A Palaeolimnological Assessment of Human and Climate Influences on Chironomid Communities in Western Ireland
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Climate warming and biological response are prominent themes in Irish environmental research. It is widely acknowledged that certain species can be used as indicators of climate variables, and can be used to reconstruct environmental conditions spatially and temporally. The examination of ecological records to test the predictive response of certain species to climate change is ongoing and essential in the advancement of our understanding of global change. It is being increasingly recognised that chironomids (non-biting midge, Insecta; Diptera) are reliable indicators of summer temperatures. However, the breadth of palaeolimnological studies using chironomids to reconstruct temperature in Europe have tended to focus on high-latitude or high-altitude locations, or the late Glacial to early Holocene transition. It is not known if chironomids can reliably reconstruct Holocene temperature in less climatically-extreme locations such as Ireland. For the last 6,000 years, human influence has been a prominent feature on the Irish landscape and, as a result, lakes on the island are rarely isolated from such impacts. This study set out to test the sensitivity of chironomids to recent temperature and land-use change, and assess their potential as palaeotemperature indicators across various timescales. In order to investigate climate change and human impacts on lakes in western Ireland, two main lines of enquiry were employed: the exploration of temperature data from a previously unexplored climate record in Markree Observatory (1842 to 2009), and chironomid-based palaeolimnological analysis. Results from the Markree reconstruction show the unique temperature regime for the County Sligo region, with Markree exhibiting characteristics of an 'inland' site despite its coastal location. As this record has been largely absent from past analyses of Ireland's long-term temperature trends, it helps to increase the spatial coverage of the extended Irish climate chronology. The newly reconstructed climate time-series for the study region allowed for greater accuracy in comparing recent temperature change with palaeolimnological changes through time. Chironomids were extracted from the centre of four different lakes. Three of these lakes were used to assess chironomid sensitivity to recent climate change and land-use histories through redundancy analysis and direct time-series comparisons with the Markree record and land-use records from the recent past. The chironomid communities from two of these lakes proved to be climatically sensitive, with chironomid-inferred temperatures following instrumental temperature trends, despite low impact human activities in the lake catchments. The chironomid community from the third lake was found to be more responsive to changes in lake level due to the presence of large, shallow shelves within the lake, which de-coupled the chironomid-temperature relationship. The fourth lake was used to investigate chironomid sensitivity to long-term Holocene climate fluctuations. This study is the first chironomid-based quantitative temperature reconstruction over the Holocene in Ireland. The temperature change inferred from this lake is largely in agreement with climate trends and events inferred from other palaeoclimate records in Europe and Ireland, including a Holocene Thermal Maximum at 9,600 cal. yr BP, a cooler trough between 7,800-7,500 cal. yr BP, mid-Holocene warmth peaking at 6,000 cal. yr BP, with subsequent cooling as Neoglacial conditions establish, with notable cooler phases between 4,000 and 3,600 cal. yr BP, and between 1,800 and 1,650 cal. yr BP. The overall findings of this study show the potential for chironomids to be used as palaeotemperate indicators across various timescales, and as a tool to tease apart human and climate impacts on Irish lakes through time.
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