Large suprabenthic Daphnia middendorffiana from an Alpine Lake in the Canadian Rocky Mountains

Abstract

Exceptionally large (up to 4.0 mm) Daphnia middendorffiana were collected with a bottom-closing net while sampling benthos in Pipit Lake, Alberta. Such large D. middendorffiana have not been observed in plankton samples collected from the lake over the past 30 years, nor have such large individuals been reported in the literature. Stable nitrogen isotope ratios (δ15N) of the large Daphnia were 4.5‰ higher than smaller Daphnia from higher in the water column, indicating that the spatially separated Daphnia may use different food sources. The large Daphnia had Hesperodiaptomus resting eggs and nauplii in their guts, while the smaller Daphnia higher in the water column had mainly phytoplankton in their guts. Our findings highlight the need to use several different kinds of sampling gear to completely assess the fauna of a lake.