Population dynamics of the scyphomedusa Aurelia aurita in Southampton Water

The population dynamics of the scyphomedusa Aurelia aurita inSouthampton Water is characterized. Strobilation, indicatedby the presence of 1 mm ephyrae, occurred from the end of Januaryto the middle of March. Maximum abundances of up to 8.71 m^–3occurred soon after ephyrae release, after which numbers declinedsteadily until the end of June, when the population was absentfrom Southampton Water. The residence time of 3–4 monthsis somewhat less than that reported in many other areas, includingKiel Bight and Gullmarfjord. The carbon biomass of A.auritaaccounted for 92–97% of the predominant gelatinous biomass(A.aurita, Pleurobrachia pileus and Phialidium hemisphericum)in the upper estuary, and this reached a maximum of 30.2 mgC m^–3 in May 1990 and 27.6 mg C m^–3 in June 1991.Coincident with increased water temperature and mesozooplanktonabundance during May, growth rates increased from 0.02–0.30mm day^–1 to a peak of 4.8 mm day^–1, with a maximumbell diameter of 120–140 mm reached in late May/earlyJune. Size to maturity was variable, although the smallest medusaobserved to be ‘ripe’, i.e. containing dividingeggs and planula larvae in the brood sacs on the oral arms,were 64–71 mm. Aurelia aurita is believed to be endemicto Southampton Water, but because of the double high water inthe area, short flushing rates of between 4.5 and 20 days maybe responsible for such short residence times of Aurelia medusae.The effects of strong NE winds were considered as factors governingthe distribution of medusae in years of atypical temporal abundance.