Influence of physical forcing on mesozooplankton communities at the Ushant tidal front

The pelagic ecosystem around Ushant tidal front was studied over a spring-neap tidal cycle in September 2009. The system was divided in a mixed coastal zone, the frontal zone and a stratified oceanic zone, which remained stable from spring to neap tide. Standing stock, size structure and composition of the phytoplankton and mesozooplankton communities varied more intensely between zones than with the tidal cycle. Localized changes in chlorophyll a occurred due to forcing by wind and tides. At the front, a diatom bloom intensified from spring to neap tide, but generally phytoplankton communities maintained stable proportions and were numerically dominated by pico- and nanophytoplankton. Highest mesozooplankton diversity was encountered in the frontal zone where organisms from both sides are blended; yet highest biomass was found in the stratified zone 30 nm west of the front. Temporal changes in the abundance of several zooplankton groups occurred and mirror trophic dynamics, physical processes and phytoplankton production. Differences between normalized biomass spectra from LOPC and image analysis were strongest during spring tide. The spectral slope steepened from a theoretical equilibrium value in the stratified zone to <−1 in the mixed zone reflecting the greater importance of small sized mesozooplankton in coastal marine ecosystems.