Using invertebrate remains and pigments in the sediment to infer changes in trophic structure after fish introduction in Lake Fogo: a crater lake in the Azores

Fish introduction may have marked effects on the trophic dynamics and ecological state of former fishless lakes, but due to scarcity of historical data this can seldom be documented. We used remains of cladoceran, chironomid and pigment assemblages in the sediment archive to unravel the effect of introduction of carp (Cyprinus carpio), rainbow trout (Oncorhynchus mykiss) and a cyprinid (Chondrostoma oligolepis) in Lake Fogo, the Azores (Portugal). The stratigraphical record showed two major shifts in community assemblage coinciding with the time of introduction of carp (AD ca. 1890) and trout (AD 1941), respectively. Carp introduction was followed by an abrupt and major decline in the abundance of chironomids, a shift in the cladoceran community from a benthic to a more pelagic dominated community, and Daphnia size was significantly reduced. Pigment assemblages also indicated a shift from a benthic to a pelagic dominated ecosystem, as cryptophytes became markedly more abundant at the expense of benthic diatoms. Trout introduction was followed by a return to a more benthic cladoceran and benthic algae (pigments) dominated state, which we attribute to trout predation on carp leading to improved water clarity. A steady increase in the abundance of pigments and cladoceran remains followed, suggesting enhanced productivity, which may be attributed to enhanced atmospheric nitrogen deposition and introduction of C. oligolepis. We conclude that fish introduction has profoundly altered the trophic dynamics and the relative importance of benthic and pelagic production in this species poor and natural fishless lake in the Azores, and likely in most others lakes at the archipelago islands as fish stocking has been a widespread practice.