PRODUCTION AND DOWNWARD FLUX OF ORGANIC-MATTER AND CALCITE IN A NORTH-SEA BLOOM OF THE COCCOLITHOPHORE EMILIANIA-HUXLEYI

In July 1993, an extensive study was made of a large bloom of the cocc olithophorid Emiliania huxleyi in the North Sea halfway between the Sh etland Islands and Norway. Here we report on the hydrography, producti on and sedimentation of particulate organic carbon (POC) and calcite c arbon (calcite-C) at 4 stations occupied for 24 h, 2 inside the bloom and 2 just outside. The coccolithophorid bloom was confined to North S ea waters, where a stable shallow mixed layer had been formed. Bloom d evelopment had entered the decaying phase, judged by the relatively lo w living cell number (maximally 1200 cells cm(-3)), the high number of loose coccoliths (up to 350 000 coccoliths cm(-3)), and the fact that sedimentation of calcite-C exceeded production. In the top 15 m at th e bloom stations, the mean daily production of coccoliths was 17 per c ell. At the 2 stations outside the bloom, the dominant coccolithophore was a holococcolithophorid (up to 1400 cells cm(-3)), with insignific ant amounts of calcite produced per cell. At these stations, nutrients were present in non-limiting concentrations and production of POC was twice as high as at the bloom stations. In the bloom, mixed layer nit rate levels were below 0.2 mu M. Faecal pellets collected in the sedim ent traps contained large numbers of coccoliths of E. huxleyi. Althoug h the numbers of grazers at the 2 stations outside the bloom were not lower than those in the bloom, the volume of faecal matter sedimenting at 50 m was about 70 times lower. It is hypothesized that faecal pell ets outside the bloom were so Light in weight that they did not sink v ery far before degradation, whereas the pellets produced in the E. hux leyi bloom in general were exported rapidly due to their heavy load of calcite. This implies that recycling of materials in the mixed layer of this bloom is relatively low due to high downward flux rate. The ra tio at which POC and calcite-C were sedimenting amounted to 1.3 on ave rage for the 2 bloom stations at 50 m water depth.