Subfossil Chaoborus mandibles confirm historical fish decline in two acidified lakes, SW Sweden

摘要

Sediment cores from acidified Lake Gaffeln and Lake Lilla Hastevatten, SW Sweden, were analysed to test if the occurrence of subfossil Chaoborus mandibles were coupled to extinction of fish in the early 1970s. Mandibles of C. flavicans were found throughout the sediment cores while mandibles of C. obscuripes occurred in more recent sediments only. The appearance of C. obscuripes mandibles was accompanied by an increase of C. flavicans mandibles in both lakes. indicating reduced fish predation and eventual fish loss. The increase in Chaoborus flavicans density confirms earlier studies that this species is favoured by the loss of fish in acidified lakes. Since C. obscuripes is a strong indicator of fish-free conditions, we propose that this species can be used in paleostudies as an indicator of fish absence in the same way as C. antericanus in North America. However, mandibles of C. obscuripes were scarce in the sediment cores, and it will likely be of best use as indicator in shallow and humic lakes, i.e. its preferred lake type.