Issues in Ecology Number 12 Summer 2004 Among the new organic contaminants of greatest concern the greatest concern PBDes areused in thousands of consume are synthetic musk fragrances,PBDE flame retardants,and products from fire-resistant textiles and upholstered fumiture to fluorinated surfactants. computers and televisions.Global demand for theseadditives Synthetic Fragrances.Syntheticmusk fragrances aresemi increased from 40,000 tons in 1992 to 67,125 tons in 1999. Theteta-andpertaBDEsTeEandne DE)are of greates wildlife TeBDE and PeBDE a icals writh erties similar to those of some PCBs.A highe synthetic fragrances areon the U.S.HPVC list but haveonly recently been studied as contaminants in any natural systemin solid phase hemical,but it may degrade in sunlight and in the ragrances used are edia form eks knov HHCB em adian A 0f1981to200.2 Flugrinated surfactants scientists have recently documented four synthetic compo nds wereproduced in19 for useas widespread contamination of wildlifeand the general human ds ted isa term early 21 ns of synth be organic mole me sin th n bonds.Th compounds.Recent measurements of the econ pounds in s region, chr disruption in fish.(Hormonally actives chemical that mimicor interfere with hormone function and can distort properties of perfluorinat ce I era rey ted anon sphys mu d PF the polveyclic musk fragrance AHINalld n dis ent.Worldwide diss nination of perfluorinated acids must therefore oour by way xylene were effectively banned from use as fragrances in 2002 of an airbomeneutral derivative that yields the free acid wheni U.S.HPV Wi espread detection of precursors of PFOSand their ngev h heUnited Sta m n nds ha aha y whic donot havetorenort how much thevuse ormanufacture.They also donot have to report any estimates on how much synthetic fragrance may ultimately be discharged into theenvironmen PFOS has been detected in the blood of ringed seals from the eco mpacts of s cond of n rthern f a,d cts of the nds in Eu in polar bear livers range fron wtweight.makniheanhaogn ram o for these fragrances. contaminant in these mammals.? duct n ve me chemicals as pote Mercury fateand in methodology,especially in the caseof fluorinated organics. globalpollutant and can bemobilized into the atmosphere from many human activities,including municipal trash incineratior ppagtpo burning of high sulfu coal (whic
5 Issues in Ecology Number 12 Summer 2004 Among the new organic contaminants of greatest concern are synthetic musk fragrances, PBDE flame retardants, and fluorinated surfactants. Synthetic Fragrances. Synthetic musk fragrances are semivolatile and lipophilic (literally “fat-loving” because they are attracted to fatty tissues) compounds that are added to a wide range of personal care products, including perfumes, cosmetics, soaps, and shampoos as well as laundry detergents.15 These synthetic fragrances are on the U. S. HPVC list but have only recently been studied as contaminants in any natural system in this country. The most common synthetic fragrances used are two nitro musks called musk xylene and musk ketone and two polycyclic musks known as HHCB (hexahydrohexamethylcyclopentabenzopyran) and AHTN (hexamethyltetraline). In Europe, approximately 6,500 metric tons of these four synthetic compounds were produced in 1999 for use as consumer product additives.16 In the early 1980s, concentrations of synthetic musk fragrances were discovered in animal tissues for the first time. Since then, there has been an increasing awareness of the ubiquitous distribution and possible toxicological effects of these compounds. Recent measurements of these compounds in wastewater effluent and in air and water in the Great Lakes region, for instance, have illustrated that ecological exposures are chronic and likely to be increasing.17 This is cause for concern because both HHCB and AHTN have been shown to exhibit hormonal disruption in fish.18 (Hormonally active substances are chemicals that mimic or interfere with hormone function and can distort normal reproductive development, alter behavior, and impair disease resistance in wildlife and humans.) Several studies with cell cultures indicate that musk xylene, musk ketone, p-aminomusk xylene (a major breakdown product of musk xylene), and the polycyclic musk fragrance AHTN all demonstrate estrogenic activity in laboratory tests.In Europe, musk ketone and musk xylene were effectively banned from use as fragrances in 2002 because of their reported toxicities.19 Although HHCB and AHTN are both on the U. S. HPVC list, their use in personal care and household products is privileged information in the United States, and companies that use them do not have to report how much they use or manufacture. They also do not have to report any estimates on how much synthetic fragrance may ultimately be discharged into the environment. Because of this, ecological impacts of these compounds can only be identified through field and toxicological studies conducted long after exposures have begun. Fortunately, thanks to the intense interest in the fate and impacts of these compounds in Europe, analytical methods have been developed and standards are available for these fragrances. For the vast majority of high production volume chemicals identified as potentially bioaccumulative and persistent, however, there are no trace analytical methods available for tracking their fate and impacts.20 Many of the recently initiated measurements of organic chemicals have been made using advances in analytical methodology, especially in the case of fluorinated organics. Flame retardants. Among the newly emerging chemical contaminants of aquatic environments, the PBDE flame retardants and the perfluorinated surfactants discussed below have generated the greatest concern. PBDEs are used in thousands of consumer products from fire-resistant textiles and upholstered furniture to computers and televisions. Global demand for these additives increased from 40,000 tons in 1992 to 67,125 tons in 1999.21 The tetra- and pentaBDEs (TeBDE and PeBDE) are of greatest concern, and their concentrations are increasing in humans and wildlife.22 TeBDE and PeBDE are multimedia chemicals with physical properties similar to those of some PCBs. A higher brominated product, decabromodiphenyl ether (DecaBDE), is a solid phase chemical, but it may degrade in sunlight and in the tissues of fish to these lower brominated multimedia forms.23 Researchers measured a nine-fold increase in PBDEs in the tissues of ringed seals from the western Canadian Arctic over the period of 1981 to 2000.24 Fluorinated surfactants.Scientists have recently documented widespread contamination of wildlife and the general human population with perfluorinated acids.25 “Perfluorinated” is a term used to describe organic molecules that are fully fluorinated, meaning fluorine atoms have replaced all hydrogen atoms in the carbon-hydrogen bonds. The most widely known perfluorinated acids are perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA); however, similar compounds having longer or shorter perfluorinated chains are also produced or exist as impurities within manufactured formulations. These important industrial chemicals fall into the category of surfactants because they are surfaceactive agents that repel water and oil or resist heat or other chemicals. The major use of PFOS is in treating fabric surfaces for stain resistance. The existing database describing physical properties of perfluorinated acids, including PFOS and PFOA, is severely limited because of their anomalous physical and chemical behavior. The properties of PFOS and PFOA suggest that they are poor candidates for long-range airborne transport, yet they have been discovered throughout the global environment. Worldwide dissemination of perfluorinated acids must therefore occur by way of an airborne neutral derivative that yields the free acid when it degrades.26 Widespread detection of precursors of PFOS and PFOA in the air in North America is providing increasing evidence that this is indeed the means by which these nonvolatile compounds have become such widespread contaminants.27 Over the past decade, researchers have found PFOS in birds, fish, and marine and land mammals around the world. For example, PFOS has been detected in the blood of ringed seals from the northern Baltic Sea, the eastern Canadian arctic, and Svalbard; the blood and liver of northern fur seals from Alaska; and the livers of polar bears from northern Alaska.28 PFOS concentrations in polar bear livers range from 1 to 5 micrograms per gram of tissue (wet weight), making it the most prominent organohalogen contaminant in these mammals.29 Mercury Mercury is a metallic element (Hg) that has been extracted for centuries from sulfide ore or cinnabar (HgS). It has become a global pollutant and can be mobilized into the atmosphere from many human activities, including municipal trash incineration, burning of high sulfur coal (which contains cinnabar) in coalfired power plants, metal smelting, chlorine-alkali plants, cement
Issues in Ecology Number 12 Summer 2004 making,and gold extraction,as well as from use of mercury- mercury (Hg)s Theremaining balanceofthe mercury exists a based fungicides in latex paints and the paper and pulpindustry. RGM,as particulate complexes of divalent mercury,and in the Mercury in itselemental state has low reactivity and a long organic form asmonomethylmercury. atmospheric resi a s have be deposited on land and waterby snow and rainfall.Particulate threefold sincethe beginningof the industrial ageThisestimate forms of mercury fall as dry deposition. has been supported by data from several field-based studies of and wetland Themass balar at5 00to6,0 cury input by sited on the have been declining since then It has been estimated that oceans via runoff.Refinement of mass balance calculations human activities contribute 70to8percent of the total annual has led some researchers to conclude that dry deposition of mercury emissions to the atmosphere and that more than95 percent of mercury vapor in the atmosphere exists aselementa he total mercury input to the ocear ospheric nitrogen compounds(the major chemical forms of atmosphericnitrogen ReducedNitrogen Agricultural Ammonia/Ammorium(NH/NH) Livestockwaste(volatilizedNH) ion&landdearing Urban Rural(non-agricultural) atertreatment(volatilizedNH) Nahdamnimamecaicomutes Wastew Biomassbuming(forestandgrassfires) Dust and aerosols Oxidized Nitrogen Urban Rural (non-agricultural) NitrogenOxides(NO/NO./NO.) Biomassbuming PhotolysisofN,O(air,land,water) 8ay Agricultural Natural ??=possible,but little knownabout,sources 6
6 Issues in Ecology Number 12 Summer 2004 making, and gold extraction, as well as from use of mercurybased fungicides in latex paints and the paper and pulp industry. Mercury in its elemental state has low reactivity and a long atmospheric residence time, thus allowing it to be mixed in the atmosphere on a global scale, while the oxidized forms are removed by wet and dry deposition.30 Oxidized reactive gaseous mercury (RGM), for example, is very soluble in water and is effectively deposited on land and water by snow and rainfall. Particulate forms of mercury fall as dry deposition.31 The total mass of mercury in the atmosphere has been estimated at 5,000 to 6,000 metric tons, and approximately half of that was generated by human activities.32 Atmospheric concentrations of mercury peaked in the 1960s and 1970s and have been declining since then.33 It has been estimated that human activities contribute 70 to 80 percent of the total annual mercury emissions to the atmosphere and that more than 95 percent of mercury vapor in the atmosphere exists as elemental mercury (Hg).34 The remaining balance of the mercury exists as RGM, as particulate complexes of divalent mercury, and in the organic form as monomethyl mercury.35 Although atmospheric concentrations have been declining for several decades, mass balance calculations that relate net mercury accumulation in the atmosphere with net loss indicate that human inputs of mercury to the atmosphere have increased threefold since the beginning of the industrial age.36 This estimate has been supported by data from several field-based studies of dated sediment cores from lakes and wetlands.37 The mass balance calculations also suggest that a legacy of mercury inputs is stored in terrestrial landscapes since only 5 percent of the atmospheric mercury deposited on the land is carried to the oceans via runoff. Refinement of mass balance calculations has led some researchers to conclude that dry deposition of RGM from the atmosphere can represent up to 35 percent of the total mercury input to the ocean.38 Table 1 - Natural and anthropogenic sources of atmospheric nitrogen compounds (the major chemical forms of atmospheric nitrogen compounds are the reduced, oxidized and organic forms). Sources (in approximate order of importance) Agricultural Livestock waste (volatilized NH3 ) Chemical fertilizers (volatilized NH3 ) Biomass burning Dust from deforestation & land clearing Urban & Rural (non-agricultural) Wastewater treatment (volatilized NH3 ) Fossil fuel combustion (from automobile catalytic converters) Natural Biomass burning (forest and grass fires) Decomposition of organic matter Dust and aerosols Volcanism Urban & Rural (non-agricultural) Fossil fuel combustion mobile & stationary engines powerplants & industrial Natural Biomass burning Lightning Photolysis of N2 O (air, land, water) Dust and aerosols generated by storms Microbially-mediated volatilization Agricultural Dust and volatilization of wastes?? Urban & Rural (non-agricultural) Dust/aerosols?? Natural Atmospheric photochemical and lightning Biological production in oceans?? ?? = possible, but little known about, sources Chemical Form Reduced Nitrogen Ammonia/Ammonium (NH3 /NH4 + ) Oxidized Nitrogen Nitrogen Oxides (NO/NO2 - /NO3 - ) Organic Nitrogen (Dissolved and Particulate)
Issues in Ecology Number 12 Summer 2004 while the mass balancehas identified themagnitudeof the flux to the north atlantic Ocean basin is approximately 11.2 various fluxes and pools of mercury and possible pathways for teragrams(trillion grams)per year and accounts for 46 to57 contamination ofland and water,it doesnotprovideinformation percent ofits"new"nitrogen input.This is comparableto the on the true partitioning of various forms of mercury in the new"nitrogen inputs delivered totheoceanby rivers. Indeed ngo orth American continenta atmosphere excee nitrogen se arriving by rivers potential for atmospheric deposition of mercury.for example depends upon the distribution of various forms of mercur in Table2-Estimated contributionsofatmosphericdeposition of nitrogen to"new"nitrogen inputs in diver se estuarine,coasta d,th sources,whil and opene sources (we and/or ) pherically dep instrumentation allows for real-time measur mercury as RGM,particulate mercury,and gaseous mercury at the picogram or sub-picogram level.The simultaneous mea surement of the RECEIVINGWATERS DEPOSIED ns,an t mercury near point sources,at din remote are BalticSea(Proper) Nutrients W+D,I 1060%W,1 Waquoit Bay,MA.USA 29%W,1+0 and mar sphericdeposition,either as rain Ne York Bicht USA plant growth (primary production)beca setheir concentratior Bamnegat Bay,USA 40%W,I+O control thegrowth of algae,which form thebaseof aquatic food Chesapeake Bay,USA deRiver,MD,US trace s such a ngan copper, d, d um By 409%W+D,】 mo Sarasota/Tampa Bay,FL,USA 30%W+D,1 ine estuarine anda few fresh Mississippi River Plume,USA 2-5%W+D,I+0 Nitrog deposition.In the marineenvironment iron hasbeen thesubject Excessivenitrogen loading toestuarineand coastalwaters is sing interest because recent studies have shown that this sprimary pro he open nd y,hyp PepOshaeecivedatentiominfreshwatere and associated habitat loss which are most oftenphosphoruslimited. As asignificant source of"new"nitrogen,atmospheric Early studiesonhuman-generated contaminantsdelivered to deposition is both a local and regional issue becauseemission ystems via ere id aftecte am-an my. id. nosty generat ing adiv in the Unitedstates with reducdnit array of human activities and,toa lesser extent natural pro making up the rest.Rapidly expanding livestock(swine,cattle (Table 1).These compounds include inorganic reduced forms andpoultry)operations in the Midwest and Mid-Atlanticregions have accelerat ds).Du fold d H)d .which c deposition at the National deposition ranges from 400 to more than 1,200 kilograms per Acid Deposition Programnetwork site in Duplin County,North hectareeach year and represents 40percent Carolina,alocation that hasexperienced a rapid rise in animal waters(a opean operationsd e2)."Onalargerscale,nitrogen
7 Issues in Ecology Number 12 Summer 2004 While the mass balance has identified the magnitude of the various fluxes and pools of mercury and possible pathways for contamination of land and water, it does not provide information on the true partitioning of various forms of mercury in the atmosphere. This information is vital for predictive modeling of global mercury cycling and the effectiveness of mercury reduction strategies, and it continues to be an active topic of research. The potential for atmospheric deposition of mercury, for example, depends upon the distribution of various forms of mercury in emissions and plumes. Both particulate mercury and RGM are likely be deposited closer to their local or regional sources, while gaseous mercury is expected to be transported long range and have a one to two year residence time in the atmosphere. Current instrumentation allows for real-time measurement of atmospheric mercury as RGM, particulate mercury, and gaseous mercury at the picogram or sub-picogram level. The simultaneous measurement of these various atmospheric forms has allowed for analysis of phase distribution of mercury near point sources, at offshore oceanic stations, and in remote areas. Nutrients A significant and increasing source of nutrients to freshwater and marine ecosystems is atmospheric deposition, either as rain or snow or as dry deposition of particles and gases. The nutrients that have received most attention are those that are essential for plant growth (primary production) because their concentrations control the growth of algae, which form the base of aquatic food webs. These nutrients include nitrogen, phosphorus, iron, and trace elements such as zinc, manganese, copper, cobalt, molybdenum, boron, and selenium. By far, the greatest attention has been focused on nitrogen because it is the most common limiting nutrient in marine, estuarine, and a few freshwater systems. Nitrogen is also a highly significant component of atmospheric deposition.39 In the marine environment, iron has been the subject of increasing interest because recent studies have shown that this metal limits primary production in some open ocean waters.40 Iron can also act synergistically with nitrogen to enhance algal production in coastal and ocean waters.41 Both nitrogen and phosphorus have received attention in freshwater ecosystems, which are most often phosphorus limited. Early studies on human-generated contaminants delivered to ecosystems via the atmosphere identified nitrogen as a major nutrient constituent of both rain- and dry-fall.42 Atmospherically deposited nitrogen provides aquatic systems with a variety of biologically available nitrogen compounds, reflecting a diverse array of human activities and, to a lesser extent, natural processes (Table 1). These compounds include inorganic reduced forms (ammonia, ammonium), inorganic oxidized forms (nitrogen oxides, nitrate, nitrite), and organic forms (urea, amino acids, and unknown compounds). During the past century, atmospherically deposited nitrogen has increased tenfold, driven by trends in urbanization, industrial expansion, and agricultural intensification.43 Nitrogen deposition ranges from 400 to more than 1,200 kilograms per hectare each year and represents from 10 to more than 40 percent of the “new” nitrogen coming into North American and European inland and coastal waters (Table 2).44 On a larger scale, nitrogen flux to the North Atlantic Ocean basin is approximately 11.2 teragrams (trillion grams) per year and accounts for 46 to 57 percent of its “new” nitrogen input.45 This is comparable to the “new” nitrogen inputs delivered to the ocean by rivers.46 Indeed, in the waters of the North American continental shelf, nitrogen inputs via the atmosphere exceed those arriving by rivers.47 Excessive nitrogen loading to estuarine and coastal waters is the key cause of accelerating eutrophication and the associated environmental consequences, including algal blooms, decreases in water clarity, toxicity, hypoxia or anoxia (oxygen-depleted or “dead zones”), fish kills, declines in submerged aquatic vegetation, and associated habitat loss.64 As a significant source of “new” nitrogen, atmospheric deposition is both a local and regional issue because emission sources may be situated either within or far outside affected watersheds.65Nitrogen oxides, mostly generated by fossil fuel combustion, account for 50 to 75 percent of nitrogen pollution in the United States, with reduced nitrogen and organic nitrogen making up the rest. Rapidly expanding livestock (swine, cattle and poultry) operations in the Midwest and Mid-Atlantic regions have accelerated the generation of nitrogen-enriched wastes and manures, and 30 to 70 percent or more of this may be emitted as ammonia (NH3 ) gas. This has led to local and regional increases in ammonium (NH4 + ) deposition, which can be seen in a twodecade analysis of atmospheric nitrogen deposition at the National Acid Deposition Program network site in Duplin County, North Carolina, a location that has experienced a rapid rise in animal operations during this period (Figure 3).66 In Western Europe, where animal operations have dominated agricultural production for the Table 2 - Estimated contributions of atmospheric deposition of nitrogen to “new” nitrogen inputs in diverse estuarine, coastal and open ocean waters. When identified, the sources (wet: W and/or dry deposition: D) and chemical forms (inorganic: I and/ or organic: O) of atmospherically deposited nitrogen are indicated48. RECEIVING WATERS Baltic Sea (Proper)49 ~ 30 W+D, I Kiel Bight (Baltic)50 40% W, I North Sea (Coastal)51 20-40% W+D, I Western Mediterranean Sea52 10 60% W, I Waquoit Bay, MA, USA53 29% W, I+O Narragansett Bay, USA54 12% W, I+O Long Island Sound, USA5 20% W, I+O New York Bight, USA56 38% W, I+O Barnegat Bay, USA57 40% W, I+O Chesapeake Bay, USA58 27% W, I+O Rhode River, MD, USA59 40% W, I+O Neuse River Estuary, NC, USA60 35% W, I+O Pamlico Sound, NC, USA61 ~ 40% W+D, I Sarasota/Tampa Bay, FL, USA62 30% W+D, I Mississippi River Plume, USA63 2-5% W+D, I+O PERCENT OF “NEW” NITROGEN THAT IS ATMOSPHERICALLY DEPOSITED
Issues in Ecology Number 12 Summer 2004 betterpartof thepast century,ammonium is themost abundant months whenplant nutrient demands are highest,phosphorus formof atmospherically deposited nitrogen inputs fromsurface runoff are minimal.At the same time,dry Phosphorus is acomponent of atmospheric deposition,buti and windy conditions ter dtofavor und to dust parti tha such as dust and periods.Further windblown soils. Accordingly,in 450 ● agricultu reg 400 45 ded r20.63 .73 and 4 350 arid regionswhere 35 osition rates in soils are readily 30 various geo transported by graphic regions 25 ve to to bemost highly 量200 ●。 enriched with case of nitrogen phosphorus. 150 ● emissions from Even in these 100 1 菌营鹿蓉套富喜落空索雷 菌雪商营喜店草落露 rally highes gen inputs Year Year Fromaneco In the case logical perspec Figure3-A20yearNationalAcdDeposit ofiron and other epo tive, NH metals,atmos phosphorus thannitrogen is required for supplies of these nutrients to balanced plant growth.Therefore,inphosphorus-limited lakes, coastaland open ocean waters.Iron can be transported over rivers,reservoirs,and evensomemarinesystems suchas theeastem great distances,as demonstrated by the iron-enriched Saharar Mediterranea Sea,atmosphericphosphorus inputs can bea gtutiemt contributes from 5to 15 ne of the externally su rated b volcanicemissions and by various continental pollution sources concentrations of total dissolvedphosphorus in rainfall ranged including power plant,automotive,and industrialemissions.? perliter at nine sites,and total wet While thereisuncertainty about the hemicalforms and behavio deposition rangec ms per square mete ally deposited iron thatenters theocean,there i rninaboutperntofhewholelke'sphosphorus budget.Inalpine Lake Tahoeon theCalifornia-Nevada border, EMISSIONDEPOSIIONANDEATEPROCESSESANDSCALES 25percentofannualpho-phorusinputs whi hep The th ajor atmosph icpathways by whichper nt of the enter v es su as sea are as (1)wet den osition via rain,snow,and fog,(2)dry of total phosphorus loadings to water bodies from all sources.It deposition of particles,and (3)gaseous exchange between the remains unknown whether phosphorus transported into aquatic air and water(Figure4).Many urbanindustrialcentersarelocated systems by river or air differs in its availability for stimulating on or near coastal estuaries and the Great Lakes.Emissionso plant growth ants into theu tmosphere are refle varies withthe ericdep osition of both nit P s Fo ample,dur onthiandaoeratr reover and ab
8 Issues in Ecology Number 12 Summer 2004 better part of the past century, ammonium is the most abundant form of atmospherically deposited nitrogen.67 Phosphorus is a component of atmospheric deposition, but it typically occurs at concentrations less than a few percent those of nitrogen.68 This is especially true in regions where wet exceeds dry deposition, since phosphorus is usually bound to particles such as dust and windblown soils. Accordingly, in agricultural regions where phosphorus is applied as a fertilizer, or in arid regions where soils are readily transported by wind, atmospheric deposition tends to be most highly enriched with phosphorus.69 Even in these situations, phosphorus inputs rarely exceed nitrogen inputs. From an ecological perspective, however, phosphorus may be of considerable importance since far less phosphorus than nitrogen is required for balanced plant growth.70 Therefore, in phosphorus-limited lakes, rivers, reservoirs, and even some marine systems such as the eastern Mediterranean Sea, atmospheric phosphorus inputs can be a significant nutrient source. For example, in Mid-western lakes, including the Great Lakes, atmospheric deposition of phosphorus contributes from 5 to 15 percent of the externally supplied phosphorus.71 In a recent study of the Mid-Atlantic coastal region, concentrations of total dissolved phosphorus in rainfall ranged from 4 to 15 micrograms per liter at nine sites, and total wet deposition ranged from 3.9 to 14 milligrams per square meter per year across the region.72 Annual total phosphorus loading to Lake Michigan in 1976 was 1.7 million kilograms per year, representing about 16 percent of the whole lake’s phosphorus budget.73 In alpine Lake Tahoe on the California-Nevada border, atmospherically deposited phosphorus accounts for approximately 25 percent of annual phosphorus inputs, while in the phosphoruslimited eastern Mediterranean Sea, atmospheric deliveries represent about 10 percent of the “new” phosphorus. Overall, it appears that airborne phosphorus typically accounts for 10 to 20 percent of total phosphorus loadings to water bodies from all sources. It remains unknown whether phosphorus transported into aquatic systems by river or air differs in its availability for stimulating plant growth. Atmospheric deposition of both nitrogen and phosphorus varies with the seasons. For example, during the dry summer months when plant nutrient demands are highest, phosphorus inputs from surface runoff are minimal. At the same time, dry and windy conditions tend to favor transport of dust. Since phosphorus is often bound to dust particles, it is possible that atmospherically deposited phosphorus assumes a more important role as a source of “new” phosphorus during these crucial growth periods. Further investigation and quantification are needed of absolute and seasonal atmospheric phosphorus deposition rates in various geographic regions relative to other phosphorus input sources. In the case of nitrogen, emissions from agricultural, urban, and industrial sources are generally highest in summer. In the case of iron and other metals, atmospheric deposition, mainly in the form of dust, is a major source of “new” supplies of these nutrients to coastal and open ocean waters.74 Iron can be transported over great distances, as demonstrated by the iron-enriched Saharan dust storms that travel thousands of kilometers over the subtropical North Atlantic to “fertilize” iron-deficient and nutrient-poor waters as far away as the Caribbean Sea and the Eastern Seaboard of the United States.75 Iron and trace metals are also generated by volcanic emissions and by various continental pollution sources, including power plant, automotive, and industrial emissions.76 While there is uncertainty about the chemical forms and behavior of atmospherically deposited iron that enters the ocean, there is little doubt that it represents an important source of “new” iron in an environment that is otherwise free of external iron inputs.77 EMISSION, DEPOSITION, AND FATE PROCESSES AND SCALES The three major atmospheric pathways by which persistent organic pollutants enter water bodies such as the Great Lakes, Chesapeake Bay, other coastal estuaries, and the coastal and open sea are as (1) wet deposition via rain, snow, and fog, (2) dry deposition of particles, and (3) gaseous exchange between the air and water (Figure 4). Many urban industrial centers are located on or near coastal estuaries and the Great Lakes. Emissions of pollutants into the urban atmosphere are reflected in elevated local and regional pollutant concentrations and also in areas of intense localized atmospheric deposition that are over and above Figure 3– A 20 year National Acid Deposition Program Network (NADP) nitrogen depositional record for monitoring station NC35 in Duplin Co., NC, showing increases in NH4 + deposition over time. This area has experienced an increase in livestock operations during this period