A metalimnetic oxygen minimum indirectly contributing to the low biomass of cladocerans in Lake Hiidenvesi - a diurnal study on the refuge effect

The diurnal vertical migrations of smelt (Osmerus eperlanus), larvae of pha ntom midge (Chaoborus flavicans) and cladoceran zooplankton in eutrophic La ke Hiidenvesi were studied in order to clarify the factors behind the low z ooplankton biomass. In the study area, an oxygen minimum occurred in the me talimnion in the 10-15 m depth. No diurnal fluctuations in the position of the minimum were observed. Cladocerans inhabited the epilimnion throughout the study period and their vertical movements were restricted to above the thermocline and above the oxygen minimum. C. flavicans conducted a diurnal migration. During the day, the majority of the population inhabited the 12- 15 m depth just in the oxygen minimum, while during darkness they were foun d in the uppermost 8 m. Smelts started ascending towards the water surface before sunset and reached the uppermost 3 m around 23:00. During daytime, t he majority of smelts inhabited the depth of 7-9 m, where the water tempera ture was unfavourably high for them (18 degreesC). Smelts thus probably avo ided the steep oxygen gradient in the metalimnion, whereas Chaoborus used t he oxygen minimum as a refuge against predation. Those smelts that were fou nd in the same water layers as Chaoborus used the larvae as their main prey . The metalimnetic oxygen minimum thus seemed to favour the coexistence of vertebrate and invertebrate predators, leading to a depression of cladocera n zooplankton.