Seasonal Zooplankton Distributions in Quantico Creek and the Adjacent Potomac River

Zooplankton collected with a 153-μm mesh net in Quantico Creek and the adjacent Potomac River were dominated bv the rotifer Brachionus- plicatilis, the cladoceran Bosmina longirostris, and the copepodites and adults of cyclopoid copepods. Eurytemora affinis copepodites and adults, Brachionus calyciflorus, Diaphanosoma brachyurum, and at least five other cladoceran genera and two other rotifer genera were also relatively abundant. Bimonthly zooplankton samples contained fewest organisms (186/m3) in February and maximum numbers (286,000/m3) in June. Rotifers were the most numerous taxon in April, cladocerans dominated the collection in December and June, and copepods were a majority of the organisms collected in February; in August and October species composition was more evenly distributed among the major groups. During periods of psak abundance, zooplankton were concentrated in Quantico Creek and the sampling locations closest to the mouth of the creek as opposed to the more open Potomac River stations. The high densities of rotifers in Quantico Creek nearly coincided with maximum numbers of egg and larval white perch. Early larval white perch have been shown to feed on rotifers such as Brachionus spp. Obvious advantages accrue to fish species such as white perch, which inhabit areas of increased zooplankton densities, relative to species such as striped bass, which remain mostly in the mainstream Potomac River.