Is food availability the main factor controlling the abundance of Euchlanis dilatata lucksiana Hauer in a shallow, hypertrophic lake?

Abstract

Visual observations and experiments on food preference of Euchlanis dilatata lucksiana show that this euchlanid can feed on blue-green algae not consumed by the most planktonic animals. Nevertheless, even in lakes with blooms of blue-green algae, E. d. lucksiana occur infrequently and generally in low numbers. The paper is an attempt to explore into the causes for the rare occurrence of Euchlanis in the pelagial. A comparison of threshold food concentrations calculated from N and P excretion rates (Gulati et al., this volume) with the concentrations of seston in the Lake Loosdrecht shows that the latter were several times higher during study period in 1984. This implies that the food requirements of Euchlanis were always satisfied in this lake. The time needed for the consumption of the total food fraction in a liter of lake water by a concentration of 50 Euchlanis l⁻¹ was also calculated. This time varied from 70 to 200 days, so a Euchlanis population even at its maximum density will not cause major changes in blue-green algae biomass by grazing. Thus, food limitation cannot be viewed as a factor controlling the Euchlanis densities in Loosdrecht Lakes. There is some evidence that Euchlanis is heavily predated in Loosdrecht Lakes, losses in its biomass accounting for 126% of the production. Adaptation of this species to the littoral zone, as expressed by the deposition of eggs on plants, can also limit the occurrence of the lucksiana form to water bodies with blooms of blue-green algae.