Lake-climate interactions: hydrological forcing and ecological response

The extent to which 20th century climate affects temperate lakes and their biota is difficult to determine. This is due to the lack of long-term monitoring data and the over-riding effects of other anthropogenic stressors, notably acidification and eutrophication. Lake sediment records, however, provide an excellent archive of palaeolimnological change, particularly of biological change and can be used to extend the monitoring timescales. This project aims to determine if it is possible to examine the response of the biota using Chironomidae, diatoms and fossil pigments in Danish lakes to climate forcing (e.g. the 1975/6 drought) against a background of other changing environmental conditions. The response of lakes to multiple stressors at different temporal scales is also a key theme. Ravn Sø is the focus of a high resolution (~1 yr) study where biological changes can be compared with independent annual climate records and monitoring data. At Sarup Sø the objective is to assess the impact of climate and environmental change against a background of cultural change during the Mesolithic-Neolithic transition (~5900 cal. yr BP) and the Neolithic when humans first began to significantly alter the environment with the introduction of agriculture. Few studies statistically assess the impacts of different forcing factors on temperate lowland lakes over varying timescales (e.g. the mid-Holocene and the 20th century). Thus this thesis fills a gap in knowledge, particularly in Europe where much of the research is focused on high latitude or high altitude lakes. Multivariate analysis comprising PCA, RDA, variance partitioning and DCCA using proxy and instrumental climate data enabled the effect of the different variables to be analysed. At Ravn Sø the biota responds to the decreased precipitation in 1975/6 through fewer nutrients entering the lake. Due to an increased retention time there is internal loading releasing authigenic phosphorus from the sediment enhancing the in-lake phosphorus concentration. The biota at Sarup Sø reflects lake-level fluctuations due to precipitation variability (Chironomidae) and eutrophication as a result of anthropogenic landscape disturbance during the mid-Neolithic. Landscape modification was evident through a two-fold increase in the diatom-inferred TP compared with earlier periods supported by increased algal productivity (pigments) and archaeological evidence.