Dynamics and Trophic Impact of Mesoplanktonin the Shannon River Estuary System, Ireland

Mesoplankton (netplankton >250 µm) were sampled 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. Principal Component Analysis extracted dimensions D1, D2, and D3 accounting for 26%, 17%, and 12% of the variance, respectively. In the D1-D3 plane, Spr and Sum were positioned ~90° apart. Theapproximate trophic impact of by major taxa was estimated from abundance and published clearance rates. Overall, the mean herbivorous/detritivorous clearance by mesoplankton was 54 L·m⁻³·d⁻¹. Of this, mysids, Mesopodopsis slabberi (predominantly April–November), contributed 96.3% and the appendicularian Oikopleura dioica (May–October) 2.0% (nano- and pico-plankton) and copepods only 0.98%. The ctenophore Pleurobrachia pileus (present April–October) cleared an 2.0% (carnivorous clearance). Mysids and copepods contributed additional unquantified carnivorous clearance. These data, collected 45 years ago, provide a valuable baseline for assessing subsequent ecological changes.