Zooplankton are a diverse group of heterotrophic organisms that consume phytoplankton, regenerate nutrients via their metabolism, and transfer energy to higher trophic levels. Over the past 40 years, few studies have specifically targeted zooplankton communities of the York River estuary and tributaries. However, several studies targeting specific taxa, and time series of multiple taxa, provide an emerging view of York River zooplankton community composition and how zooplankton communities change seasonally, and over longer time scales. Microzooplankton communities are dominated by ciliated protozoa, and rotifers are important in fresher water regions. In the lower Bay microzooplankton abundance peaks in spring, and in mid-summer to early fall. The mesozooplankton community is dominated by calanoid copepods Acartia tonsa, Acartia hudsonica, and Eurytemora affinis. Mysids undergo diel vertical migrations and are important food for many fish species in the Bay. Some taxa such as chaetognaths are not endemic to the bay but are transported in from the continental shelf. Various meroplankton such as larvae of decapods, bivalves, and gastropods become abundant at times. A striking seasonal change in the zooplankton community composition occurs in spring when large gelatinous zooplankton such as the ctenophore Mnemiopsis leidyi and (subsequently in summer) the scyphomedusa Chrysaora quinquecirrha (sea nettle) “bloom.” Mnemiopsis blooms now appear earlier in the York River compared to 40 years ago, correlated to earlier warming in spring water temperatures. Humans may be influencing zooplankton populations in the York River via introduced species and eutrophication-induced hypoxia, as well as via input of contaminants. Future research priorities and monitoring needs include long-term monitoring of zooplankton communities, increased studies of the dynamics of microozooplankton and of gelatinous zoopankton, diel and seasonal cycles and grazing rates of some of the lesser studied groups (e.g., other than copepods), and use of new technology such as underwater digital video systems.
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