Issues in Ecology Number 12 Summer 2004 Output:2070 kg/year dataeven suggest that estuarine food chains (a)Input:310 kg/vear dominated by surface runoff of pollutants may depson e8on gradation air-watere consideration is thatair-waterexchangedeliver rivers 35.7% organic pollutants to the water column in the form of 100 percent bioavailable dissolv (b)Input:57kg/year Output:557 kg/year gases,wr from riven cednydrieoied deposition by organisms. In summary,atmosphere-water interactions are critically important in the cycling and residence times of persisten outflow 13 8% and the global ocean.In remote aquatic systems or those with large surface areas tmosphericdeposition in general,and air-wate ere movalofatrazine fromlarge water bodies,and in-lakelosses are al andevenin so dominatedby degradatior are significant The nutrient statu As in case PCE ever,air-wate of water bodies and the cycling oforganic material through the tood webalso play crtical roles in determining the fate and impac pollutants induding PCBs.PAHs chlori of persistent organic pollutants in aquatic ecosystems y ECOLOGICALRESPONSES environments,persistent organicpollutantssorb(adhereor bind) toparticulateorganicmatter,and a fraction of thism Theenvironmental factorsdiscussed abovedeterminewhether into deeperth aquatic life will be ionalfactors,however,determinewhethe a In contam ants will ha high Conversely most chemicals also have threshold concentrationsbelow which no appreciable adverseeffectson aquaticlife are expected.From the perspective ofidentifying Input:11.8tons/vear Output:11.8 tons/year of organic contan mtrnentsahsofthewati (trace) taminant is ava uea时 ourial 0.2% contaminants from the water lumn sink to the bottom. nvers 76.3% Asshown in Figure4,the processesof air-w degradation75.3% ate d water of remote freshwaterex as wellas largelakes and theglobaloceans.Recent ntaminant byp 10 10 Issues in Ecology Number 12 Summer 2004 moval of atrazine from large water bodies, and in-lake losses are dominated by degradation and water outflow. As in the case of PCBs noted above, however, air-water exchange has been shown to dominate contaminant deposition and loss processes in many aquatic systems for a wide range of persistent organic pollutants, including PCBs, PAHs, chlorinated hydrocarbons (HCHs), toxaphene, and PCDDs/Fs.89 In aquatic environments, persistent organic pollutants sorb (adhere or bind) to particulate organic matter, and a fraction of this material sinks into deeper waters and sediments. Once organic pollutants are sequestered in the sediments, they are effectively removed from participating in dynamic air-water exchange. In marine waters, this process represents a major sink controlling the surface recycling and impact of persistent organic pollutants. However, the role of sinking particles and other biogeochemical processes, such as algal uptake, on the global dynamics of persistent organic pollutants has so far not been assessed. We now know that algal uptake and air-water exchange behave as coupled processes in aquatic environments.90 That is, atmospheric deposition to surface waters supports the concentration of organic contaminants in algal biomass, and the nutrient status of the waters influences how much of the contaminant is available for volatilization to the air.91 For instance, eutrophic (nutrient enriched) conditions lead to faster algal uptake and removal of contaminants from the water column as algae die and sink to the bottom. As shown in Figure 4, the processes of air-water and water-algal exchange may promote the introduction of persistent organic pollutants into the aquatic food chain. This is the dominant process for contamination of remote freshwater ecosystems and their food webs, as well as large lakes and the global oceans.92 Recent data even suggest that estuarine food chains dominated by surface runoff of pollutants may also experience food chain contamination from air-water exchange, especially where local and regional emissions create high atmospheric concentrations of pollutants.93 A key consideration is that air-water exchange delivers organic pollutants to the water column in the form of 100 percent bioavailable dissolved gases, whereas contaminants from riverine sources and in stirred and resuspended sediments may not be readily available for use by organisms. In summary, atmosphere-water interactions are critically important in the cycling and residence times of persistent organic pollutants and the contamination of food webs in lakes, estuaries, coastal waters and the global ocean. In remote aquatic systems or those with large surface areas, atmospheric deposition in general, and air-water exchange specifically, dominates total inputs. Moreover, air-water exchange is the likely mode of contaminant entry into the food chain where inputs from surface runoff are minimal, and even in some cases where local surface loadings are significant. The nutrient status of water bodies and the cycling of organic material through the food web also play critical roles in determining the fate and impact of persistent organic pollutants in aquatic ecosystems. ECOLOGICAL RESPONSES The environmental factors discussed above determine whether aquatic life will be exposed to atmospherically transported chemicals. Several additional factors, however, determine whether these contaminants will harm aquatic organisms or the animals and humans that consume them. Essentially, all chemicals can be toxic to aquatic life if the exposure concentrations are sufficiently high. Conversely, most chemicals also have threshold concentrations below which no appreciable adverse effects on aquatic life are expected. From the perspective of identifying (a) Input: 310 kg/year Output: 2070 kg/year (b) Input: 57kg/year Output: 557 kg/year Figure 5 – Mass balance of persistant organic pollutants in Lake Superior. (a) PCBs; (b) Toxaphene.88 Dry deposition includes the contaminant deposition associated with atmospheric particle fallout; wet deposition includes the removal of gas phase and particle-bound contaminant by precipitation such as rain and snowfall. dry deposition 10.4% wet deposition 40.6% rivers 35.7% other inputs 13.3% dry deposition 11.1% volatilization (net) 91.8% degradation 0% burial 5.3% outflow 2.9% wet deposition 22.0% other inputs (trace) rivers 66.9% volatilization (net) 70.0% degradation 0% burial 16.2% outflow 13.8% Input: 11.8 tons/year Output: 11.8 tons/year Figure 6 – Mass balance of Atrazine in Lake Michigan.94 Dry deposition includes the contaminant deposition associated with atmospheric particle fall￾out; wet deposition includes the removal of gas phase and particle-bound contaminant by precipitation such as rain and snowfall. dry deposition 10.4% wet deposition 22.0% rivers 76.3% gas absorption 0.3% volatilization (trace) degradation 75.3% burial 0.2% outflow 24.5%