Issues in Ecology Number 12 Summer 2004 the regional signal.for example.the southern basin of Lake organicpollutants Thatisbecausethelakeiscold nutrientpoor ha Michigan and northern Chesapeake Bay are subject to a large surface area that oovers most ofits watershed and the urban contaminationby airpollutants(PCBs,polyaromatichydrocarbons and industrialdensity in the area islow.Cold water and alarge (PAHs),mercury,and tracemetals)becauseoftheir proximity to surfacearea enhancethe lake'ssensttivity to atmospheric inputsan rializ ed and olatilization.Duringth nd coastal Iake Michig and in the air tially at about 20 Chesapeake Bay near Baltimorecompared to the regionalsigna Higheratmospheric concentrationsof pollutants are ultimately particles and sink to thelakebottom this sedimentation proces reflected in increased precipitation and dry particle inputs o contan inants to the lakeor toestuarinewate hance walerairewdhangeisthedopimamto ay are in appr the pathways tooverallwae must beevaluated in terms of The twoto five fold highe of toxaphene(an insecticide banned in the United Statesince 1990)than PCBs in facilities,pollution fromupstream river flow,and mobilization of Lake Superior has been attributed to a lower sedimentation rate sediments er w del relativeto theotherGreat Lakes me le.The half-fe PCB ofth decline in LakeSt ed to12ye twoexamples Intum,polluted waterbodies may becomesources odegonalatm be faster were it not for the higher atmosphericconcentrations d for PCBsi e air. has Lake Mi e and deliver is believed tobe minimal Although atrazine for PCBsand PAHsin the Chesapeake Bay. ershed-to-wate tend e tem are most sensitive to outary inputs the lakee mpa Aquatic and Terrestrlal Ecosystem controlled by the long watershed serve as Link residence times in wate minatontc anmdhrloratrs nt of the in the Mid-Atlantic fields is lost by runof States,forexample. to riversand lakes and nce of atmosph another 1 percent to sources of contamin quantities of this ation is hest demon pesticide that are stratedby chemicalmas applied combine with Superior, Figure4-Schematicshowingthepathways,distributionand foodwebinterac of the d prime example of an aquaticsystemin which wet and dry de significant accum They can alsobe deposited on terrestrialsystemsand thenenter aquaticsystem ulation of atrazine in ia snow melt and run-off.Pollutantscan also re-enter the atmosphere,where they can be transported and begin the cycle again. atmosph sot persister or no role in the re9 Issues in Ecology Number 12 Summer 2004 the regional signal. For example, the southern basin of Lake Michigan and northern Chesapeake Bay are subject to contamination by air pollutants (PCBs, polyaromatic hydrocarbons (PAHs), mercury, and trace metals) because of their proximity to industrialized and urbanized Chicago and Baltimore, respectively. Concentrations of PCBs and PAHs are significantly elevated in Chicago and coastal Lake Michigan78 and in the air over Chesapeake Bay near Baltimore79 compared to the regional signal. Higher atmospheric concentrations of pollutants are ultimately reflected in increased precipitation and dry particle inputs of contaminants to the lake or to estuarine waters, as well as enhanced air-water exchange of organic compounds such as PCBs and PAHs.80 Of course, the relative importance of these atmospheric pathways to overall water pollution must be evaluated in terms of other inputs, including discharges from wastewater treatment facilities, pollution from upstream river flow, and mobilization of pollutants from sediments. All three atmospheric pathways deliver pollutants directly to the water surface. This is especially significant for water bodies that have large surface areas compared to the area of the watershed that supplies their runoff. The Great Lakes and coastal seas are two examples. In turn, polluted water bodies may become sources of contaminants to the local and regional atmosphere as gases are lost from the water column to the air. This has been demonstrated for PCBs in the Great Lakes regions of southern Lake Michigan and Green Bay;81 for PCBs, PAHs, polychlorinated dibenzo-p-dioxins and –dibenzofurans (PCDD/Fs), and nonylphenols in the New York-New Jersey Harbor Estuary;82 and for PCBs and PAHs in the Chesapeake Bay.83 In contrast, many aquatic systems have large watershed-to-water area ratios. In these systems, deposits of atmospheric pollutants onto forests, grasslands, crops, paved areas, and other land surfaces in the watershed serve as important sources of runoff contamination to down-stream lakes and estuaries. This is true of most lakes and estuaries in the Mid-Atlantic States, for example. The relative impor￾tance of atmospheric deposition versus other sources of contamin￾ation is best demon￾strated by chemical mass balances (Figures 5 and 6). Lake Superior, the largest and most pristine of the Great Lakes, is a prime example of an aquatic system in which the atmosphere must play a dominant role in inputs and losses of persistent organic pollutants. That is because the lake is cold, nutrient poor, has a large surface area that covers most of its watershed, and the urban and industrial density in the area is low. Cold water and a large surface area enhance the lake’s sensitivity to atmospheric inputs and air-water exchange through outgassing or volatilization. During the 1980s, for instance, the PCB burden in Lake Superior decreased exponentially at about 20 percent a year, primarily because of outgassing losses to the air.84 Although some PCBs bind to organic particles and sink to the lake bottom, this sedimentation process does not provide permanent removal of these contaminants from the water column. Thus, water-air exchange is the dominant loss mechanism. PCBs in the water column today are in approximate equilibrium with atmospheric concentrations. The two-to-five fold higher concentration of toxaphene (an insecticide banned in the United State since 1990) than PCBs in Lake Superior has been attributed to a lower sedimentation rate and colder water temperatures relative to the other Great Lakes.85 Outgassing is an important loss mechanism for toxaphene, just as it is for PCBs, but on a longer time scale. The half-life for PCB decline in Lake Superior waters is 3.5 years compared to 12 years for toxaphene.86 Clearance of toxaphene by volatilization would be faster were it not for the higher atmospheric concentrations generated by continued outgassing of toxaphene from agricultural soils in the southern states upwind from Lake Superior. The pesticide atrazine provides a counter example to PCBs and toxaphene since it is delivered to water bodies mainly by riverine transport of agricultural runoff, and the role of atmospheric delivery is believed to be minimal. Although atrazine has a 30- to 90-day half-life in soils, transport into rivers and lakes significantly extends its half-life. Lake Michigan and other large aquatic systems are most sensitive to tributary inputs of atrazine, but the long￾term impacts on the lake environment are controlled by the long residence times in water and the slow rates at which the compound is transformed.87 Only about 1 percent of the atrazine applied to crop fields is lost by runoff to rivers and lakes and another 1 percent to aerial transport. Never￾theless, the large quantities of this pesticide that are applied combine with efficient transport, slow transformation rates, and long residence times in water to cause significant accum￾ulation of atrazine in aquatic systems. The atmosphere plays little or no role in the re￾Figure 4 – Schematic showing the pathways, distribution and food web interactions of persistent organic pollutants entering and leaving aquatic systems (modified from D. Muir). Pollutants can be bound to particles or in gaseous phase and can be deposited directly on aquatic ecosystems via both wet and dry deposition. They can also be deposited on terrestrial systems and then enter aquatic systems via snow melt and run-off. Pollutants can also re-enter the atmosphere, where they can be transported and begin the cycle again