Clearance potential of jellyfish Aurelia aurita, and predation impact on zooplankton in a shallow cove

T h e potential predation impact of jellyfish Aurelia aurita on zooplankton in a shallolv cove was investigated.Population clearance potential was calculated from laboratory clearance rates and from measurements of medusae size and abundance during 1991 and 1992 in the shallow cove Kert~nge Nor, Denmark.Clearance by A. aurita as a function of medusae size and water temperature was measured in the laboratory using copepods (Acartia tonsa) and rotifers (Brachionus pl~catilis) as prey.Clearance rate increased (from 0.01 to 2.3 1 h-' ind.-') with increasing medusae diameter (from 4 to 44 mm) when rotlfers were used as prey.For medusae > 5 mm, clearance on copepods and rotifers was similar Water tempei-ature had marked effect on clearance, which increased from 0.16 1 h-' ind:' at 5°C to 0.58 1 h-' ind:' at 14 for a 20 mm medusa feeding on A .tonsa.Between 14 and 21°C clearance was constant, and at 30°C the medusae died.In spring, when medusa were small, the population in Kertlnge Nor could potentially have cleared the volume of water in the cove less than 0.1 tlmes each day.This value was 3.5 tlmes each day in late summer when biomass was high.Blooms of rotifers were observed in Kertlnge Nor, but only during periods of low population clearance potential of A. aurita, and the abundance of holozooplankton in general was extremely low in Kertinge Nor during 1991 and 1992.High predat~on pressure by medusae appears to control zooplankton in the shallow cove, at least dunng summer and early fall, and A. aurita may thus b e considered as a keystone species in the control of the trophlc structure in Kertinge Nor.