Edinburgh Napier University

  The abundance and productivity of phytoplankton and bacterioplankton in the region of Jones Bank (Cel-tic Sea) were assessed in relation to potential physical and chemical drivers of the marine microbial community during July 2008 on RRS James Cook cruise JC25. A major storm dominated the early part of the cruise; this influenced the microbial community with a decrease in diatom abundance being evident due to mixing of cells to sub euphotic zone depths. In the euphotic zone, phytoplankton was dominated by Phaeocystis cells which, with time, became constrained within a chlorophyll maximum that was located near the top of the nutricline. Multivariate statistical analyses found that temporal changes in diatom and dinoflagellate communities were significantly related to water temperature (diatoms) and nitrate concentration (dinoflagellates). Post storm, primary productivity was greatest in the Phaeocystis dominated 2–20 lm size fraction, but with no obvious topographically generated differences between on and off bank stations. Use of a mathematical model demonstrated the likely enhancement of productivity by the storm event, and also that this storm would have masked any topographic influence on productivity. The primary production:bacterial production ratio decreased significantly with time during the cruise, suggesting active competition by bacteria with phytoplankton for mineral nutrients. This was confirmed by a deck board enrichment experiment that demonstrated a rapid increase in bacterial productivity following supplementation of the euphotic zone microbial community with nutrient rich water collected below the mixed layer.