Shifts in phenology of Bythotrephes longimanus and its modern success in Lake Maggiore as a result of changes in climate and trophy

In Lake Maggiore, the density of the invertebrate predator Bythotrephes longimanus increased following lake re-oligotrophication in the late 1980s. This “invasion” was followed by dramatic changes in the pelagic food web, consistent with those that followed the establishment of B. longimanus in North American lakes where it is not native. In this contribution, we explore the modern success of B. longimanus in Lake Maggiore by investigating its phenology and population density, and their correlations with abiotic and biotic factors during the period from 1981 to 2003. A 10-fold increase in the abundance of B. longimanus followed an earlier start and longer duration of annual population growth. Increased prey resources and decreased predation pressure were not observed during the B. longimanus density increase. Instead, a rise in lake temperature may have altered the reproductive cycle of this species. Furthermore, the depth and duration of a refuge from visually orienting fish predators increased during these 20 years, as a result of changes in the thermal stratification regime of the lake. This case study provides a timely example of how climatic changes may interact with biotic drivers (e.g. fish predation) to influence the density and phenology of an invertebrate predator.