Western Lake Erie Clean Up:phosphorus Control or Zebra Mussel Effect?-aquatic Science Stb No.aqss-11 PDF
Preview Western Lake Erie Clean Up:phosphorus Control or Zebra Mussel Effect?-aquatic Science Stb No.aqss-11
1 Ontario MINISTRY OF ENVIRONMENT AND ENERGY SCIENCE AND TECHNOLOGY BRANCH AQUATIC SCIENCE SECTION STBTechnicalBulletinNo.AqSS-1 August1996 Western Lake Erie Clean up: Phosphorus Control or Zebra Mussel Effect? - An Example of the Value of Long-term Data BACKGROUND These two interventions, one planned (phosphorus loadingcontrol)andtheotherinadvertent(zebramussel Threedecadesago,LakeEriegainedareputation invasion) have had a combinedimpact on Lake Erie. in thepopularpress as a "deadLake". Becauseitwas When only relatively recent data (5-10 years) are "first in line" to receive most of the phosphorus analyzed, thereis a tendancyto associatethe present deliveredtothelakefromthemajorsources,LakeErie's improvedstateofwaterqualityofwesternLakeErie(in western basin topically experienced all ofthe classic regardstoclarity,"greenness",levelofalgalinterference symptomsofadvancedeutrophyduringthe 1960'sand withwatertreatment,recreationaluse,etc.)solelywith early 1970's. In particular,thewestern basin suffered thearrivalofzebramussels. A much longertermdata frompea-soupyalgalbloomswhichrenderedthewater set is needed toplace the water quality effects ofthe turbidandmalodorousandinterferedwithrecreational zebramusselinvasioninitsproperperspectiverelative useandtreatmentformunicipaldrinkingwatersupplies. to the effects of phosphorus loading control. The Thisconditionwastheresultofexcessiveproductionof purposeofthis"bulletin""istoillustratethedifferences phytoplankton in response to phosphorus loadings of ininterpretationofchangethatresultwhenevaluations 30-40 metric tons per day. originating in municipal ofashort-term(5-10years)datarecordarecontrasted sewageandagriculturaldrainage. withalonger-termdataset. Thedatarecordconsistsof nearly 30 years ofplanktonic Chlorophyceae (green SignificantprogresstowardsrehabilitationofLakeErie algae) density data (and two dominant chlorophyte was initiated under the Great Lakes Water Quality genera, Pedwstrum and Scenedesmns) measured in Agreements of 1972 and 1978 (incl. amendments of samplescollectedweeklyfromtheUnionWaterSystem 1983 and 1987). whereby target phosphorus loading intake (north shore, western Lake Erie, at Kingsville, reductions were set for both Canada and the United Ontario).TheChlorophyceaewereusedinthisanalysis States. As aresult,declinesinphytoplanktonbiomass because representatives of this group have been havebeenreportedduringthepasttwodecades abundanthistorically(secondonlytodiatoms)andare likelymorefoodwebfunctionalthanthelargecolonial Thearrivalofthezebramussel(Dreissenapolymorpha) diatomswhichhavedominatedwesternLakeErieover inthelate 1980'sincreasedthepotential"improvement" the past several decades. Planktonic chlorophues are ofwaterqualityowingtoitsfilterfeedinghabit. Several thereforeveryrelevanttoquestionsof"eutrophication investigatorshavereporteddramaticallyimprovedwater reversal" and trophic interactions as potentially clarity and significant reductions in Lake Erie influenced by changes in phosphorus loading and phytoplanktonafterthearrivalofzebramussels. establishmentoffilterfeedingzebramussels. R ESULTSAND DISCUSSION summerdensities ofScenedesmus andPediastrum of about400and2000A.S.U.ml"1 respectively,duringthe , During the mid-1980's, prior to the arrival of late1960's(Fig.2).Thepatternsofdeclineinthesetwo zebramusselsinwesternLakeErie,theChlorophyceae generawereclearlydifferent.Scenedesmusestablished developedsummer-fallmaximaof100-150A.S.U.ml"1 anewbaselinedensityafter1971 whichpersisteduntil separatedbywinter-springlowsof<20A.S.U.ml"1 (Fig. 1). After the establishment of zebra mussels in late 1988,thesummerpeakswereeithernon-distinguishable (1990 and 1992) or greatly reduced in duration and magnitude (1989, 1991, 1993 and 1994). There is a clear inflection point in the cumulative sum oftotal chlorophytedensity(Fig. 1) which coincides with the establishmentofthemusselpopulation. 2500 i 150 66 70 74 78 82 86 90 94 Year Fig.2 Monthlymean densitiesofScenedesmusand Pediastruminsamplescollectedweeklythroughthe Union WaterSystemintake 1980,afterwhichtherewasafurtherdeclinetoverylow Fig. 1 Monthly mean andcumulative densities of values in 1983, some minor increases again through total Chlorophyceae in samples collected weekly 1984-87beforethecrashattributedtozebramusselsin throughtheUnion WaterSystemintake, 1984-1994. 1988-89.Incontrast,densitiesofPediastrumremained high through the 1967-1977 period and then steadily The timing ofthis effect is consistent with the sharp declinedtotracelevelsby 1984(Fig2). Atthisscale,a declines reported in chlorophyll a and total case for a zebra mussel impact on Pediastrum is not phytoplankton for this location. The annual average plausible and for Scenedesmus only a little more chlorophyte density decreased from 38.3 A.S.U. ml"1 convincing. Clearly, the most important trend is the duringthepre-zebramusselyearsof1984-1988toonly progressivedeclinefromtheveryhighvaluesofthelate 5.7A.S.U. ml"1 duringthepostzebramusselyears of 1960's and early 1970's for both genera. Annual 1989-1994, a reduction of85%. There is reasonable average total chlorophyte density during the 3-year certainty that this decline was caused by Uie filter- periods 1970-1972and1983-1985was708A.S.U.ml"1 feedingzebramussels. and 44 A.S.U. ml"1, respectively. It is significant that this major reduction (94%) was achievedpriorto the Two "weedy" genera, Pediastrum and Scenedesmus arrivalofzebramussels. havedominatedthechlorophyteclass fortheperiodof record. Annual averages for both Scenedesmus and A plot ofthe cumulative sum(CUSUM) ofthetotal Pediastrum weresignificantly(P<0.05)lowerduring chlorophytedensities(Fig.3)revealsaninflectionpoint the post zebra mussel period than during pre-zebra in theearly 1970's coincidingwith mitialreductionof musselperiod(t-testsonlog-transformedA.S.U.values phosphorus at sewage treatment plants and with afterremovalof1988becauseitwasatransitionyear). legislation controllinglevelsofphosphatesin laundry detergents. A more important inflection point in the Although these apparent effects ofthe zebra mussel CUSUM curve appears at about 1981 (Fig. 3). This invasion on Pediastrum and Scenedesmus were accelerated rate of decline of the western basin dramatic, a longer termviewofthe data set suggests Chlorophyceaeafter 1980isapparentlyinresponseto thatthezebramusselimpactwasbutatinyaberration a marked decrease in Lake Erie total phosphorus onamuchmoresignificanttrend. Earlydatarevealpeak concentrationsbeginningataboutthesametime.
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