Long-Term Population Dynamics and Production of Pisidium (Bivalvia) in the Profundal of Lake Esrom, Denmark

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Abstract
During 21 yr (1953-1972, 1976-1977) of routine quantitative sampling, 3 spp. of Pisidium (P. casertanum, P. subtruncatum and P. henslowanum) were present in the profundal of the eutrophic Lake Esrom, Denmark. From 1955-1962 P. casertanum was numerically dominant (peak density of 3273 ind m-2 was attained in 1957); at other times P. subtruncatum dominated (peak 3025 ind m-2 in 1955). After 1957-1958 the densities of both species were below 1000 ind m-2. The peak density of P. henslowanum [370 (individuals) m-2] was attained in 1057. All the species laid their eggs in spring (April-May), but the young were not released until the following winter, the exact time depending on the temperature and O2 conditions. Only some of the eggs laid developed into young, and the size of the brood depended on parental size. The young reached maturity at the age of 2 yr, with a probably maximum life-span of 4-5 yr. The regularly occurring O2 eficiency in the hypolimnion in summer caused heavy mortality in the dominant species and halted growth of individuals. Both the standing crop and annual production were lower in the population of P. subtruncatum, which is a smaller species. The annual P/B[production/biomass]-ratios were circa 1 in both the dominant populations.