Nanomolar phosphate supply and its recycling drive net community production in the subtropical North Pacific

Seasonal drawdown of dissolved inorganic carbon (DIC) in the subtropical upper ocean makes a significant contribution to net community production (NCP) globally. Although NCP requires macronutrient supply, surface macronutrients are chronically depleted, and their supply has been unable to balance the NCP demand. Here, we report nanomolar increases in surface nitrate plus nitrite (N+N, similar to 20nM) and phosphate (PO4, similar to 15nM) from summer to winter in the western subtropical North Pacific. Molar ratios of upward fluxes of DIC:N+N:PO4 to the euphotic zone (< 100m) were in near-stoichiometric balance with microbial C:N:P ratios (107 similar to 243:16 similar to 35:1). Comparison of these upward influxes with other atmospheric and marine sources demonstrated that total supply is largely driven by the other sources for C and N (93 similar to 96%), but not for P (10%), suggesting that nanomolar upward supply of P and its preferential recycling play a vital role in sustaining the NCP. Primary productivity in the oligotrophic ocean sustains Earth's ecosystems, but nutrient concentrations are vanishingly low. Here the authors measure nanomolar macronutrient concentrations in the North Pacific and find that net community production is sustained through high rates of phosphorus recycling.

Automated summary

The seasonal drawdown of dissolved inorganic carbon in the subtropical upper ocean contributes significantly to net community production globally. While surface macronutrients are chronically depleted, nanomolar increases in surface nitrate plus nitrite and phosphate from summer to winter in the western subtropical North Pacific are found to be in near-stoichiometric balance with microbial C:N:P ratios. These findings suggest that the nanomolar upward supply of phosphorus and its preferential recycling play a vital role in sustaining net community production in oligotrophic ocean environments.