Restoring lakes through external phosphorus load reduction: The case of Lake Pusiano (Southern Alps)

Abstract

We estimated the external phosphorus load (EPL) between 1960 and 2015 in Lake Pusiano, a midsized subalpine lake that reached its maximum trophic state in the mid-1980s. Using historical data we also estimated the internal phosphorus load (IPL) between 1972 and 2015. EPL reached its maximum in the mid-1980s with phosphorus (P) values ~21 t yr-1 compared to the current value close to 6 t yr-1. IPL was one order of magnitude less and ranged between 0.25 t yr-1 (2015) and 3.6 (1985) t yr-1. The strong reduction of the P load determined a marked decrease of both P (from 200 to 23 μg L-1) and chlorophyll a (from 18 to 8 μg L-1) in-lake concentrations. The process of eutrophication and subsequent recovery, however, showed hysteresis between P load and in-lake P concentrations. In recent years, in particular, the P concentrations at winter overturn seem to be independent from the P load, related to modifications in the hydrological management of the lake that favored P flushout in October-November, when the EPL is maximal. This process led to a marked decrease of the total P concentrations at winter overturn (23 μg L-1), which recently resulted in lower-than-target concentrations (30 μg L-1) established by the Lombardy Region. The lake, nevertheless, suffers from the presence of the toxic cyanobacterium Planktothrix rubescens, and eradicating this species and further improving water quality will likely require an additional abatement of the EPL.