Decadal trends of pelagic algal biomass capacities in Lago Maggiore (N. Italy)

During the last two decades Lago Maggiore, the second largest Italian lake, located in the subalpine lake district, moved from meso-eutrophy during the mid- 1970s to an oligotrophic condition as a consequence of change in the nutrient load (MANCA et al. 1992, DE BERNARDI et al. 1996). Following the decrease of inlake TP concentration, notable changes in the plankton assemblages occurred (MANCA & RUGGIU 1998, RUGGIU et al. 1998) after a period of resilience of the plankton communities against falling phosphorus. Different trophic states affect the capacity of a system and the biomass a lake can support (REYNOLDS 1997). This study evaluates algal levels, measured as chlorophyll a, in Lago Maggiore during its recent trophic evolution. We calculated potential algal carrying capacity based on phosphorus and solar radiation, representing nutrient and energy limitation, respectively. We analysed the relationship between these two resources during the lake's trophic evolution by dividing the data series into three periods corresponding to the response of the lake to decreasing phosphorus load.