Redfield revisited 1. Regulation of nitrate, phosphate, and oxygen in the ocean

The ratio of phosphate and nitrate concentrations in the deep ocean matches closely the Redfield ratio required by phytoplankton growing in the surfac e ocean. Furthermore, the oxygen available from dissolution in ocean water is, on average, just sufficient for the respiration of the resulting organi c matter. We review various feedback mechanisms that have been proposed to account for these remarkable correspondences and construct a model to test their effectiveness. The model's initial steady state is close to the Redfi eld ratios and stable against instantaneous changes in the sizes of the nit rate and phosphate reservoirs. When classic flux estimates are adopted, nit rate responds to perturbation in 1000-2000 years and phosphate in 40,000-60 ,000 years. However, recently increased estimates of the input and output f luxes of nitrate and phosphate suggest that they respond more rapidly to pe rturbation, nitrate in 500-1000 years and phosphate in 10,000-15,000 years. Nitrogen fixation tends to maintain nitrate close to Redfield ratio with p hosphate, while denitrification tends to keep nitrate as the proximate limi ting nutrient and tie it in Redfield ratio to dissolved oxygen. Under incre ases in phosphorus input to the ocean, the relative responsiveness of nitro gen fixation and denitrification determine whether nitrate remains close to Redfield ratio to phosphate or to oxygen. If nitrogen fixation is strongly limited (e.g., by lack of iron), increasing phosphorus input to the ocean can cause phosphate to deviate above Redfield ratio to nitrate. Hence nitro gen dynamics can control phosphate behavior and nitrate can potentially be the ultimate limiting nutrient over geologic periods of time. When nitrate and phosphate are coupled together by responsive nitrogen fixation, negativ e feedbacks on organic and calcium-bound phosphorus burial stabilize their concentrations. If anoxia suppresses organic phosphorus burial, the resulti ng feedbacks on phosphate (positive) and oxygen (negative) improve regulati on toward the Redfield ratios. Variants of the model are forced with a glob al record of phosphorus accumulation in biogenic sediments as a proxy for c hanges in phosphate input to the ocean over the past 40 Myr. Nitrate is gen erally regulated close to Redfield ratio to phosphate, despite large change s in phosphorus input. If nitrogen fixation is strongly limited, then there is one interval (similar to 15 Myr ago) when a very rapid increase in phos phate input forces phosphate above Redfield ratio to nitrate. Decreases in phosphorus input cause phosphate and nitrate to quickly deviate below Redfi eld ratio with oxygen, removing anoxia from the ocean, while increases in p hosphorus input rapidly increase anoxia. Hence we conclude that there appea rs to be an element of chance in observing today's ocean "on the edge of an oxia" with nitrate, phosphate, and oxygen all close to the Redfield ratios.