A drastic and rapid reduction in the abundance of zebra mussel was observed in the Skoszewska Cove, an area in the River Odra estuary. As a result, the core of the recovering population consisted of young individuals of a lowered condition. The substantial decrease in the total biofiltration capacity of Dreissena resulted in a significant alteration of some physical, chemical, and biological parameters of the Cove: the phytoplankton biomass, chlorophyll a content, and oxygen saturation of the water increased and water transparency decreased. Ammonia and nitrate nitrogen concentrations decreased as a result of a marked reduction in the zebra mussel excretion and in remineralisation of nutrients bound in the Dreissena pseudofaeces. The major cause of the ecological catastrophe described was most probably the excessive density of the zebra mussel population and the resultant depletion of the available food resources. Prior to the reduction, the zebra mussel resources in the area of its densest occurrence, i.e., within 2.0-3.5 m depth (ca 52% of the Cove's bottom surface), was estimated at about 19 thou. t., abundance and biomass averaging 4.700 ind./m 2 and 1760 g/m 2 , respectively. The total zebra mussel population was then theoretically capable of filtering the entire volume of the Cove during 2.5 days. averaged ca 3 thou. ind./m 2 and more than 1 kg/m 2 , respec- tively. The total zebra mussel resources in the area were estimated at 22 thou. t! The zebra mussel was an extremely important component of the Cove biocoenosis and con- trolled the trophic status of the area. The survey conducted in 2005 revealed a drastic reduction in the zebra mussel abundance in the Cove. The size/age structure of the local population was changed as well, and the mussel's condi- tion was observed to have deteriorated. Certain chemical parameters of the water as well as the phytoplankton stand- ing stock were clearly altered as well; in addition, Carlson's TSI was observed to have significantly increased. The literature contains numerous descriptions of the Dreissena-habitat interactions during a rapid increase in the population size, particularly during zebra mussel's in-