PRODUCTIVITY OF PICOPLANKTON COMPARED WITH THAT OF LARGER PHYTOPLANKTON IN THE SUB-ARCTIC REGION

We determined the productivity (mu g C mu g(-1) Chl a h(-1)) of size-f ractionated phytoplankton in the northern North Pacific and the Bering Sea in summer and winter. Picoplankton (<2 mu m) were more productive than larger sized phytoplankton (2-10 and 10-200 mu m) in the subtrop ical region, where the in situ temperature was >10 degrees C; whereas picoplankton in the subarctic region were similar in productivity or l ess productive than larger sized plankton, where the in situ temperatu re was <10 degrees C. The result from the subtropical region in this s tudy agrees with previous results from tropical and subtropical waters , which indicate that phytoplankton productivity tends to decrease wit h increasing cell size. The result from the subarctic region, however, differs from previous results. We observed a positive linear regressi on for in situ temperature and picoplankton productivity, but this tre nd was not seen in the larger sized phytoplankton. The results show th at the productivity of picoplankton is markedly influenced by in situ temperature compared with that of larger sized plankton. Low temperatu re appears to account largely for the observation that the productivit y of picoplankton is not significantly higher than that of larger size d phytoplankton in the subarctic region.