Microplankton assemblages at Station KNOT in the subarctic western Pacific, 1999–2000

The microplankton communities on filter samples from three shallow (0–300 m) hydrocasts, conducted in August 1999, January 2000, and May 2000 at Station KNOT in the Western Subarctic Gyre of the NW Pacific, have been analyzed by scanning electron microscopy (SEM). The results clearly show that small (<10 μm) phytoplankton, such as coccolithophorids, Parmales, and the pennate diatom Fragilariopsis pseudonana, play important roles in seasonal productivity. The Parmales were the most dominant mineralized phytoplankton group in May and August, and their year-round presence in scaly form suggests they are not cysts as previously thought. As the Parmales are rare or absent in the underlying sediment traps or surface sediments, their siliceous scales are undoubtably being efficiently recycled in the euphotic zone where silica levels are never depleted. The phytoplankton community structure observed by SEM is different to those reported previously, in which more traditional light microscopical techniques were used, with the consequence of missing the smaller phytoplankton. This study, regardless of its biogeographic location, highlights the potential problem of enumerating marine phytoplankton assemblages purely by light microscopy. An inability to determine the contribution made by these small phytoplankton can lead to an overestimation of the importance of large phytoplankton, and an underestimation of total productivity.