Invasions and re-emergences: an analysis of the success of Bythotrephes in Lago Maggiore (Italy)

Bythotrephes was first seen in North America in 1982 (single individual, Lake Ontario) and noticed elsewhere in the Laurentian Great Lakes in abundance in 1985. Starting from 1987 it sharply increased in the open-water zooplankton of Lago Maggiore, reaching values never recorded in the past 40 years. Despite being native, the species' impact on Lago Maggiore was somewhat comparable to that observed in invaded North American lakes. The re-emergence led to an overall increase in invertebrate predation which became permanent five years after Bythotrephes' establishment, causing direct and indirect effects on the food web. Changes included a sharp decrease in the large filter-feeder Daphnia, whose mortality rate increased, and consequently an increase in phytoplankton cell density; an increase in abundance and size of colonial rotifers with a decline in Leptodora abundance, and an overall decrease in zooplankton biomass were also observed. The increase of Bythotrephes in Lago Maggiore, however, was related to a shift in population phenology, which resulted in seasonal growth starting earlier and lasting longer. Daphnia phenology changed only later, suggesting that the impact was consequent to a temporary decoupling between Bythotrephes and its preferred prey. The shift in density and phenology became permanent. Over the long-term, however, potential competitors and prey were able to recover, reaching levels of abundance comparable to those recorded before the Bythotrephes' re-emergence. Such a response is likely attributable to behavioural adaptation mechanisms resulting in temporal and spatial displacement of potential competitors, predators and prey.