PHYTOPLANKTON PHOTOSYNTHESIS PARAMETERS ALONG 140-DEGREES-W IN THE EQUATORIAL PACIFIC

The role of trace metal limitation in the maintenance of the high nutr ient and low chlorophyll condition in eastern equatorial Pacific surfa ce waters remains controversial. On the EqPac Survey II cruise, photos ynthesis versus irradiance (P-I) and phytoplankton spectral absorption were measured along 140 degrees W from 12 degrees N to 12 degrees S i n the eastern tropical Pacific to determine the spatial variability of P-I parameters in relation to the light, macronutrient, and trace met al regime. The latitudinal patterns of the maximum quantum yield of ph otosynthesis (phi(m)) and carotenoid-corrected spectral absorption cov aried with the concentration of NO3 at 60 m, a proxy for the macronutr ient supply rate. At the equator, the maximum quantum yield of phytopl ankton photosynthesis was less than half that expected for nutrient-re plete phytoplankton. This indicates that phytoplankton photosynthesis was nutrient limited even though NO3 concentrations at the equator exc eeded 6 mu mol kg(-1). When phi(m) was corrected for absorption by pho toprotective carotenoids, maximum values were found at 12 degrees N an d values declined linearly with latitude to 12 degrees S. This pattern is coincident with the putative atmospheric iron flux and the distrib ution of dissolved iron. These observations support the hypothesis tha t the photosynthesis of equatorial phytoplankton is regulated by iron availability. Phytoplankton photosynthetic performance and in situ chl orophyll-normalized primary production along 140 degrees W appeared to be regulated by the interaction of macronutrient and trace metal supp ly rates on phytoplankton photoacclimation processes.