SEASONAL DYNAMICS AND PRODUCTION OF THE HYDROMEDUSAN CLYTIA-HEMISPHAERICA (HYDROMEDUSA, LEPTOMEDUSA) IN SOUTHAMPTON WATER

In 1990 and 1991, a zooplankton sampling program revealed the presence of the hydromedusan Clytia hemisphaerica (Hydromedusa: Leptomedusa) f rom early May through late September throughout Southampton Water. The most notable aspects of the population dynamics of C. hemisphaerica i n Southampton Water were the considerable interannual variation in pea k abundance-10.65 m(-3) in 1990 and 2.44 m(-3) in 1991-and temporal pa tterns of mean bell diameter. The suggestion of almost continuous repr oduction in 1990, as indicated by high abundance and large size range on each sampling date, is in contrast to 1991, where at the Cracknore site, five cohorts were produced at 21 d to 80 d intervals. The maximu m bell diameter attained in each generation was typically 10-11 mm. At the Cracknore site in 1991, in situ bed diameter growth was linear wi thin each cohort. A trend of increased growth rate with each subsequen t cohort, from 26 mu g DW d(-1) to 106 mu g DW d(-1) was coincident wi th increasing water temperature. Annual production ranged from 0.98 mg C m(-3) yr(-1) to 1.08 mg C m(-3) yr(-1) in 1990 and 0.35 mg C m(-3) yr(-1) to 0.48 mg C m(-3) yr(-1) in 1991. Daily P:B ratios varied in e ach cohort from 0.05 to 0.35.