Montel, M.L., N. Lair
Abstract
The summer development of Daphnia longispina living in a stratified eutrophic lake was examined both in the field and in laboratory experiments. The demographic parameters were analysed in relation with the growth of algae, heterotrophic nanoflagellates and picocyanobacteria with particular attention to the spatial distribution of the different food components. Within two months, the maximum densities of this cladocera alternate with those of the HNF. The positive correlation between the HNF density, fecundity and birth rates suggest these highly energetic heterotrophic protists may enhance the daphnids reproductive effort. From July 7 to August 8, 1994 the increase in available phytoplankton, supports both positive increase and fecundity rates. From August 11 to September 1, the inedible fraction represents a large standing crop and only HNF and PCB densities can support the cladoceran fecundity expenses. During that second period the fecundity rate remains high, while the average population growth rate was negative, leading to the mid-summer decline of Daphnia. Larger individuals remained concentrated in the metalimnion during the day which suggests both vertebrate predator avoidance and search for food supply. Life table experiments on Daphnia were run in the laboratory with epilimnetic and metalimnetic water as food supply, without predators and under similar abiotic conditions. The HNF density of the epilimnion (86 +/- 10.10^3 cell.l^-1) is lower than that in the metalimnion (167 +/- 20.10^3 cell.l^-1) but globally the density of the edible components remains similar in both zones. The epilimnetic food largely supports their growth and fecundity expenses, while, in contrast the metalimnetic food is associated with detritus and bacteria, which support only the daphnids growth before their death. Such results tend to sustain that the food distribution does not act as a component of the vertical migration, and that the highly energetic HNF globally enhance the population fecundity. However, during the second period if the PCB contribute to sustain the population fecundity rate, the decrease in the available food induces the mid-summer decline. Not only the morphology as well as the nutritional value of the food, but also the non-living environment represented by detrital sources play an important role in the food uptake by the non selective filter feeders.
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