Productivity of Daphnia longispina in a highly humic boreal lake

Abstract

The development of the Daphnia longispina (O. F. Müller) population in a highly humic boreal lake was followed throughout one growing season, and the amount of secondary production was estimated in relation to primary production and available food resources. The growth rate method was applied in the secondary production measurements. Daphnia longispina did not appear in the water column until 16 May, after which the animals were present throughout the growing season. The population showed three density peaks; the first appeared in early June, and the second and third in mid-July and at the beginning of September, respectively. Somatic production followed a seasonal pattern, with highest production rates in midsummer. The maximum value of 127 mg C m-2 day-1 was recorded at the beginning of July. The total annual net production of D. longispina was 7.9 g C m-2. During most of the growing season, the primary productivity in the lake was well below 100 mg C m-2 day-1 and the total annual productivity of photosynthetic algae was 5.0 g C m-2. We conclude that in this lake the zooplankton population did not rely on phytoplankton primary production as a sole carbon source, but that most of the carbon must have originated from bacterial production either directly or through a microbial loop.