Phytoplankton photosynthesis parameters along 140°W in the equatorial Pacific

The role of trace metal limitation in the maintenance of the high nutrient and low chlorophyll condition in eastern equatorial Pacific surface waters remains controversial. On the EqPac Survey II cruise, photosynthesis versus irradiance (P-I) and phytoplankton spectral absorption were measured along 140°W from 12°N to 12°S in the eastern tropical Pacific to determine the spatial variability of P-I parameters in relation to the light, macronutrient, and trace metal regime. The latitudinal patterns of the maximum quantum yield of photosynthesis (φm) and carotenoid-corrected spectral absorption covaried with the concentration of N03 at 60 m, a proxy for the macronutrient supply rate. At the equator, the maximum quantum yield of phytoplankton photosynthesis was less than half that expected for nutrient-replete phytoplankton. This indicates that phytoplankton photosynthesis was nutrient limited even though N03 concentrations at the equator exceeded 6 μmol kg−1. When φm was corrected for absorption by photoprotective carotenoids, maximum values were found at 12°N and values declined linearly with latitude to 12°S. This pattern is coincident with the putative atmospheric iron flux and the distribution of dissolved iron. These observations support the hypothesis that the photosynthesis of equatorial phytoplankton is regulated by iron availability. Phytoplankton photosynthetic performance and in situ chlorophyll-normalized primary production along 140W appeared to be regulated by the interaction of macronutrient and trace metal supply rates on phytoplankton photoacclimation processes.