Trophic Modifications in Lake Como (N. Italy) Caused by the Zebra mussel (Dreissena Polymorpha)

A large scale study on the western basin of Lake Como (N. Italy) was started in 1995 to examine the effects of the zebra mussel colonization which began in early’ 90. Our results have been related to’ 91–92 data (pre-Dreissena period), before the maximum colonization of zebra mussel. In spring and summer of the post-Dreissena period total phosphorus, P-PO₄,, nitrate and chlorophyll values decreased, while ammonium and transparency increased at every sampling station.Zebra mussel does not modify the trophic state of this sub-basin but it plays an important role in nutrient cycling. The entire population can filter epilimnetic waters 2.1 times per year and can produce 2.9 × 104 t/y of pseudofaeces which are transferred to sediments.