The biology of gelatinous predators and their impact on the mesozooplankton community of Southampton Water

The mesozooplankton community of Southampton Water was characterized by Spring and Autumn peaks of abundance, with densities of up to 8000 m-3 in late April/early May, and a smaller peak of 4000 m-3 from late September to early October. The mesozooplankton community was dominated by calanoid copepods, particularly Acartia spp., which accounted for up to 98% of total mesozooplankton abundance during the peaks. During June, there was a dramatic 77-94% decrease in mesozooplankton abundance, particularly the calanoid copepods, while cirripede larvae increased up to 2400 m-3, and co-dominated with the calanoid copepods. Coincident with the late Spring-early Summer decline in mesozooplankton abundance, was a succession of gelatinous predators, principally the scyphomedusa Aurelia aurita from early March to late June, the ctenophore Pleurobrachia pileus, and hydromedusa Phialidium hemisphericum from April through to September, and the chaetognath Sagitta setosa, which was present from September through to February. The temporal and spatial patterns of abundance of the four major gelatinous species within the estuary were described, with maximum densities of 8.71 m-3, 2 m-3, 15.68 m-3 and 3.69 m-3 recorded. In order to interpret the patterns of abundance, the life histories of each major species were examined through size-frequency distributions, and histological examination was carried out on A.aurita gonadal tissue.