Limnological and ecological sensitivity of Rwenzori mountain lakes to climate warming

Abstract

An increasing number of studies forecast that anthropogenic climate change poses serious consequences for the biodiversity and ecosystem functioning of high-elevation mountain lakes, through a series of both direct and indirect effects. The impacts of future climate warming on alpine ecosystems are of particular concern, given that warming is expected to be most pronounced at high elevations around the globe. Here, we evaluate the limnological and ecological sensitivity of high-elevation lakes in the Rwenzori Mountains (Uganda-D. R. Congo) to climate change. This is done by comparing the species assemblages of larval chironomid remains deposited recently in lake sediments with those deposited at the base of short cores (dated to within or shortly after the Little Ice Age) in 16 lakes. Chironomid-based reconstructions of mean annual air temperature (MATemp) are made using a variety of inference models (with transfer functions based on weighted averaging, weighted-averaging partial least squares, and a weighted modern analogue technique), and two different calibration data sets, one covering the full regional temperature gradient and one comprising only high-elevation Rwenzori lakes and ponds. The reconstructed historical temperature change ranges between a cooling of -2.03A degrees C and a warming of +3.22A degrees C (with n = 16 lakes x 3 models x 2 calibration data sets). However, excluding the atypical mid-elevation lake Mahoma (2,990 m altitude), we find a three-to-one ratio of cases of inferred warming against inferred cooling, and of the 24 Delta MATemp values exceeding 0.60A degrees C, 23 are positive and only one is negative. Chironomid-inferred temperature changes mostly fall within the error range of the regional temperature inference models. A generalized linear mixed model analysis of the combined result from all lakes (except Mahoma) nevertheless indicates significantly warmer MATemp (on average +0.38 +/- A 0.11A degrees C) at present compared to between similar to 85 and similar to 645 years ago. Inferred temperature changes are independent of whether lakes are located in glaciated or non-glaciated catchments, and of the age of the core base, suggesting that at least part of the signal is due to relatively recent, anthropogenic warming. The direction of faunal change at the lakes in relation to established species-environment relationships suggests that part of the observed shifts in species composition reflect lake-specific evolution in habitat features other than temperature, such as nutrients, pH or oxygen regime, which in our present calibration data set co-vary with temperature to a greater or lesser extent. The fairly uniform and marked historical warming trend in Rwenzori lakes documented by this study highlights their ecological vulnerability and their value as early warning systems for detecting the limnological and ecological effects of global warming

Modern and pre-Industrial Age distributions of Cladocera in Italian and Swiss Alpine lakes

“…Knowledge of reference conditions is very important to the characterization of ecosystem responses to stress and to establishing realistic restoration targets. Paleolimnology has become an important tool for determining baseline conditions (Rühland et al, 2003;Sweetman et al, 2008;DeSellas et al, 2011), but single samples from bottom sediments must be seen only as snapshots from the past; they can represent also the naturally changed assemblages e.g., during the cold Little Ice Age (Eggermont et al, 2010;. Community dynamics are influenced by natural as well as anthropogenic variables, and this phenomenon dictates that more continuous analysis of cores over a period of decades to centuries should be undertaken to reveal long-term averages and degree of fluctuation.…”

“…Cladocerans in Alpine lakes have been subject to many anthropogenic stresses, the most severe of which are believed to be air-borne industrial pollutants (including acid deposition) and fish introductions (Cammarano & Manca, 1997;Manca & Armiraglio, 2002;Guilizzoni et al, 2006a). In the current study, we use the ''top-bottom'' approach of paleolimnology (Smol, 2002;Sweetman et al, 2008;Eggermont et al, 2010;DeSellas et al, 2011) to assess modern and pre-Industrial Age cladoceran communities in the Italian and Swiss Alpine lakes. This approach involves comparison of sediments deposited shortly before the Industrial Age (pre *1850 AD) with those deposited at the sediment surface and representing modern conditions.…”

Paleolimnological reconstruction of the trophic state in Lake Balaton (Hungary) using Cladocera remains

“…Consequently, palaeolimnological information is crucial to evaluate the impact of future perturbations in these systems. Analyses of Cladocera remains and other biological proxies preserved in lake sediments are useful tools for the evaluation of a lake's response to environmental changes (Korhola & Rautio, 2001;Buczkó et al, 2009) such as eutrophication (Brodersen et al, 1998;Jeppesen et al, 2003), acidification (Hořická et al, 2006), trophic structure (Jeppesen et al, 2002) and climate change (Hofmann, 2000(Hofmann, , 2001Eggermont et al, 2010).…”

Cladocera response to Late Glacial to Early Holocene climate change in a South Carpathian mountain lake

“…Climate change in mountain areas is a major concern especially as temperature at high altitude is increasing more rapidly than in the lowlands (Battarbee, 2010). Warming can have a direct effect on mountain lake biodiversity by-for example-causing a reduction in habitats available for cold stenothermic taxa (Č iamporová-Zaťovičová et al, 2010;Eggermont et al, 2010). Climate warming can also affect lake biota indirectly by modifying the behaviour of pollutants (Battarbee, 2010).…”