MODIFICATIONS IN LAKE-COMO (N ITALY) CAUSED BY THE ZEBRA MUSSEL (DREISSENA-POLYMORPHA)

A large scale study on the western basin of lake Come (N. Italy) was s tarred in 1995 to examine the effects of the zebra mussel colonization which began in early '90. Our results have been related to '91-92 dat a (pre-Dreissena period), before the maximum colonization of zebra mus sel. In spring and summer of the post-Dreissena period fetal phosphoru s, P-PO4(4), nitrate and chlorophyll values decreased while ammonium a nd transparency increased at every sampling station. Zebra mussel does not modify the trophic stale of this sub-basin but it plays an import ant role in nutrient cycling. The entire population can filter epilimn etic waters 2.1 times per year and can produce 2.9 x 10(4) t/y of pseu dofaeces which are transferred to sediments.