The recent palaeolimnology of a remote Scottish loch with special reference to the relative impacts of regional warming and atmospheric contamination

Abstract

Summary 1. High‐resolution palaeolimnological multiproxy data were obtained from an unacidified remote Scottish loch, Loch Coire Fionnaraich (LCFR). The sediment record of spheroidal carbonaceous particles (SCPs) was used to define the onset of low‐level atmospheric industrial contamination (c. 1830 AD) and discriminate between “pre‐industrial” and “industrial” eras. 2. The diatom and chrysophyte cyst fossil assemblages indicated that LCFR was ecologically stable at a decadal to centennial scale prior to the 19th century. However, the species composition of both proxies changed progressively over the last 200 years. Diatom species shifts were taxonomically consistent with those reported for other remote oligotrophic lakes where regional climate warming has been proposed as the dominant driver. 3. We examined the character of species variation across the full core record and within pre‐industrial and industrial periods in isolation to test competing hypotheses of climate (air temperature) and air pollution as drivers of biological change in this remote lake. We also applied SCP flux and Central England Temperature Series (CET) data as potential explanatory variables of species variation. 4. Marked decadal‐ to centennial‐scale variability in climate is not unique to the industrial era. We hypothesised, therefore, that if climate change was the principal driver of species change in LCFR, the species diagnostic of change should provide evidence of similar counter‐variation in the pre‐industrial section of the core. Alternatively, were anthropogenic atmospheric contamination of primary importance, then the species dominating variation in the pre‐industrial and industrial sections should differ markedly. 5. The principal component of variation in both proxies was significantly correlated with a range of CET variables. However, SCP flux was a far stronger predictor of change in diatom composition and CET variables did not account significantly for any residual variation. In contrast, while SCP flux was also the strongest individual predictor of change in chrysophyte cyst composition, CET variables remained significant when SCP flux was held covariable. 6. Species dominating the principal component of variation in diatoms in the pre‐industrial period were unrelated to those dominating the principal change in the flora, but were closely related to the second component in the full core. Similarity between the second component and diatom inferred pH and DOC, therefore, indicated tentatively that pre‐industrial variation in species was dominated by hydrologically driven variation and that this influence had persisted as the sub‐dominant mode to the present. 7. While the overall trajectories of community change in the two assemblages were similar, changes in diatom community (largely benthic) could be linked only to the effects of atmospheric deposition, possibly through effects of nitrogen enrichment. However, shifts in the chrysophyte community (largely planktonic) were most likely explained by an interaction between atmospheric pollutants and warming. We conclude that, although the two proxies exhibit broadly similar patterns of long term change, only change in the planktonic proxy can be linked to climate change, and even here chemical contamination from the atmosphere is likely to have dominated the biological response.