ABUNDANCE AND FEEDING OF MEDUSAE AND THEIR POTENTIAL IMPACT AS PREDATORS ON OTHER ZOOPLANKTON IN BEDFORD BASIN (NOVA-SCOTIA, CANADA) DURING SPRING

Only five hydromedusan species (Sarsia princeps, Euphysa tentaculata, Rathkea octopunctata, Cosmetira sp., and Aglantha digitale) and one scyphomedusan (Aurelia aurita) were found in the upper 30 m of Bedford Basin during spring 1988. Medusan carbon weight and that of total zooplankton varied reciprocally, suggesting significant population control by gelatinous predators. In March, when zooplankton concentration was low (5-9 mg C.m-3) and medusan weight less than 1% of this total, daily ingestion by medusans did not exceed 10% of zooplankton carbon weight. In May, when mesozooplankton concentration was greater than 10 mg C.m-3 and predators more abundant, their potential daily impact was at least 59%. From March to June, mean impact was 15.8%. During this period, as medusans increased and copepods decreased, the proportion of copepods consumed continued to increase, while the fraction of eggs consumed, primarily those of potential competitors such as ctenophores and chaetognaths, decreased. Predation between different taxa of medusans was also found. A population decrease in Rathkea octopunctata, the dominant spring hydromedusan, may have been attributable to predation by Aurelia aurita.