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.