Long-term trends in mesozooplankton abundance in Chesapeake Bay, USA: influence of freshwater input

Analyses of a 16 year time-series (1985-2000) of mesozooplankton abundance in the Chesapeake Bay reveal the influence of freshwater flow on species composition and abundance. Trend analysis and linear mixed-effects regression models were used to assess long-term variation in, and influence of water-quality parameters (modulated by freshwater input) on, the monthly mean abundance of the 2 dominant copepod species Acartia tonsa and Eurytemora affinis. There were no long-term trends in abundance of either copepod species, with the exception of a slight downward trend for A. tonsa in the mesohaline region of the northern Chesapeake Bay. There were no long-term trends in temperature, but there was a downward trend in salinity related to a wet period in the late 1990s. Linear mixed-effects models showed a negative correlation between freshwater input and A. tonsa abundance in the oligohaline region, and no significant relationship between other water-quality parameters and A. tonsa abundance in the mesohaline region. A. tonsa abundance was positively correlated with temperature in the polyhaline region. E. affinis abundance in the oligohaline region was negatively correlated with biovolume of the ctenophore Mnemiopsis leidyi and positively correlated with phytoplankton abundance. A negative correlation with salinity and a positive correlation with turbidity were found for E, affinis in the mesohaline region. While years of above-average freshwater input were characterized in the spring by high abundances of E. affinis in the mesohaline region and low abundances of A. tonsa in the oligohaline region, the former may show a lagged response depending on the time and magnitude of the input. Freshwater input appears to be mainly influencing habitat parameters specific to each copepod species and top-down control by predators.