Limnological aspects of an Apennine shallow lake

We present the results of a four-years monitoring program of Lake Duchessa, a small shallow subalpine lake situated at 1788 m a.s.l. in central Apennines (Italy). From 1997 to 2000, during snow free periods, we regularly measured conductivity, pH, macrophyte cover and water transparency. Contemporarily, 4 water samples were collected to determine nutrient (NH3-N, NO2N, NO3-N and PO4-P) and phytoplanktonic chlorophyll a concentrations. Taxonomic compositions of aquatic macrophytes, zooplankton and macroinvertebrates were also determined. Inter- and intra-annual variation of water chemistry and phytoplankton biomass were addressed. Lake Duchessa showed some limnological aspects proper to lowland eutrophic situations due to livestock grazing in the watershed pastures and watering in the lake. In particular, nutrient concentration and phytoplankton biomass reached extremely high values, and lake community was relatively poor. However, despite the high nutrient loading, phytoplankton biomass collapsed in August 1997. In the same occasion, lake water became transparent and remained clear until the end of 1999. The occurrence of this clear water phase and its surprising span may be related to the reduction of tench population (due to a natural fish kill observed during the study) and to the extension of Potamogeton pectinatus L. and Myriophyllum spicatum L. cover. Our results support early observations that water transparency (i.e. environmental quality) of eutrophic shallow lakes can be improved also when nutrient loading is not reduced. Many of the Apennine lakes and ponds situated at high altitudes show conditions similar to those of Lake Duchessa. Therefore this study highlights the ecological processes to be considered in a reliable management of these freshwater systems.