Changes in midge assemblages (Diptera Chironomidae) in an alpine lake from the Italian Western Alps: the role and importance of fish introduction

Introduction of exotic species is one of the major human impacts for decline in autochthonous biodiversity. In freshwater lentic environments, alien fish introduction heavily shapes macrobenthic invertebrate structure, with special regard for Chironomidae (Diptera) community. These effects could be more evident in alpine lakes whose environments are considered as remote and undisturbed, but extremely susceptible and vulnerable to both natural and anthropogenic impacts. In this context, modern and subfossil chironomid assemblages were studied in a high-altitude lake (Balma Lake, Piedmont, Italy) which was not previously investigated and subject to brook trout introduction for recreational fishing at the end of the twentieth century (after 1970). Seasonal samplings (summer, autumn) were performed in five littoral and three deep sites, while a core sample was extracted from the deepest point of the lake. The analysis highlighted significant differences in subfossil chironomid communities before and after fish introduction and between subfossil and modern communities, with notable decrease in recent diversities. Dissimilarities were mainly related to Corynocera oliveri, Zavrelimyia, Micropsectra, Metriocnemus, and Heterotrissocladius marcidus type. Therefore, this study highlights the importance and effects of anthropic environmental events, such as fish introduction, in climate reconstructions and their interpretation, especially regarding the last 100/200-year period, when human environmental issues have become more significant.