Plankton Dynamics andAurelia auritaProduction in Two Contrasting Ecosystems: Comparisons and Consequences

The population dynamics and production of the ScyphomedusaAurelia aurita(Lam.) were examined within two contrasting ecosystems on the south coast of England; Southampton Water, an open productive estuary, and Horsea Lake, an enclosed, brackish, man-made body of water. AnnualA. auritaproduction in Southampton Water ranged between 31 and 58 mg C m−3year−1in different years (1990–94); while at Horsea Lake, annual production was 76 mg C m−3year−1in 1994. The potential impact ofA. auritaon the mesozooplankton communities was estimated through measurements ofin situsecondary production and transfer efficiency between the two populations. Using literature-based estimates for mesozooplankton production, the annual transfer efficiency between the mesozooplankton andA. auritapopulations in Southampton Water ranged between 9 and 26% in different years, and in Horsea Lake was 73% in 1994. These data imply that the abundantA. auritapopulation may have a significant impact on the mesozooplankton community of Horsea Lake, and in turn, the very smallA. auritasizes observed indicate a severely food limited population. The ecological role ofA. auritain Horsea Lake is discussed in view of the very low chl aand mesozooplankton numbers present.