Seasonal production of photosynthetic picoplankton and nanoplankton in the Celtic Sea

Primary production by picoplankton and other phytoplankton has been measured in all seasons in the Celtic Sea. Picoplankton production was greatest during the summer but was always less than that of small nanoplankton which accounted for most of the primary production throughout the year. Large phytoplankton (> 5 ym) was quantitatively the most important only for a short period of the spring diatom bloom. In winter, small nanoplankton (t 5 to > 1 pm) accounted for almost 70 % and picoplankton for 13 % of the daily primary production. Annual phytoplankton production was 102 g C m-2 yr-I (excluding production of dissolved organic carbon, which was about 10 to 1 5 % of the particulate production). Phytoplankton > 5 p m fixed 37.89g C m-2 yr-' but half of this production occurred during the spring diatom bloom in April. Small nanoplankton (< 5 to > 1 pm) fixed 41.86g C m-2 yr-l and picoplankton (< 1 pm) fixed 23 0 6 g C m r 2 yr-'