Combining subfossil Chironomidae and Cladocera remains to evaluate the effect of fish introductions on palaeolimnological records in moutain lakes
Résumé
Mountain lakes are cold and nutrient-poor ecosystems with scarce buffering capacity which are very sensitive to changes both in air temperature and large-scale nutrient transport. Also, mountain lakes have developed a significant role for the economy of human local populations during the past centuries, particularly in regards to their usage as fish suppliers. As such, not only temperature and nutrient transport, but also fish introductions, may play a significant role in shaping the ecological communities inhabiting mountain lakes. Because fish introductions in previously fishless lakes have been performed for centuries (in some cases dating back to the XIV century), it is interesting to analyse their effects on palaeolimnological records, compared to that of climate and/or nutrient enrichment. This question is relevant to adjust management strategies for mountain lakes, as well as for deciphering the effects of fish, climate, and nutrient loading on palaeolimnological records. Here, we propose the use of subfossil remains (i.e. Chironomidae and Cladocera in combination) to evaluate the effects of fish introductions on the invertebrate communities in lakes. We first evaluated the abundance and size of the ephippia of selected Cladocera species, which may vary in response to fish predation pressure. We then analysed the changes in chironomid communities that could be potentially attributed to fish introductions. We hypothesised that these changes may be triggered directly (i.e. direct predation of fish on larger and more visible chironomids) or indirectly (i.e. caused by fish predation on zooplankton, hence indirectly increasing phytoplankton and subsequently the organic matter content in sediments from in-lake productivity). We accompanied these results with estimates on the organic matter content in sediments (e.g. LOI, C/N) and further stratigraphic analyses. Results are presented for four small lakes in the Pyrenees located in the Haut Vicdessos valley, where fish introductions began during the mid XX century.