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

The previously unknown effects of wind-induced Lagmuir circulations on the distribution of phantom midge larvae (Chaoborus flavicans) were studied by echosounding. The study was carried out in the eutrophic Lake Hiidenvesi, w here chaoborids use a metalimnetic oxygen minimum as a daytime refuge again st fish predation. At a wind velocity of 8-9 m s(-1), the upwelling water c irculations snatched clouds of chaoborids from the metalimnion (12-15 m dep th) to the mon oxygenated epilimnion. The average density of C. flavicans i n the clouds was 790 ind. m(-2), whereas, elsewhere in the epilimnion, it w as 380 ind. m(-2). Planktivorous fish (smelt Osmerus eperlanus) were active ly aggregated in the upwellings. When the wind velocity decreased to 3 m s( -1), chaoborids disappeared from the epilimnion. The results suggest that L angmuir circulations may affect the abundance of chaoborids by disturbing t heir low-oxygen refuges.