Climate Change in the Champlain Basin - The Nature Conservancy

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Climate Change in the Champlain BasinV. Findings: Impacts onconservation targetsThe Nature Conservancy’s conservation actionplan “Conserving Lake Champlain’s BiologicalDiversity,” completed in 2005, provides aframework for evaluating local environmental threatsposed by climate change. Referencing key ecologicalattributes identified in the report, we focus on componentsof the Champlain Basin ecosystem that arepotentially vulnerable to future changes in temperatureand hydrology:• Tributary systems• Wetlands and shorelines• Littoral zone• The deep lake• Native fish assemblages• Native mussel assemblagesManagement strategies that effectivelyaddress future climate-driven threats tothese systems will help conserve LakeChamplain’s full diversity of species andhabitats.Tributary systemsTributary systems encompass a diversityof features, including headwatersand adjacent wetlands, floodplains anddeltas. They are important sources of water, nutrients andsediment for the lake, and harbor a wealth of living things.Riverine habitats are important refuges for rare mussels andthe basin’s designated rare, threatened or endangered fish(including eastern sand darter, northern brook lamprey,American brook lamprey and stonecat). They also providespawning grounds and nurseries for walleye, sturgeon, redhorses,quillback, Atlantic salmon and other predominantlylake-dwelling fish that spend part of their life cycle in runningwaters (R. Langdon, personal communication).Future warming may alter the geographic ranges of aquaticspecies that currently live in the Champlain Basin, but predictingthose changes is hampered by topographic variablesas well as a lack of temperature tolerance data for manyorganisms. However, an analysis of baseline water temperaturesfrom 1,700 U.S. Geological Survey stream-monitoringstations concluded that stream habitat for 57 cold- andcool-water fish such as brook trout and slimy sculpin coulddecline by as much as 50% in the United States as temperaturesrise during this century (Eaton and Scheller, 1996).Rising temperatures may also exacerbate late-summer lowflows by increasing evapotranspiration through vegetationand evaporation from land and water surfaces. The U.S.Fish and Wildlife Service’s New England aquatic baseflowpolicy currently defines 0.5 cfs/square mile as the Augustmedian flow of New England streams and also considers itto be the threshold required to maintaincritical life cycle functions of many floraand fauna (Lang, 1999). In addition,reduced water volumes can increase theconcentrations of toxic pollutants thatcollect in riparian wetlands (EnvironmentCanada, 2009).The quantity of precipitation that falls ona watershed over a given time periodstrongly affects stream discharge (Figure6), but so do physical features of thelandscape. The basin is heavily forested,which slows the rate at which watermoves through it, but its fine-grainedsoils have generally low permeability,and many of the streams have steep gradients, features thattend to increase runoff rates.Courtesy of J. Ellen MarsdenThe timing of precipitation and runoff is also important. Historically,the highest levels of stream flow in the watershedhave occurred in spring under the influence of snowmelt,low evaporation rates and saturated ground. Dischargerecords maintained by the United States Geological Surveyshow that many rivers north of the 44th parallel have experiencedprogressively earlier winter/spring high flows duringthe 20th century (Hodgkins and Dudley, 2006a). This isconsistent with regional warming trends and suggests thata similar change may be expected for Lake Champlain.20
What natural resource managers can expect and doby the floodwaters that are nowconfined within their channelscan result in greater channel incisionand bank erosion. Lack offloodplain access also reducessediment and nutrient storagefunctions at a watershed scale,because floodplains often storesediment and the nutrient pollutionthat may be bound to it(Vermont ANR, 2008). Projectedincreases in the intensityof storm events would be likelyto further exacerbate theseproblems.Figure 6. Annual river discharge (cubic feet per second)and total annual precipitation (inches) in theAdirondacks, 1929–2002 (after Stager et al., 2009).As winters become warmer and snow is more oftenreplaced by rain, river flow may become higher and morevariable during the cold months of the year, increasing themagnitudes and frequencies of mid-season floods and icejams. However, the overall reduction of snow cover mightalso reduce the volume of potential meltwater left to bereleased later on during spring floods.In a warmer future, seasonal buildup of river ice shoulddecrease, with important consequences for the ecology ofriver corridors. More frequent winter rains could increasethe number of runoff pulses and ice releases throughoutthe season, but less snow and ice overall could also reducethe buildup of meltwater behind ice barriers and weakenspring flows. Furthermore, lighter ice loads and dischargevolumes during early spring could reduce the scouringeffects of fast-moving ice, which is central to the developmentand maintenance of riparian wetlands and “icemeadow” habitats.Normally, high river flows spill into floodplains, whichprovide natural water-storage capacity and sustain riparianforests and wetlands (Opperman et al., 2009). But widespreadchannel modifications (e.g., straightening, dredging,armoring and berming) in the Champlain Basin, especiallyin the more agrarian regions of the valley, have largelyeliminated river access to floodplains that would otherwiseattenuate ice and high water flows during spring runoff andlarge storm events. The increase in stream power causedMore than 20 Lake Champlaintributaries are considered“impaired,” under state andfederal standards, by stormwaterpollution, which carries nutrients that cause undesirableplant and algae growth (LCBP, 2008; Vermont ANR, 2010).Heavier river discharge can also deliver more toxic pollutants,including agricultural chemicals and atmosphericcontaminants, to riparian and lake habitats. Much is alreadybeing done to reduce sources of river-borne pollutants,but progress is offset by inadequate implementation ofbest management practices and by continuing landscapedevelopment. In 2002, for example, a phosphorus-loadingbudget was established for Lake Champlain as part of aNew York/Vermont pollution control plan, but as of 2005–2006, streams still delivered more than twice the targetamount of phosphorus (ca. 1,000 metric tons) into the lake(LCBP, 2008).Records dating back to 1991 show a direct correlation betweenriver flow and nonpoint phosphorus loading (LCBP,2008), with the amount of phosphorus that is carried bylocal tributaries rising dramatically when flow rates andvolumes increase (Smeltzer and Simoneau, 2008; Sullivanet al., 2009). Concentrations in Lake Champlain itself donot increase by the same proportion, for various complexreasons (LCBP, 2008), but anticipated increases in annualprecipitation and winter runoff in the watershed suggestthat nutrient delivery to the lake is likely to be enhanced byfuture climatic change.21
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