Abundance, distribution and community composition of small gelatinous zooplankton in southern Irish coastal waters

Gelatinous zooplankton play important roles in coastal seas, from predators of fish eggs and larvae to competitors of fish for copepods. Despite this, they are often an underestimated component of coastal marine ecosystems. In Ireland, there has been limited research focussed on the gelatinous zooplankton communities in coastal waters. In the present study, 21 stations in the coastal waters to the south and west of Ireland were sampled, providing the first broad-scale description of the gelatinous zooplankton communities along over 600km of the Irish coastline. In total, 23 species/genera of gelatinous zooplankton were identified from six taxa: hydromedusae, siphonophores, ctenophores, chaetognaths, urochordates and polychaetes. Only a few of the stations sampled were highly stratified in terms of temperature or salinity; these were shallow stations close to freshwater inputs. Thermo-and halocline intensity had little influence on the community composition observed. The community structure was however found to be highly dependent on temperature at depth and surface salinity, which varied with latitude and longitude, respectively. The similarity in the community composition between stations was largely driven by three highly holoplanktonic abundant species: the hydromedusa Aglantha digitale; the ctenophore Pleurobrachia pileus; and the larvacean Oikopleura sp. These species reached densities of 214, 83 and 492 individuals m^-3, respectively.