Protist abundance and carbon concentration during a Phaeocystis-dominated bloom at an Antarctic coastal site

Summary Changes in the concentrations of bacteria, phytoplankton, protozoa, dissolved organic carbon (DOC), particulate organic carbon (POC), particulate carbohydrate (PCHO) and particulate organic nitrogen (PON) were followed throughout the summer at an Antarctic coastal site. The colonial prymnesiophyte Phaeocystis pouchetii was the first major phytoplankton species to bloom, reaching concentrations of 6 × 10⁷ cells · 1^–1 and remained numerically dominant for most of the summer. During the P. pouchetii bloom the concentration of most other autotrophs did not increase. Microheterotroph abundance peaked during or immediately after the Phaeocystis bloom. Their peak coincided with very high concentrations of organic carbon, particularly DOC which exceeded 100 mg · 1^–1, and low bacterial abundance. Maximum bacterial abundance was reached after the decline in microheterotroph numbers. Bacterial utilization of carbon substrates and microheterotroph grazing of bacteria and uptake of DOC may form an important link to higher trophic levels during Antarctic Phaeocystis blooms.