Population Dynamics and Secondary Production of Hyalella azteca (Amphipoda) in Typha Stands of a Subtropical Florida Lake

Abstract

We used field and laboratory studies to assess the influence of warm temperatures on life history, population dynamics, and secondary production of the freshwater amphipod, Hyalella azteca (Saussure) in three Tyhpa stands of a eutrophic, subtropical lake. Our results were compared with those from temperate habitats. Mean egg development period of laboratory females (11 d) was 2-4 d longer than for temperate populations but mean fecundity (laboratory = 3.91; field = 2.48) was comparable. Birth rates of field females were low (0.01-0.06 per capita/d and 0.07-0.25 per mature female/d) with lowest rates during the summer when water temperatures were 25-34°C. Annual mean density, mean standing stock, and annual production estimates from Lake Thonotosassa (12,354 m², 2.46 g/m², and 26.9 g m^-2 yr^-1) were markedly higher than those seen in temperate studies. However, among-site variation in Lake Thonotosassa was large. Our annual production: biomass ratio (P/B̄) was 10.9 and exceeded that of temperate populations, but cohort P/B̄ was 3.9 and comparable. The Lake Thonotosassa population of H. azteca was multivoltine with a cohort CPI of 129 d whereas temperate populations usually are univoltine with CPI intervals >365 d. Subtropical temperature regimens, continuous reproduction and recruitment of amphipods, and year-around abundance of Typha and associated epiphytic algal communities appear to be the factors responsible for high densities and production in this subtropical lake.