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.

Discriminating the diel vertical migration of fish andChaoborus flavicanslarvae in a lake using a dual-frequency echo sounder

“…Analysis of fish in the presence of Chaoborus has been a long unsolved problem making it difficult to perform fish stock assessment (Eckmann 1998;Malinen et al 2001;. We have demonstrated that using a 38 kHz frequency removes echoes from Chaoborus to such an extent that an unbiased fish assessment can be obtained.…”

“…The larva possesses two pairs of air-sacs and is a strong acoustic scatterer at the echosounder frequencies traditionally used in freshwater (Northcote 1964;Eckmann 1998;Wagner-Döbler and Jacobs 1998;Malinen et al 2001Malinen et al , 2005Knudsen et al 2006). When they co-occur with fish, Chaoborus display a diel vertical migration, where they are found in the sediments or in the hypolimnion during the day and ascend into the epilimnion to feed in the evening (Malueg and Halser 1966;Nilsen 1974;Voss and Mumm 1999).…”

“…Living Resour. 22, 273-280 (2009) Methods based on thresholding have been suggested as a means of solving the Chaoborus-fish separation problem (Eckmann 1998;Malinen et al 2001Malinen et al , 2005, but none have considered the possibility of using multiple frequencies to distinguish fish from Chaoborus. The acoustic scattering from Chaoborus is dependent on echosounder frequency (Jones and Xie 1994;Knudsen et al 2006).…”

“…Indirect methods are available (Eckmann 1998;Malinen et al 2001Malinen et al , 2005, but routine acoustic surveys would benefit from direct and rapid methods. This is also in line with recent EU freshwater directives that aim to establish routines for effective and non-invasive monitoring of all components of the freshwater ecosystem.…”

The Role of Light and Oxygen inChaoborus punctipennis(Insecta: Diptera) Diel Vertical Migration

“…Often acting as the most common planktivore besides fish (Wissel et al 2003), they contribute to high biomasses in eutrophic lakes (Malinen et al 2001) and can greatly affect zooplankton community structure (Moore 1988). Certain populations of Chaoborus often exhibit diel vertical migration (DVM), a behavior characterized by staying in deeper waters during daytime hours and ascending nocturnally to feed (Dodson 1990).…”