Productivity of picoplankton compared with that of larger phytoplankton in the subarctic region

We determined the productivity (µg C µg^–1Chi a h^–1) of size-fractionated phytoplankton in the northernNorth Pacific and the Bering Sea in summer and winter. Picoplankton(<2 µm) were more productive than larger sized phytoplankton(2–10 and 10–200 µm) in the subtropical region,where the in situ temperature was >10°C; whereas picoplanktonin the subarctic region were similar in productivity or lessproductive than larger sized plankton, where the in situ temperaturewas <10°C. The result from the subtropical region inthis study agrees with previous results from tropical and subtropical waters, which indicate that phytoplankton productivitytends to decrease with increasing cell size. The result fromthe subarctic region, however, differs from previous results.We observed a positive linear regression for in situ temperatureand picoplankton productivity, but this trend was not seen inthe larger sized phytoplankton. The results show that the productivityof picoplankton is markedly influenced by in situ temperaturecompared with that of larger sized plankton. Low tem peratureappears to account largely for the observation that the productivityof picoplankton is not significantly higher than that of largersized phytoplankton in the subarctic region.