorus P that enter watenrvays 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 quotdead zonesquot 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 Pollutionquot concludes that 0 Nutrient overenrichment of coastal ecosystems generally triggers ecological changes that decrease the biologi cal diversity of bays and estuaries 0 While moderate N enrichment of some coastal waters may increase fish production overenrichment generally degrades the marine food web that supports commercially valuable fish 0 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 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 Research during the past decade confirms that N is the chief culprit in eutrophication and other impacts of nutrient overenrichment in temperate coastal waters while P is most problematic in eutrophication of freshwa ter lakes 0 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 0 Both agriculture and the buming of fossil fuels contribute significantly to nonpoint flows of N to coastal waters either as direct runoff or airborne pollutants o 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 IO percent of its degraded coastal systems by 20 O 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 overfertilization of agricul tural fields and nutrient pollution from animal wastes in livestock feedlot operations 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 in qu her Fall atone 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 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 pointsource pollution of toxic materials oxygenconsuming 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 watenrvays from dispersed or nonpoint sources such as agricultural and urban runoff or windbome 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 overenrich ment because both N and P are vital to plant growth A wide range of problems plaguing nearshore 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 I Xhile 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 underregulated 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 Pollutionquot 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 nonspecialists 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 Consequencesquot I 997 and Nonpoint Pollution of Surface Xaters with Phosphorus and Nitrogenquot I998 ECOLOGICAL DAMAGE FROM NUTRIENT POLLUTION Nutrient overenrichment 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 nutrientinduced 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 bottomdwell ing benthic communities For instance blooms of harmful 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nmhmsmm us m mum 044m mp5 The ux 04 Ms W their Mt olmmgtn xmmn and Ihtse i a g mama smags mg oambamsna dummy 3th mang 3L nobauma m1 0 x m shmagz Pmmary mwyoms wan remams mm byamw ams R Thxs has hm demonstrated m WHDLE m expmm ms byvsmxmg a wan Wm a m7 mnlamounlo over smrax r5 Forlhz rst 42w wars m wan 3 90 mm mamsz Ngh eve solN Itrlwhmr soth we ramooINJ was abovs ms mm mm umer m7 muons nommgsnnxamn mum m m m The ex nmmla mamm was New sum that m wan r2me m a m an amount MN so that m mpm w be mm mm m Numgsmmg organ sms muddy appmmd and ma up m N de r m Estuams and eulrophm nasta waler pro masa smkmg wires MN MS whenm W q ag that mlmgm mum m mam sysssms 7 as am 035m saas as we as man waltrs 7 dwmxca bxongm and pwsmx damn R2 nobauema m 4m mam am am I m th s s h a r x a 4 mm m I s M s 4 m m N W m and by grazmg at Mum r pnnl d from NRC 1000 mmme mum s omsrsxmms b29625 N and P 3 90 can N hmmamon mm my m ssmanss and P hmmamon mm have a mam mums m m structure daqumm comm Mker m W25 For mam y w H NAM 5mm and 035m gowns and naghbonng mm wams The mamam sums at nulrmls 4m ssmanss mm or m mums on m Man rats ofa ga growth m an aqua sysxsm m Wm 5mm 5 abunda l mamms are m 39 Wm 5mm Scams and to have an NP ramo we mow m Rama am thanks to m armmy at demmfymg mama m m mmmma 5M4 lam mogm a pmss that fetuses N m m atmosphtm Thus 5 s more Webm be N mm man are lakts Fmshwaxsr am and mm mam emsysxsms 3 90 s m ow Supp y mm dassss ofa gae mums m phy lop ankxm ommumly wa wans am argay 4mm my an at gods and ssmmmxs dams m asnwy 04 umb t 5mm 0 masca mam sys was by spum g mlmphmnn upsmm mm was to lime in Ewlg trap silica before it reaches the coast Thus the concentra tion of silicate in Mississippi River water entering the Gulf of Mexico decreased by 50 percent from the I 950s to the I 980s even as nitrate fluxes and concentrations increased Even the silica that reaches the coast can quickly become unavail able if eutrophication leads to diatom blooms As the dia toms die and fall to the bottom their silicarich shells remain stored long term in bottom sediments A decrease in silica availability particularly if com bined with increases in N may encourage some harmful al gal blooms as competition with diatoms is decreased In all cases where longtem1 data are available on silica in coastal waters a decrease in silica relative to N or P has been corre lated with an increase in harmful algal blooms SOURCES OF NUTRIENTS TO COASTAL ECOSYSTEMS Human activities profoundly influence the global cy cling of nutrients especially movement of nutrients to estu aries and other coastal waters For instance human activity has more than doubled the rate of P delivery from land to the oceans Our effect on N cycling is equally immense and the rate of change in the human N use pattern is much greater Our increased reliance on synthetic inorganic fertilizers has caused the single largest change in the global N cycle Bur geoning fertilizer use accounts for more than half of the total humandriven alteration of the N cycle The process for making inorganic N fertilizer was invented during World War l but was not widely used until the I950s The rate of fertilizer use increased steadily until the late I 980s when the collapse of the former Soviet Union led to great disruptions in agriculture and fertilizer applica tions in Russia and much of Eastern Europe These disrup tions led to a slight decline in global N fertilizer use for a few years By I995 however global use of inorganic N fertil izer was again growing rapidly with much of the growth driven by China Annual fertilizer use by I996 totaled ap proximately 83 teragrams Tg where one teragram equals a million metric tons of N Approximately half of the inor ganic N fertilizer that has ever been used on earth has been applied during the past I5 years Although fertilizer production and use is the human activity that mobilizes the largest amount of N worldwide other humancontrolled processes also convert atmospheric N into biologically available forms These activities include the buming of fossil fuels such as coal and oil and extensive plantings of nitrogenfixing crops such as soybeans peas and alfalfa Human fixation of N from all these activities increased by two to threefold from I960 to I990 and continues to grow By the mid I990s human activities made new N available at a rate of some I40 Tg per year matching the natural rate of biological nitrogen fixation on all the land Number 7 Pg soon surfaces of the earth Thus the rate at which humans have altered N availability far exceeds the rate at which we have altered the global carbon cycle driving up the carbon diox ide concentration in the atmosphere and perhaps setting in motion a warming of the earth s climate Humandriven changes in nutrient cycling have not occurred uniformly around the world The greatest changes are concentrated in the areas of high human population den sity and intensive agricultural production Some regions of the world such as the Hudson s Bay area of Canada have experienced very little change in the flows of either N or P to the coast while other places have experienced tre mendous changes Human activity is estimated to have in creased N fluxes down the Mississippi River by more than 4 fold into the coastal waters of the northeastern United States generally and Chesapeake Bay specifically by some 6 to 8 fold and into the rivers draining to the North Sea by I Ifold Figure 6 Trends over time also vary among regions For example while the global use of inorganic N fertilizer contin ues to increase its use in the United States has risen very little since I985 Wastewater us Nonpoint Sources of Nutrients Pointsource wastewater flows can sometimes be the major source of N to an estuary when the watershed is heavily populated and small relative to the size of the estuary itself Even in some estuaries fed by runoff from larger watersheds sewage wastewater can be the largest source of N if the watershed is heavily populated For example wastewater contributes an estimated 60 percent of the N that enters Long Island Sound largely because of sewage discharges into the sound from New York City In most estuaries how ever N and P inputs from nonpoint sources are greater than those from wastewater particularly in estuaries that have relatively large watersheds and thus more rural land devoted to crop and livestock production as well as more area to trap N pollution from the atmosphere For example only one quarter of the N and P inputs to Chesapeake Bay come from wastewater treatment plants and other such point sources For the Mississippi River sewage and industrial point sources supply an estimated I0 to 20 percent of the total N and 40 percent of the total P contribution Agriculture as a Source of Nutrients For P agriculture is one of the largest sources of nonpoint pollution For N both agriculture and burning of fossil fuels contribute significantly to nonpoint flows to estu aries and coastal waters N from these sources can reach the water either by direct leaching or runoff from farm fields or indirectly through the atmosphere Some N is leached directly from agricultural fields to groundwater and surface waters A significant amount of the N from agricultural sources however travels on the wind Globally some 40 percent of the inorganic N fertilizer that is applied to fields is arm I w r Ourundushndngahkwm mmp wry mnuplua m Wm mmusmkampbi39nymxk mum quot nding mm m mm wime MspzW Mummy mun 39 mmpm Mmmwmtrst a mm m mm mm mm Wang Mme a mm mm ur39m39nns in M mum w mm mavmwm mm mu mm Wm m N w m mm m N a u 301 quotmum an R submmb daHud and wrummtd Mu mum pmdua39nn w and an m m brguiarms mm gt9 m an my and m induan mm mama av 55 01 my WWW W cw 39 r r 395 Wquot M quot H m bqu up an m m mam Wde m m m m mu mme and mm warm MN m mwgw mme m m m m mm was out M inm ma mam Sm m Mum k mm m m mp Human quotm hm mm mp was rm Mu wmr manuka m Grain mamquot muld m human withier wer mmv LA H mam in ma a in qu her Fall atone The byproducts of fossilfuel combustion princi pally exhaust from motor vehicles and electricpower genera tion are a major source of N to coastal waters in many regions This includes both direct deposition of airborne N onto the surface of coastal waters and deposition onto the landscape where it subsequently washes or leaches into riv ers or groundwater that flows into coastal ecosystems The limited evidence available indicates that direct deposition of airbome N onto the water surface alone contributes from percent to 40 percent of the total N entering an estuary depending to a large extent on the size of the estuary rela tive to its watershed In general the larger the estuary is relative to its watershed the greater the percent of N that is deposited directly onto the water For estuaries that are small relative to the size of their watersheds N deposition from the atmosphere onto the landscape with subsequent runoff into the estuary is probably a greater source than deposition of N directly onto the water surface Unfortunater the magnitude of this in put is poorly characterized for most estuaries Determining Sources of N to Speci c Estuaries rrently there are no uniformly accepted methods for determining sources of N to an estuary and thus great uncertainty remains about the importance of atmospheric deposition in delivering N to specific estuaries The amount of N deposited onto a landscape can be estimated for most watersheds although such estimates are subject to considerable error because of inadequate monitor ing and the difficulty of measuring dry deposition of N pol lutants A larger problem however is determining what portion of the N is retained in the landscape and what por tion is actually exported to rivers and ultimately the coast There are two major approaches for making this determination statistical models and processbased models In their application to estuaries both of these computer mod eling approaches are quite recent and relatively untested and there is an urgent need forfurther development and evalu ation of these techniques So far however it appears that statistical models have produced more reliable estimates of N retention in watersheds These models suggest that in areas where atmospheric deposition of N is moderately high as is true over most of the northeastern United States an aver age of 40 percent of that deposition is exported from the landscape to downstream ecosystems Process based models tend to assume that a much smaller percent of N deposition is exported downstream but these models do not consider export of all forms of N dissolved organic N as well as nitrate Yet recent evidence indrcates that for most temperate forests more N is exported in organic forms than as nitrate The lack of wellaccepted techniques for quantifying various N sources to an estuary makes it difficult to deter mine accurately the relative role of fossilfuel burning versus agricultural activity in nutrient pollution of specific coastal l3 ecosystems Nevertheless the relative importance of these two activities in controlling atmospheric deposition of N to estuaries clearly depends on the nature and extent of fami ing activities in the watershed as well as the nature and ex tent of fossilfuel combustion in the airsheds upwind of the watershed Figure 8 In estuaries fed by watersheds with little agricul tural activity but significant loads of atmospheric pollution for example the Connecticut and Merrimack rivers and most of the northeastem United States atmospheric deposition of N from fossilfuel combustion can account for up to 90 percent or more of the N contributed by nonpoint sources For many estuaries including Chesapeake Bay both agricul tural sources and fossilfuel sources are significant contribu tors of N On the other hand for watersheds such as the Mississippi River Basin where agricultural activity is high and atmospheric pollution from fossil fuel burning is rela tively low agricultural sources dominate the export of N Interestingly the major hotspots of agricultural activity that dominate the N inputs for the Mississippi and Gulf of Mexico appear to be far upstream from the Gulf in Iowa Illinois Indiana Minnesota and Ohio STEPS TOWARD SOLUTIONS According to a recently completed National Estua rine Eutrophication Assessment by the National Oceanic and Atmospheric Administration more than half of the coastal bays and estuaries in the United States are degraded by ex cessive nutrients primarily N The NRC Clean Coastal Xa tersquot report called for a 20year national effort to reverse this trend and begin the restoration of our coastal marine ecosystems At a minimum the report called for restoring IO percent of the degraded systems by 20 O and 25 per cent by 2020 In addition the report recommended that steps be taken to ensure no coastal areas ranked as healthy are allowed to develop symptoms of nutrient overenrichment Meeting these goals will require a multitude of strat egies and approaches tailored to specific regions and ecosys tems Figure 9 For some coastal systems such as Long Island Sound N removal during sewage treatment will be required Although sewage treatment in the nation has greatly improved since the passage of the Clean Water Act in I972 the major focus has been on removing organic mat ter secondary treatment Many sewage treatment plants have also instituted P removal for protection of freshwater systems Yet N removal during sewage treatment remains much less common For most coastal systems however human sewage is not the major source of nutrients and other control strat egies will be required N and P from animal wastes in live stock feedlot operations are one of the biggest sources of nutrients to coastal waters in many areas In contrast to human sewage these animal wastes receive little if any treat in qu her Fall atone ment and remain largely unregulated Yet the waste from hog production in North Carolina alone is 3fold greater than all the human sewage output from New York City Tech nologies exist for treating animal wastes to remove nutrients and for eliminating volatilization of ammonia from these wastes to the atmosphere A variety of control strategies are available to re duce fertilizer runoff from agricultural lands and emission of N compounds from fossil fuel combustion Fossilfuel emis sions are already regulated under the Clean Air Act although coastal N pollution was not considered in framing that law Greater regulation federal oversight and incentives for com pliance will all be required if these control strategies are to be better directed towards solving pollution problems of coastal rivers bays and seas ACKNOWLEDGMENTS Preparation and publication of this issue was supported by a grant from the Andrew W Mellon Foundation in support of the lntemational SCOPE Nitrogen Project and by an en dowment at Cornell University established by David R Atkinson The authors are grateful for this support This issue is based on material published in the National Research Council s report Clean Coastal Waters Understanding and Reducing the Effects of Nutrient Pollutionquot The study that produced that report was supported by the National Oce anic and Atmospheric Administration the U S Environmen tal Protection Agency the US Geological Survey and the Electric Power Research Institute SUGGESTED FURTHER READING Bricker S B C C Clement D E Pnhalla S P Orlando and D R G Farrow I999 National Estuarine Eutrophi cation Assessment Effects of Nutrient Enrichment in the Nation s Estuaries National Oceans Studies Special Projects Office National Oceanic and Atmospheric Ad ministration Silver Springs MD Carpenter et al I998 Nonpoint Pollution of Surface Xaters with Phosphorus and Nitrogen Issues in Ecology No 3 Cloem JE 2000 Our evolving conceptual model of the coastal eutrop7ication problem Marine Ecology Progress Series in press Jorgensen BB and K Richardson I996 Eutrophicaton in Coastal Marine Systems American Geophysical Union Washington DC Nixon S X I995 Coastal marine eutrophication a de nition social causes and future concems Ophelia 4 I992I9 NRC 2000 Clean Coastal Waters Understanding and Reducing the Effects of Nutrient Ibllution N ational Aca d emy Press Washington DC available on the web at httpwwwnapedu catalog 98 I 2html Vitousek PM et al I 997 Human Alteration of the Global Nitro II gen Cycle Causes and Gonsecpences Issues In EcologNo I ABOUT THE PANEL OF SCIENTISTS The authors of this article were all members of staff for or invited experts to the National Research Council s Commit tee on Causes and Management of Coastal Eutrophication and contributed to the pertinent chapters of Clean Coastal Watersquot upon which this article is based Dr Robert Howarth Committee Chair Section of Ecology and Systematics Cornell University Ithaca NY I4853 and Oceans Program Environmental Defense 6 North Market Building Faneuil Hall Boston MA 02 I09 Dr Donald Anderson Department of Biology Woods Hole Oceanographic Institution Woods Hole MA 02543 Dr James Cloem United States Geological Survey Menlo Park CA 94025 Dr Chris Elfring Senior Program Officer Water Science and Technology Board National Research Council Washing ton DC 204 8 Dr Charles Hopkinson The Ecosystems Center Marine Bio logical Laboratory Woods Hole MA 02543 Dr Brian Lapointe Harbor Branch Oceanorgraphic Institu tion Division of Marine Science 5600 US Highway North Ft Pierce FL 34946 Dr Tom Malone Director and Professor Horn Point Labora tory UMCES Cambridge MD 2 6 3 Dr Nancy Marcus Department of Oceanography Florida State University Tallahassee FL 32 306 Dr Karen McGlathery Department of Environmental Sci ences University of Virginia Charlottesville VA 22903 Dr Andrew Sharpley Research Soil Scientist US Depart ment of Agriculture Agricultural Research Service Pas ture Systems and Watershed Management Research Labo ratory University Park PA 6802 Dr Dan Xalker Study Director and Senior Program Officer Ocean Studies Board National Research Council Wash ington DC 204 8 About the Science Writer Yvonne Baskin a science writer edited the report of the panel of scientists to allow it to more effectively commu nicate its findings with nonscientists About Issues in Ecology Issues in Ecology is designed to report in language understandable by nonscientists the consensus of a panel of scientific experts on issues relevant to the environment Issues in Ecology is supported by a Pew Scholars in Conser vation Biology grant to David Tilman and by the Ecological Society of America All reports undergo peer review and must be approved by the editorial board before publica tion Editorial Board of Issues in Ecology Dr David Tilman EditorinChief Department of Ecology Evolution and Behavior University of Minnesota St Paul MN 55 I086097 Email tilmanlterumnedu Board members Dr Stephen Carpenter Center for Limnology University of Wisconsin Madison WI 53706 Dr Deborah Jensen The Nature Conservancy ISIS North Lynn Street Arlington VA 22209 Dr Simon Levin Department of Ecology 8 Evolutionary Bi ology Princeton University Princeton NJ 08544I003 Dr Jane Lubchenco Department of Zoology Oregon State University Corvallis OR 9733 I 294 Dr Judy L Meyer Institute of Ecology University of Geor gia Athens GA 306022202 Dr Gordon Orians Department of Zoology University of Washington Seattle WA 9895 Dr Lou Pitelka Appalachian Environmental Laboratory Gunter Hall Frostburg MD 2532 Dr William Schlesinger Departments of Botany and Geol ogy Duke University Durham NC 277080340 Previous Reports Previous Issues in Ecology reports available from the Ecological Society of America include Vitousek PM J Aber RX Howarth GE Likens PA Matson DX Schindler XH Schlesinger and GD Tilman I997 Human Alteration of the Global Nitrogen Cycle Causes and Consequences Issues in Ecology No I Daily GC S Alexander PR Ehrlich L Goulder J Lubchenco PA Matson HA Mooney S Postel SH Schneider D Tilman and GM Xoodwell I997 Eco system Services Benefits Supplied to Human Societies by Natural Ecosystems Issues in Ecology No 2 Carpenter S N Caraco D L Correll R X Howarth A N Sharpley and V H Smith I998 Nonpoint Pollution of Surface Xaters with Phosphorus and Nitrogen Issues in Ecology No 3 Naeem S FS Chapin III R Costanza PR Ehrlich FB Golley DU Hooper JH Lawton RV O Neill HA Mooney OE Sala AJ Symstad and D Tilman I999 Biodiversity and Ecosystem Functioning Maintaining Natural Life Support Processes Issues in EcologyNo 4 Mack R D Simberloff XM Lonsdale H Evans M Clout and F Bazzaz 2000 Biotic Invasions Causes Epidemiol ogy Global Consequences and Control Issues in Ecology o 5 Aber J N Christensen l Fernandez J Franklin L Hidinger M Hunter J MacMahon D Mladenoff J Pastor D Perry R Slangen H van Miegroet 2000 Applying Eco logical Principles to Management of the US National Forests Issues in Ecology No 6 Adz rional Copies To receive additional copies of this report 3 each or previous Issues in Ecology please contact Ecological Society of America I707 H Street NW Suite 400 washington DC 20006 202 8338773 esahqesaorg The Issues 7 co ogy series Is aso available electronically at httpesasdscedu About lssues in Ecology Issues in Ecology is designed to report in language understandable by nonscientists the consensus of a panel of scientific experts on issues relevant to the environment Issues in Ecoogyis supported by the Pew Scholars in Conservation Biology program and by the Eco logical Society of America It is published at irregular intervals as reports are completed All reports undergo peer review and must be approved by the Editorial Board before publication Issues in Ecology is an official publication of the Ecological Society of America the nation39s leading professional society of ecologists No responsibility for the views expressed by au thors in ESA publications is assumed by the editors or the publisher the Ecological Society of America Founded in l9l 5 ESA seeks to promote the responsible application of ecological principles to the solution of environmental problems For more information contact the Eco logical Society of America I707 H Street NW Suite 400 Washington DC 20006 lSSN 10928987