LENGTH-SPECIFIC CARBON CONTENT OF THE DAPHNIA POPULATION IN A LARGE SUB-ALPINE LAKE, LAGO-MAGGIORE (NORTHERN ITALY) - THE IMPORTANCE OF SEASONALITY

We compared three methods for calculating the standing stock biomass o f Daphnia hyalina galeata population in Lago Maggiore during a single year. We applied body length/carbon regression equations (LCR) obtaine d under controlled laboratory conditions and directly from field data on carbon and body length to field data on length-specific abundance. Laboratory-derived equations satisfactorily described our field estima tes of length-specific carbon content in spring. As the length-standar dized carbon content decreased exponentially along the season, we incl uded time of the year as an auxiliary variable, and tested whether it affected slope, intercept or both. The decrease had no effect on slope but reduced the intercept of the LCR. We also calculated biomass, usi ng a single mean value from the literature. By comparing the results w ith those obtained by simple conversion of individuals number into car bon we might discriminate between the effect of changes in the size fr equency distribution of the population and the decrease with time of t he length-standardized carbon content. Although we tested the differen t models for non-ovigerous females, the results did not change when we included also data on females with eggs in the first three developmen tal stages, whereas they changed substantially when we included female s with embryos. We formulate two hypotheses to explain this result and discuss possible implications for the calculation of the standing sto ck biomass of Daphnia population.