The influence of water body characteristics on phytoplankton diversity and production in the Celtic Sea

The influence of water body characteristics on phytoplankton biodiversity and production has been studied in stratified, mixed and frontal water masses in the Celtic Sea. The stations were in close geographical proximity to each other, yet very different phytoplankton assemblages were found. The factors that control phytoplankton biodiversity were investigated. Satellite remote sensing was used to map the development of stratification and fronts in spring and this placed the stations within a strong temporal context. Stations with a well-mixed water column had high nutrient concentrations, high rates of production and the phytoplankton assemblage was dominated by diatoms. At those stations with a stratified water column, nutrient concentrations were depleted but the assemblage was dominated by dinoflagellates and microflagellates. The ratios of depth-integrated primary production to depth-integrated chlorophyll concentration (P/B ratio) were higher at those stations with mixed water columns than at those in stratified waters. That is, under the low nutrient/high light conditions at the stratified station, there was less production than might have been expected for the quantity of chlorophyll present. The photosynthetic characteristics of the phytoplankton assemblages were determined at the stratified stations. P _m ^B values varied from 1.43 to 4.79 mg C [mg Chl] ^-1 h ^-1, while α ^B ranged from 0.011 to 0.047 mg C [mg Chl] ^-1 h ^-1 (μmol photons m ^-2 s ^-1) ^-1. Phytoplankton biodiversity was low at every station and did not appear to influence primary production rates; the highest rate of daily primary production measured (2.4 g C m ^-2 d ^-1) was at a station where a single species accounted for >70% of the phytoplankton biomass.