Production Dynamics of Hyalella azteca (Amphipoda) among Different Habitats in a Small Wetland in the Southeastern USA

Abstract

The life history, population dynamics, and secondary production of Hyalella azteca (Amphipoda) were studied in a small southeastern wetland pond. Habitat-specific sampling was used to estimate density, biomass, and production in each of 3 habitats (benthic, Nymphaea odorata leaves, and submerged wood). Laboratory experiments at different temperatures (10, 15, 20, 25, and 30°C) provided predictive equations for estimating growth rate and egg development times that could be applied to field populations. Size-frequency data from field sampling suggested that H. azteca had 2 major cohorts per year, but reproduction was continuous from spring through fall. Mean annual density and biomass were 904/m² and 85 mg dry mass (DM)/m². Daily growth rate was a function of both individual mass and temperature. Annual production of somatic tissues as estimated by the instantaneous growth method (675 mg DM· m^-2· y^-1) agreed well with that using the size-frequency method (714 mg DM· m^-2· y^-1). Egg production (252 mg DM· m^-2· y^-1) represented 27% of total production (927 mg DM· m^-2· y^-1). Annual production:biomass (P/B) was 7.8 without eggs and 9.8 with eggs. Habitat-specific estimates of density, biomass, and production were highest in the benthic habitat. The contribution of somatic production from Nymphaea leaves to total amphipod production was low (6.2%), but temporal patterns of production followed trends in leaf abundance. Production on leaves reached 29% in September when leaves were most abundant. Contributions of animals from submerged wood were consistently low (2.9%). Monthly densities and production peaked in May at the start of the summer cohort, decreased throughout summer, and showed a small increase in October at the start of the winter cohort. Although egg production and birth rates were highest in summer months, high death rates were responsible for the late-summer declines in density and production. These temporal trends in production suggest heavy predation or environmental stress, or both, during summer months.