On the use of subfossil remains in mountain lake sediments to trace the effects of fish introductions on invertebrate communities
Résumé
Subfossil remains (i.e. Chironomidae and Cladocera) are potentially valuable to trace the effects of fish introductions on the invertebrate communities in lakes. In this study, the abundance and size of the ephippia of Daphnia (Cladocera), and the chironomid head capsule remains, were analysed across sediment cores in four mountain lakes of the Pyrenees (Haut Vicdessos valley), where fish introductions began during the mid XX century. The aim was to elucidate changes in invertebrate communities attributable to the effect of fish introductions, but not to other anthropogenic pressures such as livestock grazing on lake catchments. It was assumed that changes in invertebrate communities may not only reflect direct predation by fish, but also regime shifts in ecosystem functioning after fish were introduced. We hypothesised that 1) the effects of fish on invertebrates, which can be driven by direct fish predation either on more vulnerable chironomids or on larger Daphnia individuals, can be traced across sediment cores; 2) indirect effects can also be visualised as fish introduced may alter lake nutrient re-cycling, which increases organic matter content in lake sediments, thus favouring some chironomids more than others; and 3) the effect of fish introduction on lake invertebrates can be distringuished from that of livestock grazing. Preliminary results indicate that both direct and indirect effects of fish on invertebrates can be traced, and may be stronger than that of livestock grazing. However, the effects of introduced fish may also depend on the type of fish introduced, fish biomass and lake size.