Population and production dynamics of the chydorid Eurycercus vernalis were studied in the laboratory and in a small wetland during a 2-y period. Laboratory growth studies were conducted to measure the effects of temperature on E. vernalis growth and reproduction and to develop a multiple-regression equation that used temperature and mass-specific growth rates to estimate secondary production in the field population. Density, biomass, and production were estimated from benthic, water-column, and Nymphaea odorata leaf habitats within vegetated (Nymphaea) and nonvegetated (open-water) areas using monthly samples from the wetland. Eurycercus vernalis exhibited optimal growth, reproductive output, and net reproductive rate when reared at temperatures between 15 and 20°C. Reproductive age and egg development time decreased with increasing temperature, and these decreases led to increased innate capacity of population increase and decreased generation time with increasing temperature. In the field studies, annual density, biomass, and production were significantly higher in the Nymphaea zone than the open-water zone during both years. Eurycercus vernalis populations developed during the fall from oversummering resting eggs and reached maximum density, biomass, and daily production in mid to late spring when water temperatures reached 18 to 21°C. Mean annual density and biomass for the wetland pond were 2134/m2 and 42.2 mg dry mass (DM)/m2 in year 1 (1993), and 1122/m2 and 19.0 mg DM/m2 in year 2 (1994–1995). Annual production and the production/biomass ratio were 2138 mg DM m−2 y−1 and 50.7/y in year 1, and 1111 mg DM m−2 y−1 and 58.5/y in year 2. High production values suggested that E. vernalis is an important component of the microcrustacean community in the Nymphaea zone, especially during winter when production of most microcrustacean species is low.
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