Chlorophyll a concentrations in the North Pacific: Does a latitudinal gradient exist?

Abstract

Chlorophyll a concentrations were measured as a function of depth from 28 to 48°N along 152°W in March 1991 with Whatman GF/F and 0.2‐µm Nuclepore filters. Surface Chl a concentrations measured with 0.2‐µm Nuclepore filters were up to fourfold higher than those measured with Whatman GF/F filters. The largest difference between the two filter types was found in subtropical waters, where picoplankton were a major constituent of the phytoplankton assemblage. Chl a concentrations integrated from 0 to 175 m showed a threefold increase (9–26 mg Chl a m−22) between 28 and 48°N when Whatman GF/F filters were used. However, integrated Chl a concentrations based on measurements with 0.2‐µm Nuclepore filters were nearly constant (25–31 mg Chl a m−2) over the transect. These results lead us to question the existence of previously reported latitudinal gradients in integrated Chl a concentrations in the North Pacific Ocean.