THE OCEANOGRAPHY SOCIETY PACIFIC BASIN MEETING, July 1994, p.59

Growth and Grazing Rates of Prochlorococcus and Synechococcus at Station ALOHA Measured by the Selective Inhibitor Technique

H. Liu and L. Campbell

A selective inhibitor method for measuring growth and grazing rates of Prochlorococcus and Synechococcus using kanamycin as the prokaryotic inhibitor has been tested both in laboratory and field experiments. Using this method, we measured the growth and grazing rates of Prochlorococcus and Synechococcus at Station ALOHA (22 45'N, 158 W). Data from profiles of the upper 175 m water column during November 1993 and February 1994 showed that Prochlorococcus grew 0.4 - 0.5 d^-1 near the sea surface and about 0.1 d^-1 at the bottom of the euphotic zone. Typically, Synechococcus was limited to the upper 100 meters and grew faster than Prochlorococcus (up to 1.0 d^-1 in the surface waters in November). However, in February, Synechococcus extended to below 150 m with abundance four times higher than measured in November and growth rates lower by half of that in November. These data indicate that the February sampling probably occured just after a small Synechococcus bloom caused by a strong mixing event. Grazing mortality rates of Prochlorococcus and Synechococcus were less than their growth rates in November, but exceeded growth rates in February. Based on these growth rates and carbon/cell conversion factors, we found that Prochlorococcus is the dominant contributor to primary production in subtropical North Pacific Ocean. In contrast, Synechococcus contributed only a few percent to primary production in this region.