Poor nutritional quality of primary producers and zooplankton driven by eutrophication is mitigated at upper trophic levels

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ Poor nutritional quality of primary producers and zooplankton driven by eutrophication is mitigated at upper trophic levels © 2022 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd. Published version Taipale, Sami Johan; Ventelä, Anne‐Mari; Litmanen, Jaakko; Anttila, Lauri Taipale, S. J., Ventelä, A., Litmanen, J., & Anttila, L. (2022). Poor nutritional quality of primary producers and zooplankton driven by eutrophication is mitigated at upper trophic levels. Ecology and Evolution, 12(3), Article e8687. https://doi.org/10.1002/ece3.8687 2022 Ecology and Evolution. 2022;12:e8687.   | 1 of 18 https://doi.org/10.1002/ece3.8687 www.ecolevol.org Received:4September2021 | Revised:22January2022 | Accepted:7February2022 DOI: 10.1002/ece3.8687 RESEARCH ARTICLE Poor nutritional quality of primary producers and zooplankton driven by eutrophication is mitigated at upper trophic levels Sami Johan Taipale1 | AnneMari Ventelä2 | Jaakko Litmanen1 | Lauri Anttila2 ThisisanopenaccessarticleunderthetermsoftheCreativeCommonsAttributionLicense,whichpermitsuse,distributionandreproductioninanymedium, providedtheoriginalworkisproperlycited. ©2022TheAuthors.Ecology and EvolutionpublishedbyJohnWiley&SonsLtd. 1DepartmentofBiologicaland EnvironmentalScience,Universityof Jyväskylä,Jyväskylä,Finland 2PyhäjärviInstitute,Ruukinpuisto, Kauttua,Finland Correspondence SamiJohanTaipale,Departmentof BiologicalandEnvironmentalScience, UniversityofJyväskylä,Jyväskylä, Finland. Email:[email protected] Funding information Thisresearchwassupportedbythe AcademyofFinlandresearchgrant 333564,awardedtoSamiJ.Taipaleand bythefoundationofKalataloudenja merenkulunkoulutuksenedistämissäätiö, awardedtoLauriAnttila. Abstract Eutrophicationandrisingwatertemperaturein freshwatersmayincreasethetotal production of a lake while simultaneously reducing the nutritional quality of food web components. We evaluated how cyanobacteria blooms, driven by agricultural eutrophication(ineutrophicLakeKöyliöjärvi)orglobalwarming(inmesotrophicLake Pyhäjärvi),influencethebiomassandstructureofphytoplankton,zooplankton,and fishcommunities.Intermsofthenutritionalvalueoffoodwebcomponents,weevaluatedchangesintheω-3andω-6polyunsaturatedfattyacids(PUFA)ofphytoplankton andconsumersatdifferenttrophiclevels.Meanwhile,thelakesdidnotdifferintheir biomassesofphytoplankton,zooplankton,andfishcommunities,laketrophicstatus greatly influenced the community structures. The eutrophic lake, with agricultural eutrophication,hadcyanobacteriabloomthroughoutthesummermonthswhereas cyanobacteria were abundant only occasionally in the mesotrophic lake, mainly in earlysummer.Phytoplanktoncommunitydifferencesatgenuslevelresultedinhigher arachidonicacid,eicosapentaenoicacid(EPA),anddocosahexaenoicacid(DHA)contentofsestoninthemesotrophicthanintheeutrophiclake.Thiswasalsoreflected intheEPAandDHAcontentofherbivorouszooplankton(DaphniaandBosmina)despitemoreefficienttrophicretentionofthesebiomoleculesinaeutrophiclakethan inthemesotrophiclakezooplankton.Planktivorousjuvenilefish(perchandroach)ina eutrophiclakeovercametheloweravailabilityofDHAintheirpreybymoreefficient trophicretentionandbiosynthesisfromtheprecursors.However,themostefficient trophicretentionofDHAwasfoundwithbenthivorousperchwhichpreycontained onlyalowamountofDHA.Long-termcyanobacterialbloomingdecreasedthenutritionalqualityofpiscivorousperch;however,thedifferencewasmuchlessthanpreviouslyanticipated.Ourresultshowsthatlong-termcyanobacteriabloomingimpacts thestructureofplanktonandfishcommunitiesandlowersthenutritionalqualityof sestonandzooplankton,which,however,ismitigatedatuppertrophiclevels. KEYWORDS benthicinvertebrates,freshwaterfoodweb,ontogeneticdietshift,perch,phytoplankton, polyunsaturatedfattyacids 2 of 18 | TAIPALE ET AL. 1 | INTRODUCTION Globally, freshwater ecosystems are challenged by land use and many factors connected to climate warming, such as changing precipitation, eutrophication (as an increase in total phosphorus) (Hasler,1947),andwaterbrowning(anincreaseofDOC)(Karlsson etal.,2009;Leechetal.,2018;O'Reillyetal.,2003).Intheboreal zone,lakewatertemperatureandprecipitationareincreasing,which may increase nitrogen (N), phosphorus (P), and dissolved organic carbon (DOC) loading, especially from agricultural and peatland- dominated catchments (Lathrop et al., 2019; Ruosteenoja et al., 2016).Changing environmentalconditions affectecosystem function and phytoplankton, zooplankton, and fish community structure(Havens,2008;Jeppesenetal.,2010,2012;Kevaetal.,2021; Sukeniketal.,2015;Venteläetal.,2016).Atthesametime,these conditions also impact the nutritional value of the phytoplankton andthustheproductionandthetransferofessentialbiomolecules through food webs (Lau et al., 2021; Müller-Navarra et al., 2004; Taipaleetal.,2016,2019). The ω-3andω-6polyunsaturatedfattyacids(PUFA)havebeen foundtohavemanyphysiologicallynecessaryfunctionsinallanimals includinghumans(Artsetal.,2009;Simopoulos,2000).Becauseanimalscannotsynthesizeω-3andω-6PUFAdenovo,theyneedto obtainthesemoleculesfromtheirdiet.Therefore,short-chainω3 andω-6PUFAofα-linolenicacid(ALA,18:3ω3)andlinoleicacid(LA, 18:2ω6)areusuallyconsideredessentialfattyacids(EFA)or“essentialnutrients”foranimals(Parrish,2009).However, eicosapentaenoicacid(EPA,20:5ω3),docosahexaenoicacid(DHA,22:6ω3),and arachidonicacid(ARA,20:4ω6)arephysiologicallymostimportant for consumers (Hulbert & Abbott, 2012; Parrish, 2009; Stanley- Samuelsonetal.,1988).Therefore,theymaybecalledphysiologicallyessentialorsemi-essentialPUFA(Taipaleetal.,2019). In marine and freshwater ecosystems, green algae and cyanobacteriaareclassifiedasnon-EPAandnon-DHA-synthesizers,while golden algae, dinoflagellates, cryptophytes, diatoms, and raphidophytesareprimaryproducersofEPAandDHA(Ahlgrenetal.,1992; Jónasdóttir,2019;Taipaleetal.,2013;Taipale,Vuorio,etal.,2016). However,EPA-andDHA-synthesizingphytoplanktontaxacanalso be found abundantly in eutrophic lakes (Lepistö & Rosenström, 1998). A clear decline in the nutritional quality of seston can be seeninhyper-eutrophiclakes(Müller-Navarraetal.,2004;Taipale et al., 2019). Therefore, it is important to monitor the abundance ofEPA-andDHA-synthesizingphytoplanktontaxa(cryptomonads, golden algae, diatoms, dinoflagellates, raphidophytes, euglenoids) throughout the summer to understand the nutritional quality of phytoplankton.Agriculturaleutrophicationhasbeenthemainreason for increased cyanobacteria blooms in boreal and temperate lakes(Jørgensen&Rast,2001).However,thegrowingabundanceof cyanobacteriabloomsintherecentpastisrelatedtoclimatechange andespeciallyduetotheincreasedtemperatureoflakes(Dengetal., 2016; Elliot, 2012; Paerl & Huisman, 2008; Pätynen et al., 2014; Rasconietal.,2017).Previousstudieshaveshownthatadecrease inthenutritionalqualityofphytoplanktonismainlyattributedtothe changes by the phytoplankton community structure, but also becausethenutritionalvalueofphytoplanktoncellsdecreasesbyeutrophication(Kevaetal.,2021;Lauetal.,2021;Taipaleetal.,2019). Herbivorous zooplankton is a key link in connecting phytoplankton and planktivorous fish and thus the nutritional value of zooplanktonisimportantforthegrowthoffishfry(Taipaleetal., 2018). However, individual zooplankton taxa differ by their nutritionalvalue(Kratina&Winder,2015).Thisisbecausecladocerans usuallyaccumulateEPAwhereascopepodsarerichinDHA(Brett etal.,2009;Hiltunenetal.,2016;Smynteketal.,2008;Taipaleetal., 2011).Moreover,zooplanktonsaregenerallyinefficientintheirabilitytobiosynthesizeALAtoEPAandDHA.Thus,theyarestrongly dependentonthefattyacidqualityintheirdiet(Elertetal.,2003; Koussoroplisetal.,2014;Taipaleetal.,2011).Herbivorouscladoceran(DaphniaandBosmina)isakeystonespeciesinmostlakeecosystems(Bergquistetal.,1985;Kerfootetal.,1988;Lynch&Shapiro, 1981).Itcandetecthighnutritionalqualitypatchesandcanselectivelyfeedonhighnutritionalqualityparticles(Hartmann&Kunkel, 1991;Schatz&McCauley,2007).Moreover,fattyacid-basedmodelinghasshownthatsestonmicrobial(includingalgae)composition doesnotnecessarilymatchwithassimilateddiet(Taipaleetal.,2019). This is because herbivorous zooplankton (Daphnia and Bosmina) favors high nutritional quality diet (Galloway et al., 2014; Taipale etal.,2019).Cyanobacteriabloomsmayleadtopoorerenergyflow inaquaticfoodwebsbecausetheypoorlysupportzooplanktonsomaticgrowthandreproduction(Bednarskaetal.,2014;Elertetal., 2003;Peltomaaetal.,2017;Porter&McDonough,1984).Theycan alsobelinkedtotheuppertrophiclevelonlybycertainzooplankton taxa(e.g.,Chydorus;Tõnnoetal.,2016).Environmentalchanges(e.g., eutrophication,browning,globalwarming)havebeenshowntohave differentimpactsonthenutritionalvalueofzooplankton(Kevaetal., 2021;Lauetal.,2021;Senaretal.,2019).Asaresult,itappearsthat thelowernutritionalvalueofphytoplanktondoesnotalwaysaffect highertrophiclevels.However,arecentstudyofproductivityand temperaturegradientinsub-arcticlakesshowedthatthezooplanktoncommunitychangedfromtheCalanoid(Eudiaptomus graciloides) dominated community towards herbivorous cladocerans (Daphnia andBosmina),resultinginadecreaseintheEPAandDHAcontentof zooplanktoncommunity(Kevaetal.,2021). Environmental changes and especially eutrophication have been known to change the structure of fish communities (Keva etal.,2021).Itiswelldocumentedthatcyprinidfish,e.g.,roach (Rutilus rutilus)and bream(Abramis brama), areultimatewinners TAXONOMY CLASSIFICATION Chemicalecology;Globalchangeecology;Trophicinteractions | 3 of 18 TAIPALE ET AL. intheeutrophicationinboreallakes,whereasvendace(Coregonus albula)andburbot(Lota lota)areknowntobelosers(Tammietal., 1999).However,itisnotwellknownhowdependentdifferentfish speciesareontheEPAandDHAcontentoftheirprey.Theability offreshwaterfishtobiosynthesizelonger-chainPUFAfromtheir precursorsisreportedlybetterthanwithmarinefish(Sargentetal., 1999).Nevertheless,thereisa paucityofstudieswithdifferent freshwaterfishspeciesarelacking.Eutrophicationandbrowning impacton theEPA andDHA contentoffishmusclearecontradicting(Ahlgrenetal.,1996;Kevaetal.,2019;Strandbergetal., 2016; Taipale, Vuorio, et al., 2016). However, some fish species couldseeminglymitigatethelownutritionalqualityoftheirprey. Ahlgrenetal.(1996)foundthatEPAandDHAcontentofroachis higherinoligotrophiclakesthanineutrophiclakes,whereasthey didnotfindasimilardifferenceintheperch,whichisincontrastto ourpreviousfindingwithpiscivorousperch(Taipale,Vuorio,etal., 2016).Chaguacedaetal.(2020)recentlyreportedthatthecontent ofARA,EPA,andDHAarestrongly regulated overontogeny in perchmusclesbasedontheirFAprofilesandcompound-specific stableisotopes(Scharnweberetal.,2021).However,itisnotclear howthelowavailabilityofDHA,causedbycyanobacteriabloomingdrivenbyeutrophicationorclimatechange,impactEPA and DHAcontentoffishatdifferenttrophiclevels. Since European perch (Perca fluviatilis) have three ontogenetic dietarystages,itisanidealfishspeciestoevaluateeutrophication's impact on the nutritional value of the same species at different trophiclevels.Perchfryeatszooplankton,fromwhichitgradually moves to thebenthosand on tofishfood(Estlanderetal., 2010, 2012; Haakana et al., 2007; Rask, 1986). Previously, it was found thatthepiscivorous(length>20cm)perchofoligo-andmesotrophiclakescontainmoreEPAandDHAthanperchineutrophiclakes (Gladyshev et al., 2018; Taipale, Vuorio, et al., 2016). Chaguaceda etal.(2020)suggestedstrongregulationofEPAandDHAinperch muscle. Here, (H1), we hypothesized that long-term cyanobacteria blooms by agricultural eutrophication increases the biomasses of phytoplankton,zooplankton,andfishcommunities,butalsochanges thestructureofplanktonandfishcommunities.Weassumedthat long-termcyanobacterialbloomdecreasesthebiomassofEPA-and DHA-synthesizingphytoplanktontaxa,favorssmallcladoceranover copepods,andincreasesthenumberofcyprinidsoverpercidsfish. Secondly,wehypothesized(H2)thatthenutritionalqualityofsestonisdecreasedbylaketrophicstatus(Kevaetal.,2021;Lauetal., 2021;Müller-Navarraetal.,2004;Taipale,Vuorio,etal.,2016).We alsoassumedthatthisdecreaseinthenutritionalqualityofprimary producersisreflectedatdifferenttrophiclevelsviachangesinthe nutritionalqualityoftheirprey.Finally,weassumed(H3)thatconsumerstrytocompensatefortheirlowernutritionalqualityofprey bymoreefficienttrophicretentionandbiosynthesisofphysiological essentialPUFA. 2 | MATERIALS AND METHODS 2.1 | Study area The research material was collected during the summer of 2017 from mesotrophic Lake Pyhäjärvi and eutrophic Lake Köyliönjärvi, which are both located in southwest Finland, as shown in Table 1. Weather conditions are similar for these two lakes,whichcanbeseeninequalsurfacetemperatureduringthe 2000s(PERMANOVA:Pseudo-F1,157 =0.33,p =.578).However, these two lakes differ in their productivity (PERMANOVA: Pseudo-F1,129 = 190.4, p = .001) and nutrients (PERMANOVA forTPandTN:Pseudo-F1,181/185 = 475/622, p =.001)basedon measurementsbetween2000and2017(Herttadatabase,Finnish EnvironmentalCentre).Basedontotalphosphorusandchlorophyll concentration,LakeKöyliönjärvicanbeconsideredaeutrophicor hyper-eutrophiclake,whereasLakePyhäjärvicanbeconsidered tobeamesotrophiclake(Bengtssonetal.,2012).Moreover,Lake Köyliönjärvi is a shallow lake (mean depth 3 m) with the deepestpointof13m,whereasthemeandepthofLakePyhäjärviis 5m,withthedeepestpointbeing26m.Bothlakessufferfroman overlyhighnutrientloadfromtheircatchments.LakeKöyliönjärvi usuallyexperienceslargecyanobacterialbloomsinsummer,which temporarily declined in the 1990s due to fish removal (Sarvala et al., 2000). Lake Pyhäjärvi has been subjected to a variety of water protection measures since the 1980s, thereby decelerating the lake's eutrophication development (Ventelä et al., 2007, 2016).Inthe2000s,climatechangeaffectedthephytoplankton community, and cyanobacteria blooms have become more frequentinLakePyhäjärvi(Dengetal.,2016).Thisdevelopmentwill Parameter Unit Mesotrophic Lake Pyhäjärvi Eutrophic Lake Köyliönjärvi 2000– 2017 2017 2000– 2017 2017 Totalphosphorus µgP/L 19± 5.2 22 ± 7.0 116 ± 36.1 77 ± 42.3 Totalnitrogen µgN/L 422 ± 50 422 ± 76 1190± 324 992± 347 Chlorophyll µg/L 7.5 ± 3.6 8.0 ± 3.1 65.1 ± 33.7 61 ± 28.7 Turbidity FNU 2.4 ± 1.1 2.4 ± 0.7 25.5 ±13.9 23.0 ± 6.7 Secchi Depth m 2.5 ± 0.6 2.3 ± 0.2 0.6 ± 0.2 0.5 ± 0.1 Temperature °C 18.4 ± 2.6 17.0 ±1.9 18.7 ± 2.3 17.0 ± 2.2 TABLE 1 Totalphosphorus,nitrogen, chlorophyll,turbidity,Secchidepth. andtemperatureformesotrophicLake PyhäjärviandeutrophicLakeKöyliönjärvi 4 of 18 | TAIPALE ET AL. befurtheracceleratedinfuturebasedonthemodeling(Pätynen etal.,2014). 2.2 | Phytoplankton and zooplankton community and fatty acid sampling Throughoutthesummermonths(June–August)of2017,thewater quality (Secchi-depth, water temperature, turbidity, chlorophyll-a, totalphosphorus, phosphatephosphorus,andtotal nitrogen),and communitycompositionofphyto-andzooplankton,andtheirfatty acidcompositionandcontent,weremonitored.Asampleof0–5m water column was taken with a tube sampler (model: Sormunen, volume 6.3 L) to analyze quantitatively the community compositionofthephyto-andzooplankton.Planktoncommunitysamples wereanalyzedbythecommerciallaboratoryLounais-Suomenvesi- ja ympäristötutkimus Oy,wherecertifiedpersons countedphytoplanktonandzooplanktonsamples.Physico-chemicalwatersamples weretakenwithaLimnostubesampler(volume2.6L)andanalyzed bytheLounais-Suomenvesi-jaympäristötutkimusOylab.ThesamplepointsinthelakeswereselectedtobeinlinewiththeenvironmentalmonitoringprogramoftheFinnishEnvironmentalInstitute, inordertoutilizethewaterqualitymaterialfoundintheHerttadatabase(www.syke.fi/avointieto).Intotal,thesummersamplingcampaignincludedsixsamplesforLakePyhäjärviandfivesamplesfor LakeKöyliönjärvi. Polyunsaturatedfattyacids(PUFA)ofseston(phytoplankton) availableforherbivorouszooplanktonwerestudiedbypre-filtering sestonwitha50µmsieveandthenfilteringaspecificamountof waterthroughGF/Ffilterpaper(Whatman).Sampledherbivorous cladoceran was majorly (>95%) Daphnia and Bosmina and contained random (<5%) Chydorus, Ceriodaphnia, or Diaphanosoma. Itwasusedtoestimatethenutritionalqualityofdietforplanktivorousperchsinceherbivorouscladoceran(especiallyDaphnia together with Bosmina)isthemajorpreyforplanktivorousperch (Estlanderetal.,2010;Ruohonen,2006).Thezooplanktonsample was collected horizontally with a 50 µm plankton net and main genera were picked up with microscope glass. Surface water (0–2mwatercolumn)wassampledwithatubesampler(model: Sormunen,volume6.3L)forthefattyacidcompositionandcontentanalysisofseston. 2.3 | Zoobenthos community and fatty acid sampling Inadditiontoseasonalphyto-andzooplanktonsampling,zoobenthoswassampledonceinthelittoralzonedepthof2–3minlate summer2017.Inbothstudylakes,asimilarsamplingprocedurefor onesamplepointwascarriedoutwithanEkmangrab.Thesamples werefilteredbya500µmscreentoremovethefinematerialand thenallmacroscopiczoobenthoswerepickedupinthelaboratory. Chironomidaelarvaeweretheonlyabundantgroupinbothlakesamples.Accordingtoearlierstudies(notpublished),inLakePyhäjärvi atleast,Chironomidaelarvaeformasignificantpartofthedietfor benthivorousperch. 2.4 | Fish community and fatty acid sampling Fish community structure and biomasses were obtained from the national fish monitoring database (Hertta/Koekalastusrekisteri) managed by the Natural Resources Institute Finland. This study coveredtheyears2012,2015,2017,and2020foreutrophicLake Köyliönjärvi.Similarly,2009,2012,2015,and2019werecoveredfor mesotrophicLakePyhäjärvi.Briefly,NORDICmultimeshsurveynets (Appelbergetal.,1995)wereusedforgillnetsampling.Gillnetsamplingfollowedrandomstratifiedsampling,includingnetsinpelagic, metalimnetic,andbenthicgillnets(Olinetal.,2016),whereasgillnet samplingwasdoneyearlyduringJulyandAugust.Theannualnumberofgillnetnightswere40foreutrophicLakeKöyliönjärviand56 formesotrophicLakePyhäjärvi.Tocomparefishbiomassesbetween lakes,weusedBPUE(wetmassperuniteffort=kgfishpergillnet night)(Rasketal.,2020)ofindividualfishspecies.Tocomparethe structureofperchcommunitiesinthesetwolakesweusedCPUE (numberoffishpergillnetnight)oftheperchgroup(dietgroup).The perchcommunitywasdividedintocategoriesincludingitsontogeneticdietshift(Estlanderetal.,2010;Estlander,etal.,2012),planktivorous(length:<15cm),benthivorous(15–19cm),andpiscivorous (>19cm).Thesecategoriesrelatetothemaindietbutplanktivorous fishmayalsofeedonbenthicinvertebrates,andbenthivorousperch feedsonsmallerfish(Amundsenetal.,2003;Estlanderetal.,2010, 2012). Perch individuals for fatty acid analysis were caught in the late summer of 2017. Perches from mesotrophic Lake Pyhäjärvi were received from professional fishers who used open-water seine fishing and gillnets for catching fish. Perch fry were also nettedfromapier.PerchinLakeKöyliönjärviwerecaughtusing theNordicgillnetseries.Duetotherapiddevelopmentofyoung fish,theyoung-of-the-yearperchwerecaughtwithintwoweeks, frombothlakes,toensurethecomparisonbetweenthelakeswas relevant.FrywerecaughtonSeptember12inLakeKöyliönjärvi andSeptember2and11inLakePyhäjärvi.Thelength,weight,and sexofeachfishweredetermined(TableS1).Agewasdetermined mainlybyusinggill-coveringbone,operculum,and,insomecases, a more precise determination was made by examining scales. Samplesforfattyacidanalysisweretakenfromthedorsalmusclesandstoredat−20°Cuntiltheywerefreeze-driedwithinone monthfromsampling.Theresearchmaterialcoveredatotalof48 fishinLakePyhäjärviand37fishinLakeKöyliönjärvi(TableS1). Inadditiontoperch,fiveindividualsofsmallroach(<10cm)were obtainedfrombothlakestoestimateifPUFAcontentofomnivorousfishandpotentialdietforpiscivorousperchdifferintheir PUFAcontent. | 5 of 18 TAIPALE ET AL. 2.5 | Fatty acid analysis Lipids were extracted from the freeze-dried seston, cladocera, Chironomidae, and fish samples in Kimax borosilicate tubes with chloroform-methanol (2:1) mixture. Fatty acids were methylated using mild sulfuric acid. Methyl esterified samples were analyzed on a Shimadzu GC-MS-QP2010 Ultra (Nishinokyo-Kuwabara-Cho, Kioto, Japan) with helium as carrier gas. Column was Zebron ZB- FAME (35 m × 0.25 mm × 0.20 µm). The temperature of the injectorwas270°Candweusedasplitlessinjectionmode(foronemin). Temperaturesoftheinterfaceandionsourcewere250and220°C, respectively.Phenomenex®(Torrance,California,USA)ZB-FAMEcolumn(30m×0.25 mm×0.20 µm)with5mGuardianwasusedwiththe following temperature program: 50°C was maintained for one min, thenthetemperaturewasincreasedat10°C/minto130°C,followed by7°C/minto180°C,and2°C/minto200°C.Thistemperaturewas heldforthreeminutes,andfinally,thetemperatureincreased10°C/ minto260°C.Thetotalprogramtimewas35.14minandthesolvent cuttimewasnineminutes.Fattyacidswereidentifiedbytheretention times(RT)andusingspecificionswhichwerealsousedforquantification(Taipale,Hiltunen,etal.,2016).Fattyacidconcentrationswere calculatedusingcalibrationcurvesbasedonknownstandardsolutions (15,50,100,and250ng)ofaFAMEstandardmixture(GLCstandard mixture 566c, Nu-Chek Prep, Elysian, MI, USA) and using recovery percentageofinternalstandards.ThePearsoncorrelationcoefficient was>0.99foreachindividualfattyacidcalibrationcurve.Additionally, we used 1,2-dinonadecanoyl-sn-glycero-3- phosphatidylcholine (Larodan, Malmö, Sweden) and free fatty acid of C23:0 (Larodan, Malmö,Sweden)asinternalstandardsand tocalculatetherecovery percentages.Thefattyacidcontentofseston(<50 µm)wascalculated basedonphytoplanktoncarbonasdescribedbyTaipaleetal.(2019). Otherwise,fattyacidcontentwascalculatedbasedonthedryweight ofzooplankton,zoobenthos,orfishmuscle. Trophic retention of ARA, EPA, and DHA by zooplankton, Chironomidae, roach, and different ontogenetic stages of perch werecalculatedbythefollowingequation(referredtoasaccumulationfactorbyHessen&Leu,2006): Trophic retention = (FAdiet/FAconsumer) – 1, where FAdiet representsARA,EPA,andDHAcontent(µgmg/C)ofdietandFAconsumer citestheircontentintheconsumers.Theaveragedietcomposition foreachconsumerwastakenfrompreviousstudies.Forherbivorous zooplankton,weusedseston,0+perch,androach.Forplanktivorousperch,weusedherbivorouscladoceran,whereasforbenthivorousperch,weused20%ofherbivorouszooplanktonand80%of Chironomidae.Weused0+ perch androach for piscivorous perch (Estlanderetal.,2010;Ruohonen,2006). 2.6 | Bulk stable isotope analysis and trophic position Approximately 0.6–1.2 mg of freeze-dried seston, zooplankton, benthic invertebrates, or fish muscle sample was weighted and encapsulatedtotincups.The15N/14NwasmeasuredusingaCarlo- Erba Flash1112serieselementalanalysis connectedto a Thermo FinniganDeltaPlusAdvantageisotoperatiomassspectrometerin continuousflowmode.Isotopicdataarepresentedinstandarddelta notationwithunitspermil(‰)andrelativetotheViennaPeeDee Belemnite(VPDB)internationalstandard.Precessionandaccuracy were determined through repeated measurements of an internal workingstandardthatwasfoundtobe0.2and0.3,respectively. Trophiclevel(TL)ofconsumers(herbivorouscladocera(Daphnia andBosmina),Chironomidae,roach,andperch)wasdeterminedby usingδ15Nvalues(Postetal.,2002). where λreferstothetrophicpositionofthebaselineorganism,δ15Nconsumernitrogenstableisotopevalueofagivenconsumer,andδ15Nbaselinenitrogenstableisotopevaluesofbaselineorganism(sestonin ourcase)instudylake.Δ15Nisatrophicfractionationfactorthatwas set3.4‰pertrophiclevelaccordingtoPost (2002).Perchweredividedintoplanktivorous(TL<3.6),benthivorous(TL3.6–3.9),andpiscivorous(TL>3.9)categoriesbasedontrophiclevels. 2.7 | Estimating the herbivorous cladoceran diet We used the measured cladoceran FA profiles to estimate relativecladocerandietcompositions(%).WeusedQuantitativeFatty Acid Signature Analysis in R (QFASAR) (Bromaghin, 2017; Iverson etal.,2004)withχ2distancemeasure(Stewartetal.,2014),which isthemostaccuratecurrentfattyacid-basedmethodforherbivorous cladoceran diet estimation (Litmanen et al., 2020). The diets wereestimatedwithanFAprofilelibraryformedofhomogeneous dietfeedingexperimentsconsistingofdinoflagellates,goldenalgae, cryptophytes,diatoms,greenalgae,euglenoids,cyanobacteria,actinobacteria,andmicrobessustainingon(terrestrial)particulateorganicmatter/detritus(Gallowayetal.,2014;Litmanenetal.,2020). The standard deviation for the diet estimates was produced with 100 sample bootstrapping in QFASAR (Table S2). The estimation was conducted with R Statistical Software v. 3.6.1 (R Core Team, 2019). 2.8 | Statistical analysis WeusedPERMANOVA(Primer7)analysisandBray-Curtissimilarity to compare phytoplankton, zooplankton, and fish community structureatclass,genus,orspecieslevel,usinglaketrophicstatus (mesotrophicoreutrophic)andmonthasfactors.Weusedthesame approachtocomparefattyacidcompositionandcontentofessential fatty acids in phytoplankton (seston), herbivorous cladoceran, benthic invertebrates, and perches. PERMANOVA with Euclidean distance as resemblance matrix was used for univariate analysis (Andersonetal.,2017).Non-metricmultidimensionalscalingNMDS TLconsumer =λ+ ( δ15 N consumer −δ 15 N baseline ) ∕Δ15N, 6 of 18 | TAIPALE ET AL. was used to separate communities’ structure, fatty acid composition, and content of essential fatty acids (Primer 7). The correlationsbetweenMDS1andMDS2andvariableswereanalyzedwith Spearman correlation analysis. Hierarchical Cluster analysis was usedtocreatesimilaritygroupsinNMDS.Weusedbubbleplotsto illustratethetotalbiomassofphytoplankton,zooplankton,andfish communitiesinNMDS. 3 | RESULTS 3.1 | Water quality and phytoplankton community Totalphosphorus,nitrogen,chlorophyll,andturbidityweresignificantlyhigherineutrophicLakeKöyliönjärvithaninmesotrophic LakePyhäjärviandlaketrophicstatusexplained66%ofthedifference( Tables1and2).Whenusingt wofactoranalysis,trophicst atus explained62%ofthevariance(PERMANOVA:Pseudo-F1,13 =71.3, p =.001)andmonthexplained9%ofthevariationofphosphorus, nitrogen, chlorophyll, and turbidity (PERMANOVA: Pseudo-F1,13 (trophicstatus/month)= 40.6./2.9, p =.001/.045).ThetemperatureofsurfacewaterwasequalinbothlakesandwhereasSecchi depthwashigherinthemesotrophicLakePyhäjärvi thanin the eutrophic Lake Köyliönjärvi during the open water season 2017 (Table2). Cyanobacteria,diatoms,andgreenalgaewerepercentuallythe threemostabundanttaxaineutrophicLakeKöyliönjärvi,whereas cyanobacteriaanddiatomswerepercentuallythemostcommon classesinLakePyhäjärvi.Thecontributionofdinoflagellatesduring summermonthswassignificantlyhigherinLakePyhäjärvithanin LakeKöyliönjärvi,butotherwise,thelakesdidnotdifferintheir phytoplanktoncompositionatclasslevel(seeTable2).However, the contribution of DHA-synthesizing taxa (cryptophytes, dinoflagellates, golden algae) was higher in the mesotrophic lake (24.3±12%ofall)thanintheeutrophiclake(6.8±7.7%).Total phytoplanktonbiomasswashigherineutrophicLakeKöyliönjärvi thaninmesotrophicLakePyhäjärvi(seeFigure1).However,comparisonatclasslevelshowedthatonlybiomassesofgreenalgae washigherineutrophicLakeKöyliönjärvithaninmesotrophicLake Pyhäjärvi,whereasdiatombiomasswashigherinthemesotrophic Lake Pyhäjärvi than in the eutrophic Lake Köyliönjärvi (refer to Figure 1, Table 2). Due to the higher biomass of diatoms in the eutrophicLakeKöyliönjärvi,thebiomassofEPA-synthesizingtaxa wasalsohigher,whereasthebiomassofDHA-synthesizingtaxadid notdifferbetweenlakes(Table2).AccordingtothePERMANOVA, each lake explained 51% of the variation in phytoplankton biomasses at class level but explained only 36% of variation at the genuslevel(Table2). NMDSoutputshowedthatphytoplanktoncommunitystructure variedgreatlybetweenthetwolakesbutvariedmore,duringsummer months, in mesotrophic Lake Pyhäjärvi than in eutrophic Lake Köyliönjä r vi(F igure1c).Mor eove r,s imila ri tya na ly sis(SI MPER )showed thatthedissimilarityofphytoplanktoncommunitiesatthegenuslevel betweenthelakeswasrelativelyhigh(89.4%).Cyanobacteriagenus ofDolichospermumandMicrocystisanddiatomgeneraofAulacoseira weremoreabundanttaxaineutrophicLakeKöyliönjärvithaninmesotrophic Lake Pyhäjärvi, whereas Aphanizomenon (Cyanobacteria) was more abundant in the mesotrophic than in the eutrophic lake. The closer comparison of EPA- and DHA-synthesizing phytoplankton genus (SIMPER: Average dissimilarity =78.7%)showedthat Aulacoseira, Acanthoceras, Uroglena, Rhodomonas, Cryptomonas, and CeratiumweremoreabundantineutrophicLakeKöyliönjärvi(explaining61.1%ofdissimilarity).WhereasDinobryon,Tabellaria,Fragilaria, Rhizosolenia,andGymnodiniumweremoreabundantinmesotrophic LakePyhäjärvi(explaining15.4%ofdissimilarity). 3.2 | Zooplankton community NMDS output revealed changes in the zooplankton community structure between the lakes but also by the season (see Figure2c).Two-factorPERMANOVAofzooplanktonbiomassesat genus level showed the following statistical difference between the lakes (PERMANOVA (lake/month): Pseudo-F1,11 = 4.3/3.3, p =.017/.009).Lakeandmonthaccountedfor21%and32%ofall variation,respectively.Accordingtothesimilarityanalysis,mostof the difference between the lakes (SIMPER: average dissimilarity 67.8%)wasexplainedbythegenusoftheChydorus,Eudiaptomus, andMesocyclops,whichweremoreabundantintheeutrophicthan inthemesotrophiclake.However,Bosminawasmoreabundantin themesotrophicthanintheeutrophiclake.Nevertheless,herbivorouscladoceranwasthemostabundantzooplanktongroupinboth lakes (Figure 2a), of which Chyrodus, Daphnia, and Bosmina were the most abundant genus. Eudiaptomus graciloideswastheonly abundantherbivorouscalanoidinbothlakesandwerethesecond- most abundant zooplankton group with predator cyclopoids in bothlakes.Megacyclops,Mesocyclops,andThermocyclopswereall abundantinmesotrophicLakePyhäjärviwhereasMegacyclops did notoccurintheeutrophicLakeKöyliönjärvi.Predatorycladoceran (Leptodoras kindtii)wastheonlyzooplanktongroupwhichdiffered statistically significantly between lakes being more abundant in eutrophic Lake Köyliönjärvi than in mesotrophic Lake Pyhäjärvi (Table2,Figure2b). 3.3 | Benthic and fish communities Our benthic invertebrate sampling was not quantitative, but our samplingintwolakesshoweddifferencesinthepresenceofvarious benthicinvertebrates.Meanwhile,wefoundonlyChironomidaein eutrophicLakeKöyliönjärvi,whileoursamplingofmesotrophicLake PyhäjärviresultedinfindingseveralindividualsofAsellus aquaticus, Ephemeroptera,Oligochaeta,Megaloptera,andPlecoptera. Figure3ashowedthattheTotalBPUE(kgfishpergillnetnight) andCPUE(numberoffishpergillnetnight)in2012–2020washigher in eutrophic Lake Köyliönjärvi than in mesotrophic Lake Pyhäjärvi | 7 of 18 TAIPALE ET AL. (Table 2). Laketrophicstatus explained83%and 79% ofthevariationin BPUE and CPUE, respectively.Thepercidswerefoundto contribute(BPUE%)20.6±6.1%and55.3.6±5.0%oftotalBPUEin eutrophicLakeKöyliönjärviandmesotrophicLakePyhäjärvi,respectively.Conversely,thecontributionofcyprinidswashigherinLake Köyliönjärvi(66.3±3.2%)thaninLakePyhäjärvi(18.6±6.5%).Roach (Rutilus rutilus)wasthemain(BPUE%= 42 ±4%)fishspeciesinLake Köyliönjärvianditsbiomasswasstatisticallyhigher(Table2)thanin LakePyhäjärvi.Correspondingly,perch(Perca fluviatilis)wasthemain (BPUE%=39±7%)fishspeciesintheLakePyhäjärvi;however,the BPUEofperchdidnotdifferbetweenlakes(Table2).Accordingto theSIMPER(averagedissimilaritybetweenlakes=50.7%,Table2) andNMDS(Figure3c),bleak,smelt,whitefish,andruffeweremore prevalent (BPUE%) in mesotrophic Lake Pyhäjärvi, whereas pike, pikeperch,bream,andwhitebreamweremoreprevalentineutrophic LakeKöyliönjärvi. Theabundanceofdifferentontogeneticgroupsofperchesdid notdifferstatisticallybetweenthelakesduetothehighvariationin mesotrophicLakePyhäjärvi(refertoFigure3b).However,planktivorousperchcontributed91±3%ofallperch(CPUE%)ineutrophic TABLE 2 StatisticalresultsforPERMANOVAbetweenmesotrophicandeutrophiclakes.%citestothecontribution,FA%tofattyacid profile,concentrationtoµgFAmg/L,%,QFASAtothecontributionoffattyacid-baseddietestimates Component Df1 Df2 PseudoF P(perm) Difference Totalnitrogen 113 23.8 0.003 Mesotrophic<eutrophic Totalphosphorus 113 15.2 0.003 Mesotrophic<eutrophic Chlorophylla 113 31.6 0.001 Mesotrophic<eutrophic Turbidity 113 87.9 0.001 Mesotrophic<eutrophic Secchi depth 113 179.5 0.001 Mesotrophic>eutrophic Temperature 113 <0.001 1Mesotrophic=eutrophic Dinoflagellates(%) 111 3.9 0.04 Mesotrophic>eutrophic DHA-synth.taxa(%) 111 8.5 0.023 Mesotrophic>eutrophic Totalphytoplanktonbiomass 110 9.3 0.0018 Mesotrophic<eutrophic Diatoms(biomass) 110 9.3 0.005 Mesotrophic>eutrophic Greenalgae(biomass) 110 526 0.002 Eutrophic> mesotrophic Phytoplanktonbiomass(class) 110 9.3 0.023 Mesotrophic<eutrophic Phytoplanktonbiomass(genus) 111 6.2 0.003 Mesotrophic<eutrophic EPA-synth.taxa(biomass) 111 12.2 0.002 Mesotrophic>eutrophic DHA-synth.taxa(biomass) 111 0.002 0.95 Mesotrophic=eutrophic Predatorycladoceran 111 3.5 0.042 Mesotrophic<eutrophic BPUE 1 7 23.2 0.035 Mesotrophic<eutrophic CPUE 1 7 28.3 0.036 Mesotrophic<eutrophic Roach(biomass) 1 7 48.1 0.033 Mesotrophic<eutrophic Perch(BPUE) 1 7 0.27 0.641 Mesotrophic=eutrophic Seston(FA%) 112 22 0.002 Mesotrophic≠eutrophic Herbivorouscladoceran(FA%) 112 4.4 0.001 Mesotrophic≠eutrophic Chironomidaelarvae(FA%) 1 5 20.5 0.004 Mesotrophic≠eutrophic Roach(FA%) 1 8 3.2 0.059 Mesotrophic=eutrophic Perch(FA%;allsizestogether) 193 7.7 0.001 Mesotrophic≠eutrophic Perch,youngoftheyear(FA%) 122 13.4 0.001 Mesotrophic≠eutrophic Perch,planktivorous(FA%) 122 6.5 0.003 Mesotrophic≠eutrophic Perch,benthivorous(FA%) 126 11.2 0.001 Mesotrophic≠eutrophic Perch,piscivorous(FA%) 113 3.1 0.024 Mesotrophic≠eutrophic SestonicEPA(concentration) 125 0.025 0.864 Mesotrophic=eutrophic SestonicDHA(concentration) 125 49.6 0.001 Mesotrophic>eutrophic Goldenalgae(%,QFASA) 110 10.2 0.024 Mesotrophic>eutrophic Crypto(%,QFASA) 110 13.3 0.013 Mesotrophic>eutrophic tPOMmicrobes(%,QFASA) 110 6.2 0.034 Mesotrophic<eutrophic Dinoflagellates(%,QFASA) 110 2.1 0.169 Mesotrophic>eutrophic 8 of 18 | TAIPALE ET AL. Lake Köyliönjärvi, but 75 ± 17% of all perch in Lake Pyhäjärvi. Moreover,benthivorousperchcontributed20±15%ofallperchin LakePyhäjärvi,butonly6±3%ofallperchinKöyliönjärvi.Thecontributionofpiscivorousperchtooverallpercheswassimilarinboth lakes(~3 – 5 % ) . 3.4 | Food web structure based on fatty acids Accordingtothetwo-factorPERMANOVAanalysis(PERMANOVA (lake/species): Pseudo-F1/12145 = 6.2/124.3, p =.001),laketype explainedonly0.5%andspecies(organism)84%ofFAvariation, FIGURE 1 Phytoplanktonbiomassof thethreemostabundantclasses(a)and threehighnutritionalqualityclasses(b) inLakeKöyliönjärvi(eutrophic)andin LakePyhäjärvi(mesotrophic).(c)Non- metricmultidimensionalscalingoutput ofbiomassesofdifferentphytoplankton genera.Vectorscitetothephytoplankton genuswiththestrong(r >.75,p <.01) Pearsoncorrelation.Greenalgaeciteto thegenusofColeastrum,Monoraphidium, Oocystis,Pediastrum,andScenedesmus; CyanoscitetothegenusofChroococcus, Cyanodictyon,Microcystis,and Woronichia.Bubbleplotsshowthetotal phytoplanktonbiomassofthesampleand dashedlinesincludesampleswitha40% similarity FIGURE 2 Zooplanktonbiomassofthe (a)herbivorouscalanoids(Eudiaptomus), cyclopoids(Cyclopoida)andcladocerans (Bosmina,Ceriodaphnia,Chyrodys,Daphnia, Diaphanosoma,Holopedium,Limnosida) and(b)predatorcalanoids(Heterocope), cyclopoids(Eucyplops,Macrocyclops, Megacyclops,Mesocyclops,Thermocyclops), andpredatorycladocerans(Leptodora kindtii)inLakeKöyliönjärvi(eutrophic)and inLakePyhäjärvi(mesotrophic).(c)Non- metricmultidimensionalscalingoutput ofbiomassesofdifferentzooplankton genera.Vectorscitetothezooplankton genuswiththestrong(r >.60,p <.01) Pearsoncorrelation.Bubbleplotsshow thetotalzooplanktonbiomassofthe sampleanddashedlinesincludesamples with50%similarity | 15 of 18 TAIPALE ET AL. 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