A key biological link between ocean geochemical cycles, ocean color and sea surface temperature (SST) is the nitrogen-to-chlorophyll a (N:chl a) ratio of phytoplankton. This is because nitrogen is the limiting factor over approximately 2/3 of the ocean surface and light absorption depends on the concentration of chlorophyll a. Recent work has demonstrated systematic variability in this ratio. Using one-dimensional models at two locations we show that varying N:Chl a, rather than keeping a fixed mean value (0--150 m), leads to lower chlorophyll a in the mixed layer, less light absorption within the mixed layer and cooler sea surface temperatures. At the equator the temperature reduction is 0.15 ¿C throughout the year whereas at 35 ¿N, a temperature difference of about 1 ¿C occurs during the summer. Although demonstrated at only two locations, this effect is likely to occur over large parts of the ocean. ¿ 2001 American Geophysical Union