THE DIURNAL ACTIVITY OF THE LARGER INVERTEBRATES AT THE SURFACE OF LAC LA RONGE, SASKATCHEWAN

Abstract

Horizontal hauls were made with a pair of large plankton nets at the surface of a large lake to determine the 24-hour emergence cycle of chironomid midges rising from different depths and to investigate the occurrence of the larger invertebrates at the surface. Chironomid pupae ascend mainly in the hours of darkness and emergence may be immediate, as in Psilotanypus rufovittatus, or delayed for several hours, as in Procladius choreus. A migration to the surface at night is demonstrated for Mysis relicta and the amphipods Pontoporeia affinis and Hyalella azteca. Chironomid larvae and a variety of other invertebrates also occur at the surface. The findings show that many benthic animals are less static in their distribution than is commonly accepted.