Recovery(?) of native bivalves following the zebra mussel invasion of the Hudson River

Populations of native North American bivalves usually decline or disappear after zebra mussels arrive.  The zebra mussel population of the freshwater tidal Hudson River exploded in late 1992, causing phytoplankton biomass to fall by ~80%.  Populations of all native bivalves (3 unionid species and 1 sphaeriid species) subsequently fell by 65-100% in 1993-1999.  Surprisingly, these declines stopped or even reversed in 2000.  Population density of the unionid Elliptio complanata more than doubled since 2000, with a 38-fold increase in the density of juveniles.  Populations of the 2 other unionids still are too sparse to be reliably evaluated by our monitoring program.  Populations of the sphaeriid clam Pisidium have recovered nearly to pre-invasion densities after falling by 94%.  The mechanisms behind these changed dynamics are unclear – the Hudson still contains large populations of zebra mussels and the phytoplankton has not recovered to pre-invasion levels.  Likewise, the long-term prospects for the Hudson’s native bivalves are unknown, but at least the catastrophic declines of the early years of the invasion did not proceed to extirpation.