Spatial distribution of zooplankton biomass size fractions in a bog lake: abiotic and (or) biotic regulation?

Abstract

Although spatial heterogeneity of zooplankton has been explained by variations in abiotic and biotic factors as well as their cascading interactions, few studies have paid attention to the relative importance of these processes in structuring the vertical and horizontal distribution of zooplankton within a lake. The main objective of this study was to determine the relative importance of abiotic and biotic processes in the regulation of the vertical and horizontal distribution of four size fractions (53-100, 100-202, 202-500, and >500 µm) of the zooplankton biomass in a bog lake (Lake Geai, Quebec). Spatial heterogeneity of zooplankton biomass was found in both the horizontal and the vertical dimensions of the lake, and spatial patterns were quite similar for all size fractions. On the horizontal axis, spatial heterogeneity was stronger in the epilimnetic stratum. Zooplankton biomass was mainly aggregated in the east side of the lake in an area of higher conductivity and greater density of small chaoborid larvae. On the vertical axis, zooplankton biomass was concentrated in the epilimnetic waters and decreased with depth. This vertical pattern of distribution might be due to the avoidance of cold and anoxic waters and a high density of large chaoborid larvae in the hypolimnion. In general, invertebrate predation has been identified as the primary factor regulating zooplankton biomass distribution in Lake Geai. However, the size structure of chaoborid predators was more important than their density in the control of the spatial distribution of zooplankton.