Seasonal Dynamics and Trophic Impact of Mesozooplankton in the Shannon River Estuary System, Ireland

Mesozooplankton (netplankton > 250 µm) were sampled during nine cruises over one year at three stations in the Shannon estuary system, Ireland. A net with three mesh sizes was used to capture a wider range of plankton sizes than a standard single-mesh net. An innovation was the incorporation factorial analysis of celestial (seasonal) variables, spring equinox (Spr) and summer solstice (Sum), together with physicochemical and biological variables, without presuming cause or effect. Over the year, water temperature and salinity were closely positively related to each other and to the occurrence of most of the taxa. The approximate trophic impact of by major taxa was estimated from abundance and published clearance rates. Overall, the mean herbivorous/detritivorous grazing by mesozooplankton was 54 L m−3 d−1. Among the mesozooplankters and mysids, Mesopodopsis slabberi (predominantly April–November) contributed 96.3% and the appendicularian Oikopleura dioica (May–October) contributed 2.0% (nano- and picoplankton), while copepods only provided 0.98%. The ctenophore Pleurobrachia pileus (present April–October) grazed 2.0% (carnivorous grazing mysids and copepods contributed additional unquantified carnivorous grazing). These data, collected 45 years ago, provide a valuable baseline for assessing subsequent ecological changes.