Langmuir circulations disturb the low-oxygen refuge of phantom midge larvae

Abstract

The previously unknown effects of wind-induced Langmuir circulations on the distribution of phantom midge larvae (Chaoborus flavicans) were studied by echosounding. The study was carried out in the eutrophic Lake Hiidenvesi, where chaoborids use a metalimnetic oxygen minimum as a daytime refuge against fish predation. At a wind velocity of 8-9 m s-1, the upwelling water circulations snatched clouds of chaoborids from the metalimnion (12-15 m depth) to the more oxygenated epilimnion. The average density of C. flavicans in the clouds was 790 ind. m-2, whereas, elsewhere in the epilimnion, it was 380 ind. m-2. Planktivorous fish (smelt Osmerus eperlanus) were actively aggregated in the upwellings. When the wind velocity decreased to 3 m s-1, chaoborids disappeared from the epilimnion. The results suggest that Langmuir circulations may affect the abundance of chaoborids by disturbing their low-oxygen refuges.