Decline in the Nutrient Inventories of the Upper Subtropical Northwest Pacific Ocean

The surface nutrient concentrations of the Northwest Pacific Ocean have shown distinct changes over the past four decades, which have been variably attributed to changes in anthropogenic nitrogen inputs or stratification. Here, we present a new data compilation that demonstrates a long-term decline in the surface nutrient concentrations caused by a significant decrease (similar to 25%) in the nutrient inventories of the permanent thermocline (100-850 m) rather than by enhanced stratification. This subsurface nutrient decrease is likely due to the northward expansion of high-temperature, low-nutrient subtropical gyre waters associated with climate change and a regional physical regime shift. Furthermore, a declining trend in the N:P ratio pointing toward greater N-limitation is observed in this research, in contrast to current assumptions inferring P-limitation by anthropogenic nitrogen inputs. Our results imply that these shifts may influence new production as well as carbon sequestration in this region.

Automated summary

The changes in surface nutrient concentrations in the Northwest Pacific Ocean over the past four decades were mainly due to a significant decrease in nutrient inventories within the permanent thermocline, possibly caused by the northward expansion of high-temperature, low-nutrient subtropical gyre waters associated with climate change and a regional physical regime shift. Additionally, a declining trend in the N:P ratio suggests a greater N-limitation in contrast to previous assumptions of P-limitation due to anthropogenic nitrogen inputs.