The importance of viruses in controlling a bloom of the coccolithophorid Emiliania huxley~ in the North Sea was investigated during summer 1993. Viral infection of E. huxleyi was highest In the decaying phase of the bloom. Up to 50V0 of E h u x l e y~ cells \yere visibly infected. For E, huxleyi, 2 types of virus-like particles are reported. Thcy differed in slze and were occassionally found within the same cell. The infection level of the large virus-like particles was never higher than 25%. Viral lysis of E. huxleyi within the nitrogen-limited decaying phase of the bloom seemed an important source of organic carbon utilized by bacteria. In addition to infection in E. huxleyi, we also found severe viral infection in Chrysochromulina sp. Our results show that in natural ecosystems viruses can be a significant source of phytoplankton mortality, influencing phytoplankton (bloom) dynamics and the microbial food web.

In July 1993, an extensive study was made of a large bloom of the coccolithophorid Emiliania huxleyi in the North Sea halfway between the Shetland Islands and Norway. Here w e report on the hydrography, production and sedimentation of particulate organic carbon (POC) and calcite carbon (calcite-C) at 4 stations occupied for 24 h, 2 inside the bloom and 2 just outside. The coccolithophorid bloom was confined to North Sea waters, where a stable shallow mixed layer had been formed. Bloom development had entered the decaying phase, judged by the relatively low living cell number (maximally 1200 cells cm-3), the high number of loose coccoliths (up to 350000 coccoliths cm-3), and the fact that sedimentation of calcite-C exceeded production. In the top 15 m at the bloom stations, the mean daily production of coccoliths was 17 per cell. At the 2 stations outside the bloom, the dominant coccolithophore was a holococcolithophorid (up to 1400 cells cn1r3), wlth insignificant amounts of calcite produced per cell. At these stations, nutrients were present in non-lim~ting concentrations and production of POC was twice as high as at the bloom stations. In the bloom, mixed layer nitrate levels were below 0.2 PM. Faecal pellets collected in the sediment traps contained large numbers of coccoliths of E, huxleyi. Although 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 pellets outside the bloom were so light in weight that they did not sink very far before degradation, whereas the pellets produced in the E. huxleyi 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 ratio at which POC and calcite-C were sedimenting amounted to 1.3 on average for the 2 bloom stations at 50 m water depth.

No abstract