Physiological and visual refuges in a metalimnion: an experimental study of effects of clay turbidity and an oxygen minimum on fish predation

Abstract

Summary1. The effectivity of elevated clay turbidity and low‐oxygen concentration in the metalimnion as refuges for chaoborid larvae against fish predation was studied in experimental water columns.2. When 70–80 nephelometric turbidity unit clay turbidity was combined with 3–4 mg L−1 oxygen concentration, prey capture rate by fish (golden orfe Leuciscus idus) was reduced by 74% compared with the control treatment with no refuges. Oxygen and turbidity refuges alone did not significantly reduce the feeding rate.3. All fish in the control treatment dwelled in the metalimnion, but 36% of the fish in the low‐oxygen treatment and 23% of the fish in the turbidity treatment stayed in the epilimnion. In the combined treatment, 54% of the fish were in the epilimnion.4. The results demonstrated that a combination of moderately elevated turbidity and lowered oxygen concentration in the metalimnion is an effective protection against fish predation, while turbidity or oxygen refuge alone are much less effective.5. In the treatment with the combined refuge, oxygen concentration limited the time fish could spend in the metalimnion and turbidity affected the detection of prey through changes in reactive distance.6. Because of the combined effects of turbidity and oxygen refuges, planktivorous fish and phantom midge larvae may co‐occur in clay‐turbid lakes in high densities. Such situation is problematic for biomanipulation, which aims to enhance the grazing rate of zooplankton through reduction of planktivorous fish.