Population dynamics of the scyphomedusa Aurelia aurita in Southampton Water

The population dynamics of the scyphomedusa Aurelia aurita in Southampton Water is characterized. Strobilation, indicated by the presence of 1 mm ephyrae, occurred from the end of January to the middle of March. Maximum abundances of up to 8.71 m−3 occurred soon after ephyrae release, after which numbers declined steadily until the end of June, when the population was absent from Southampton Water. The residence time of 3–4 months is somewhat less than that reported in many other areas, including Kiel Bight and Gullmarfjord. The carbon biomass of A.aurita accounted 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 mg C m−3 in May 1990 and 27.6 mg C m−3 in June 1991. Coincident with increased water temperature and mesozooplankton abundance during May, growth rates increased from 0.02–0.30 mm day−1 to a peak of 4.8 mm day−1, with a maximum bell diameter of 120–140 mm reached in late May/early June. Size to maturity was variable, although the smallest medusa observed to be ‘ripe’, i.e. containing dividing eggs and planula larvae in the brood sacs on the oral arms, were 64–71 mm. Aurelia aurita is believed to be endemic to Southampton Water, but because of the double high water in the area, short flushing rates of between 4.5 and 20 days may be responsible for such short residence times of Aurelia medusae. The effects of strong NE winds were considered as factors governing the distribution of medusae in years of atypical temporal abundance.