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USING TEMPORAL COHERENCE TO DETERMINE THE RESPONSE



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USING TEMPORAL COHERENCE TO DETERMINE THE RESPONSE TO CLIMATE CHANGE IN BOREAL SHIELD LAKES SHELLEY E ARNOTT1 BILL KELLER2 PETER J DILLON3 NORMAN YAN4 MICHAEL PATERSON5 and DAVID FINDLAY5 1 Department of Biology Queen s University Kingston Ontario K7L 3N6 2 Cooperative Freshwater Ecology Unit Ontario Ministry of the Environment Sudbury ON P3E 2C6 3 Trent University Peterborough ON Canada K9J 7B8 4 York University and the Ontario Ministry of the Environment 4700 Keele St Toronto ON Canada M3J 1P3 5 Freshwater Institute Department of Fisheries and Oceans 501 University Crescent Winnipeg Manitoba Canada R3T 2N6 author for correspondence e mail arnotts biology queensu ca Abstract Climate change is expected to have important impacts on aquatic ecosystems On the Boreal Shield mean annual air temperatures are expected to increase 2 to 4 C over the next 50 years An important challenge is to predict how changes in climate and climate variability will impact natural systems so that sustainable management policies can be implemented To predict responses to complex ecosystem changes associated with climate change we used long term biotic databases to evaluate how important elements of the biota in Boreal Shield lakes have responded to past fluctuations in climate Our long term records span a two decade period where there have been unusually cold years and unusually warm years We used coherence analyses to test for regionally operating controls on climate water temperature pH and plankton richness and abundance in three regions across Ontario the Experimental Lakes Area Sudbury and Dorset Inter annual variation in air temperature was similar among regions but there was a weak relationship among regions for precipitation While air temperature was closely related to lake surface temperatures in each of the regions there were weak relationships between lake surface temperature and richness or abundance of the plankton However inter annual changes in lake chemistry i e pH were correlated with some biotic variables In some lakes in Sudbury and Dorset pH was dependent on extreme events For example El Nino related droughts resulted in acidification pulses in some lakes that influenced phytoplankton and zooplankton richness These results suggest that there can be strong heterogeneity in lake ecosystem responses within and across regions Keywords climate change long term data multiple stressors phytoplankton regional drivers synchrony temporal coherence zooplankton 1 Introduction Human activities are resulting in large scale changes in aquatic ecosystems Schindler 2001 One of the most important stressors is climate change Schindler 1998 There is strong evidence of both increasing global air temperatures and increasing temperature variability Global surface temperatures have increased 0 6 C since 1861 with the 1990s being the warmest decade and 1998 being the warmest year during the instrumental record IPCC 2001 Further increases are anticipated For example mean air temperature in the Boreal Shield is expected to increase 2 to Environmental Monitoring and Assessment 88 365 388 2003 2003 Kluwer Academic Publishers Printed in the Netherlands 366 SHELLEY E ARNOTT ET AL 4 C over the next 50 years Hengeveld 2000 There is also evidence of increased climate variability associated with increased frequency of El Nino events Urban et al 2000 An important challenge is to predict how these changes in climate and climate variability will impact our natural systems so that sustainable management policies can be implemented The current and anticipated changes in climate will have complex and regionally heterogeneous impacts on the physical chemical and biological characteristics of lake ecosystems Magnuson et al 1997 There are numerous anticipated effects including potential changes in water clarity and thermal regimes associated with declines in dissolved organic carbon DOC inputs Fee et al 1996 Snucins and Gunn 2000 alteration of water chemistry including contaminants Webster et al 1996 Yan et al 1996 and changes in the distribution of organisms De Stasio et al 1996 Vander Zanden et al 1999 Leech and Williamson 2001 Direct changes in temperature and precipitation and indirect changes resulting from complex responses of the physicochemical environment are likely to impact freshwater ecosystems through changes in species abundance distribution and composition In addition to potential direct influences there is evidence that climate change may exacerbate the impact of other stressors such as invading species pollution and habitat alteration Schindler 2001 To undertake the challenge of predicting biotic responses to complex ecosystem changes associated with climate change we used long term databases to evaluate how several physical chemical and biotic variables in Boreal Shield lakes have responded to past fluctuations in climate Our data records span a two decade period where there have been unusually cold years associated with the Mt Pinotubo eruption in 1992 and unusually warm years associated with an exceptionally strong El Nino in 1998 Lakes within a region will experience similar fluctuations in climate and therefore limnological variables strongly influenced by climate will be expected to vary in a similar way through time Individual lake characteristics such as water residence time and fish population cycles may create lags and deviations from the general regional response Therefore the similarity in lake response within a region will depend on the relative importance of extrinsic i e regional characteristics versus intrinsic i e individual lake characteristics Rusak et al 1999 We used coherence analyses e g Magnuson et al 1990 Kratz et al 1998 to test for regionally operating controls on climate water temperature pH and plankton richness and biomass Coherence is calculated as the mean Pearson correlation coefficient for all lake pairs and is a measure of the synchrony or similarity in variability through time Plankton richness and biomass were chosen because previous studies have indicated they should respond to climatic signals e g George and Harris 1985 Stemberger et al 1996 and because this information was available for each of the sites We have included pH in our analyses because a relationship between El Nino events and pH has been detected for several of our study lakes in previous studies Keller et al 1992 Yan et al 1996 Dillon et al 1998 CLIMATE CHANGE IN BOREAL SHIELD LAKES 367 Figure 1 Map of the


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