Fig. 6

Simple models of nitrate consumption at the equator. Nitrate consumption increases as equatorial waters upwell towards the surface, and the degree of consumption is related to upwelling stength. Here we plot the nitrate consumption a as a percentage of the initial nitrate concentration and b in terms of nitrate concentration with varying upwelling rates. The integrated percentage of nitrate consumption in a is based on observed nitrate uptake rate with depth²¹ and total nitrate consumption (Table 1) at 0° N, 110° W from December 2004. Note that most nitrate consumption occurs in the upper 50 m, suggesting that light limitation can only play a minor role in modifying elemental requirements (e.g., lower C:N at lower light levels⁷¹). The same nitrate uptake rates were used to calculate the integrated nitrate consumption in b for a variety of upwelling rates, and we found that increasing upwelling rate decreases the integrated nitrate consumption in the water upwelling at the equator (see "Methods"). The same source water depth and iron recycling rate are assumed in all calculations