Page 1 of 35 Freshwater Biology 1 2 3 4 5 Nutrients provide a stronger control than climate on recent diatom 6 7 8 communities in Esthwaite Water: evidence from monitoring and 9 10 11 palaeolimnological records 12 13 14 15 16 XUHUI DONG *, +, HELEN BENNION * , STEPHEN C. MABERLY ++ , CARL D. 17 18 * * * 19 SAYER , GAVIN L. SIMPSON , RICHARD W. BATTARBEE 20 Copy for Review 21 * 22 Environmental Change Research Centre, Department of Geography, University 23 24 College London, Gower Street, London WC1E 6BT, UK 25 26 + 27 State Key Laboratory of Lake Science and Environment, Nanjing Institute of 28 29 30 Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, P. R. 31 32 China 33 34 ++ 35 Centre for Ecology and Hydrology, Lancaster Environment Centre, Bailrigg, 36 37 38 Lancaster, UK 39 40 41 42 43 Correspondence: Dr. Xuhui Dong email: [email protected] 44 45 46 Present address: State Key Laboratory of Lake Science and Environment, Nanjing 47 48 Institute of Geography and Limnology, Chinese Academy of Sciences 49 50 51 73 East Beijing Road, Nanjing 210008, P. R. China 52 53 54 55 56 Abbreviated title: Nutrients versus climate in Esthwaite Water 57 58 59 60 Keywords: diatoms, climate change, nutrients, limnological records, additive model - 1 - Freshwater Biology Freshwater Biology Page 2 of 35 1 2 3 4 Summary 5 6 7 1. A long-term monitoring programme on phytoplankton and physico-chemical 8 9 10 characteristics of Esthwaite Water (England) that started in 1945 provides a rare 11 12 opportunity to understand the effects of climate and nutrients on a lake ecosystem. 13 14 15 2. Monitoring records show that the lake experienced nutrient enrichment from the 16 17 18 early 1970s, particularly after 1975, associated with inputs from a local sewage 19 20 treatment plant,Copy resulting in formarked Reviewincreases in concentration of soluble reactive 21 22 23 phosphorus (SRP). Climatic variables, such as air temperature (AirT) and rainfall, 24 25 26 exhibit high variability with increasing trends after 1975. 27 28 3. Diatom analyses of an integrated 210 Pb-dated lake sediment core from Esthwaite 29 30 31 Water, covering the period from 1945 to 2004, showed that fossil diatoms exhibited 32 33 34 distinct compositional change in response to nutrient enrichment. 35 36 4. Redundancy analysis (RDA) based on diatom and environmental datasets over the 37 38 39 past 60 years showed that the most important variables explaining diatom species 40 41 42 composition were winter concentrations of SRP, followed by AirT, independently 43 44 explaining 22% and 8% of the diatom variance, respectively. 45 46 47 5. Additive models showed that winter SRP was the most important factor controlling 48 49 50 the diatom assemblages for the whole monitoring period. AirT had little effect on the 51 52 diatom assemblages when nutrient levels were low prior to 1975. With the increase in 53 54 55 nutrient availability during the eutrophication phase after 1975, climate became more 56 57 58 important in regulating the diatom community, although SRP was still the major 59 60 controlling factor. - 2 - Freshwater Biology Page 3 of 35 Freshwater Biology 1 2 3 4 6. The relative effects of climate and nutrients on diatom communities vary 5 6 7 depending on the timescale. The RDA analysis and additive model revealed that 8 9 10 climate contributed little to diatom dynamics at an annual or decadal scale. 11 12 7. The combination of monitoring and palaeolimnological records employed here 13 14 15 offers the opportunity to explore how nutrients and climate have affected a lake 16 17 18 ecosystem over a range of timescales. This dual approach can potentially be extended 19 20 to much longerCopy timescales (e.g.for centuries), Review where long-term, reliable observational 21 22 23 records exist. 24 25 26 27 Introduction 28 29 30 Lakes are important resources for humans, not only for supplying drinking water, but 31 32 also for supporting other activities such as agriculture, industry and tourism. 33 34 35 Nevertheless lakes face a variety of environmental pressures which greatly threaten 36 37 38 their ecological integrity (Vorosmarty et al. , 2000). Lakes have undergone substantial 39 40 deviations from pristine conditions over the past few centuries (e.g. Bennion, Fluin & 41 42 43 Simpson, 2004; Taylor et al. , 2006; Bjerring et al. , 2008). One key pressure is nutrient 44 45 46 enrichment, which has received wide attention and continues to dominate the science 47 48 and policy agenda (Bennion & Battarbee, 2007). Lakes have also experienced serious 49 50 51 pressure from climate change which in recent decades has proceeded at a rate beyond 52 53 54 historical natural variability (IPCC, 2007; Battarbee & Binney, 2008). The changing 55 56 climate may alter the structure and function of aquatic ecosystems directly, but may 57 58 59 also exacerbate other environmental pressures such as eutrophication (George, Hurley 60 & Hewitt, 2007; Whitehead et al. , 2009; Jeppesen et al, 2010). - 3 - Freshwater Biology Freshwater Biology Page 4 of 35 1 2 3 4 For the purposes of restoration and lake management, knowledge of how climate 5 6 7 change will affect lake ecosystems and how it may interfere with nutrient dynamics is 8 9 10 required (Jeppesen et al. , 2010). To decipher the confounding effects of climate and 11 12 nutrients on lakes, long-term monitoring datasets including ecological communities 13 14 15 and environmental variables are needed. Such records are rare, however, or may lack 16 17 18 consistency in methodology over different monitoring periods (Blenckner, 2005). 19 20 PalaeolimnologicalCopy records canfor augment Review the relatively short observed records to infer 21 22 23 historical changes in lake ecosystems over longer timescales of several centuries or 24 25 26 more (Smol & Cumming, 2000; Battarbee et al. , 2005); nevertheless, they lack direct 27 28 records of historical climate and nutrient changes and may also lack good 29 30 31 chronological resolution. Given the clearly complementary strengths and weaknesses 32 33 34 of monitoring and palaeolimnological records, they can be integrated to conduct 35 36 research on separating climate and nutrient effects on lake ecosystems (Battarbee et 37 38 39 al. , 2005). 40 41 42 Esthwaite Water (N 54°21.56', W 002°59.15') is a well-studied site where both 43 44 long-term high-resolution monitoring and numerous palaeo-environmental studies 45 46 47 based on sediment cores have been undertaken (Bennion, Monteith & Appleby, 2000). 48 49 50 Research initiated by the Freshwater Biology Association in 1945 and undertaken by 51 52 the Centre for Ecology and Hydrology and its forerunners since 1989 offers a rare 53 54 55 long-term and detailed record of key climate- and nutrient-related variables and 56 57 58 biological communities (Maberly et al. , 1994). Importantly, this monitoring record 59 60 covers both oligotrophic (prior to 1970s) and eutrophic phases (post-1970s), whereas - 4 - Freshwater Biology Page 5 of 35 Freshwater Biology 1 2 3 4 most other monitoring records capture only the nutrient enrichment phase (Battarbee et 5 6 7 al. , 2005). Consequently, this valuable record allows a direct comparison of the 8 9 10 responses to climate forcing under different trophic conditions. 11 12 Given the complex interactions among environmental variables, statistical 13 14 15 approaches have recently been developed to disentangle their relative effects. For 16 17 18 example, variance partitioning (Borcard, Legendre & Drapeau, 1992) is used 19 20 frequently to Copyidentify the major for forcing Review variables and their relative effects on aquatic 21 22 23 ecosystems over different spatio-temporal scales (e.g. Lotter & Birks, 1997; Hall et al. , 24 25 26 1999; Quinlan et al. , 2002; Bradshaw, Rasmussen & Odgaard, 2005; Kernan et al. , 27 28 2009). This method, however, can estimate only the total effect of the various 29 30 31 covariates over the time period of interest, not answering questions about when and 32 33 34 where the various covariates may be driving change in the response variable(s) 35 36 (Simpson & Anderson, 2009). To overcome this shortcoming, Simpson & Anderson 37 38 39 (2009) employed a flexible and powerful statistical tool, additive modeling (Wood, 40 41 42 2006), to elucidate the critical questions of how much, and when, do the interacting 43 44 factors affect the lake ecosystems. 45 46 47 By combining high-resolution diatom records from a sediment core and existing 48 49 50 long-term monitoring data for Esthwaite Water, we employ both redundancy analysis 51 52 and additive models to determine how climate and nutrients have affected the lake 53 54 55 ecosystem over the period 1945-2004. First, RDA was used to identify the major 56 57 58 factors driving diatom community changes and eliminate insignificant and redundant 59 60 collinear variables. Second, RDA was used to calculate the independent effect of each - 5 - Freshwater Biology Freshwater Biology Page 6 of 35 1 2 3 4 factor that can explain the main patterns of diatom variation. Finally, an additive model 5 6 7 was employed to identify the relative importance of the main controls on the diatom 8 9 10 community in greater temporal detail. Diatoms were selected since they have been 11 12 widely used to indicate trophic changes (e.g. Sayer & Roberts, 2001; Bennion, Fluin & 13 14 15 Simpson, 2004; Werner & Smol, 2005; Yang et al. , 2008) or climate-induced changes 16 17 18 in lakes such as temperature, extent of ice cover, stratification patterns and water depth 19 20 (Smol & Cumming,Copy 2000; Battarbee, for 2000;Review Mackay, 2007). They can, therefore, act as 21 22 23 useful indicators of ecological change driven by both climate and nutrients.