Recovery of native zooplankton associated with increased mortality of an invasive mussel

Impacts of alien species may change with time but there are few long-term studies of invasions. Here, we present an example of a substantial change in the impact of an alien species that appeared more than a decade after initial invasion. We studied an invasion of zebra mussels (Dreissena polymorpha) into the Hudson River (New York, USA) over a 22 year period (1987–2008) including five years of pre-invasion observations. Zebra mussels caused a substantial and sustained decline in phytoplankton, and until recently, zooplankton. However from 2005–2008, the abundance of copepods, copepod nauplii, and rotifers recovered while tintinnid ciliates partially recovered. These changes are consistent with an increased mortality of larger (. 20 mm) zebra mussels that has altered the filter-feeding impact of the population. Large mussels had a threshold relationship with the abundance of nauplii, rotifers, and zebra mussel veligers suppressing these microzooplankton when filtration by large mussels was . 0.5 m m 2 d . Zooplankton biomass declined approximately 50% after the zebra mussel invasion but has recovered to pre-invasion levels. Overall, while zebra mussels are still present and abundant in the Hudson River, their impact on zooplankton has significantly diminished.