A drastic reduction in abundance of Dreissena polymorpha Pall. in the Skoszewska Cove (Szczecin Lagoon, River Odra estuary): effects in the population and habitat

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./m2 and 1760 g/m2, respectively. The total zebra mussel population was then theoretically capable of filtering the entire volume of the Cove during 2.5 days.