January 4, 2002
Overview of historical findings (1985-2000) of the freshwater zone (0-0.5 ppt salinity)

Most stations located in freshwater habitats support diverse zooplankton communities with high average annual biomass and density. Abundant species are listed in Table 1.

Table 1. Abundant mesozooplankton of the freshwater zone in Marylandís Chesapeake Bay.

Group Species


Bosmina longirostris, Diaphanosoma leuchtenbergianus, Moina microura

Copepods (cyclopoid)

Cyclops bicuspidatus, Mesocyclops edax, Cyclops vernalis

Copepods (calanoid)

Eurytemora affinis
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Throughout the study period, cladocerans consistently dominated the warmer months while the calanoid copepod Eurytemora affinis dominated the winter season.

Versar, Inc and PBS&J, Inc. 2001. Chesapeake Bay Water Quality Monitoring Program 2000 Mesozooplankton Component. Prepared for Maryland Department of Natural Resources.

Brief Overview of Zooplankton
Monitoring Results in the Bay: 1985-1998

A sharp decline in zooplankton density has been observed in Maryland’s main stem portion of the Chesapeake Bay during the summer seasons between 1985 and 1998. Stations located between the Bay Bridge and the mouth of the Potomac River have declined between 53 and 88% in summer zooplankton density. This reduction has been accompanied by an increase in summer ctenophore biomass and a decreasing trend in bay anchovy relative abundance.

a sketch of two different ctenophores.

Ctenophores, sometimes called comb jellies, sea gooseberries or sea walnuts, may have become a particularly important predator in the food web of the Bay during the last decade. They are voracious predators on animal plankton (i.e., zooplankton eggs, juveniles and adults, larval fish, fish eggs and bivalve larvae). The ctenophores resemble jellyfish, such as the Sea Nettle (Chrysaora quinquecirrha) that many Bay swimmers will be familiar with, but they are not as they lack the stinging cells of a true jellyfish. Many ctenophores do not have tentacles. They can grow and mature quickly, and produce multiple generations each summer. Typically, the primary species of ctenophore in the Maryland portion of the Bay is Mnemiopsis leidyi which grows to about 4 inches as an adult. Jellyfish are the primary predators of ctenophores in the Bay. It has been hypothesized that lower salinity waters occurring in Chesapeake Bay in recent years produced poor habitat for the jellyfish. A decline in the jellyfish populations may have released the predation pressure on the ctenophores and allowed the ctenophore populations to increase. The increase in ctenophore abundance and their hearty appetite for zooplankton is therefore a possible link in explaining the decline of zooplankton abundances in the main stem of the Bay.

chart showing striped bass food availability

Zooplankton density and fish food availability, as measured by zooplankton abundance in the spring of the year, usually vary between the Patuxent, Potomac and Choptank Rivers. The upper Patuxent River has shown significant annual increases in zooplankton density between 1985 and 1998. Since 1994, spring season data has indicated an optimal level of zooplankton food was available for larval striped bass on the Patuxent River. The Choptank River has met the optimal food level requirements for larval striped bass during 1987, 1992, 1993, 1995, 1996 and 1998. The Potomac River only achieved optimal food availability conditions in 1995. Zooplankton density during the spring season on the Potomac River has been declining since 1995.

For more information, please contact Peter Tango at (410) 260-8651.

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