Zooplankton ecology in the Chesapeake Bay estuarine turbidity maximum, with emphasis on the calanoid copepod Eurytemora affinis

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Lloyd, Scott Steven
Roman, Michael R
The estuarine turbidity maximum (ETM) region of Chesapeake Bay, located near the limit of saltwater intrusion, is characterized by high total suspended solid (TSS) concentrations, high light attenuation, and high densities of zooplankton. Due to high light attenuation, primary production is generally low in ETMs, yet the Chesapeake Bay ETM region is often considered a 'hot spot' of zooplankton abundance within the Bay. The omnivorous copepod <em>Eurytemora affinis</em> is especially prevalent in the ETMs of Chesapeake Bay and its tributaries and in ETM regions worldwide. In order to determine the factors influencing 1) zooplankton distribution and abundance in the Chesapeake Bay ETM, 2) <em>E. affinis</em> reproduction in the Chesapeake Bay and Choptank River ETMs, and 3) zooplankton position maintenance, cruises in the Chesapeake Bay and Choptank River ETMs were conducted in 1996 and 2001-2003. Laboratory experiments examining the egg production cycle of <em>E. affinis</em> were also performed. The cruise results show that zooplankton taxa within the Chesapeake Bay ETM region tend to be distributed along a salinity gradient from up-estuary to down-estuary, with cladocerans being most common in low salinity/freshwater regions, <em>E. affinis</em> found in slightly higher salinities than cladocerans, and mysids and the copepod <em>Acartia tonsa</em> found in more mesohaline conditions. <em>Eurytemora affinis</em> appears to be contained in the ETM by freshwater limiting its up-estuary extent and biological interactions with <em>A. tonsa</em> and salinity tolerances limiting its down-estuary abundance. Grazing and egg production results indicate that <em>E. affinis</em> production is not food-limited in the ETM region and that this copepod's particle selection ability favors its success in the ETM over that of <em>A. tonsa</em>. Laboratory egg production experiments also suggest that the most accurate estimates of <em>E. affinis</em> egg production (and of all brooding copepods in general) are achieved by incorporating both a temperature-based estimate of interclutch duration (the time between successive clutches) with a temperature-based estimate of egg hatching time.