Cladocera in mountain lakes : their potential role in reconstructing climate change
Research on climate change is becoming increasingly important to understand its multifaceted impacts on ecosystems at the present day and to identify patterns of natural climate variability and their impact in the past. Many remote Northwest European mountain lakes are undisturbed by air pollution and land-use change and can potentially be used for studies of climate change. Climate change influences the mountain lake ecosystem directly via changes in patterns of ice-cover, seasonality, stratification and hydrology and indirectly via changes in alkalinity, nutrient dynamics and habitat structure. Cladocera can be used as one of the significant indicators of such change since they have well-defined taxonomy, life history behaviour, and clear ecological preferences, and the fossil remains of these fauna are well preserved in lake sediments. In this thesis, first, sampling of contemporary and surface sediment assemblages of Cladocera along various transects in Loch Coire Fionnaraich (LCFR), Scotland was carried out to understand the response of Cladocera to temporal and spatial variables at the site and to assess the patterns of fossil representation in the loch. Cladoceran assemblages varied with micro-habitat structure and seasons, while a taphonomic study indicated changes in the patterns of fossil representation in the sediments with respect to seasonality, time, lake morphometry and the source habitats. Secondly, a calibration training set was developed for surface sediment assemblages of Cladocera and major physico-chemical variables from 72 mountain lakes of Scotland and Norway. In a direct gradient analysis and the associated Monte Carlo permutation tests, summer lake surface water temperature (LSWT) explained the maximum variation for the species-environment relationships in the data-set. A Cladocera-based PLS-transfer function 0 2 for summer LSWT was then developed. A transfer function model with RMSEP=1.80 C and r =0.71 allowed the past summer LSWT of LCFR to be reconstructed. The results of Cladocera-inferred summer LSWT reconstructions were compared with the local instrumental air temperature records over the last 20 years and the Central England Temperature series over the last 110 years. Reconstruction was also assessed for taphonomic bias. Although the reconstructed summer LSWT showed a relatively similar pattern to the local instrumental temperature records, the relationship between the longer CET series and reconstructed LSWT was poor indicating either the insensitivity of the loch to temperature change prior to the recent warming of the last few decades or that the Cladocera response to temperature predominantly a more complex, indirect one. For the most recent period a rapid decline in species diversity and in the relative abundance of littoral Cladocera and a reciprocal increase in total planktonic Cladocera together with changes in loss-on-ignition of the sediment appears to be associated with the impact of recent climate warming. Again the predominant mechanisms may be indirect involving changes in trophic status and in the habitat architecture of the loch. In conclusion, it is essential to understand further the factors controlling the community dynamics of Cladocera and the overall ecological complexity of mountain lakes before reliance can be placed on simple Cladocera-temperature transfer function for climate change reconstruction.