Riverine nitrogen supply to the global ocean and its limited impact on global marine primary production: a feedback study using an Earth system model

A common notion is that negative feedbacks stabilize the natural marine nitrogen inventory. Recent modeling studies have shown, however, some potential for localized positive feedbacks leading to substantial nitrogen losses in regions where nitrogen fixation and denitrification occur in proximity to each other. Here we include dissolved nitrogen from river discharge in a global 3-D ocean biogeochemistry model and study the effects on near-coastal and remoteopen-ocean biogeochemistry. We find that at a steady state the biogeochemical feedbacks in the marine nitrogen cycle, nitrogen input from biological N-2 fixation, and nitrogen loss via denitrification mostly compensate for the imposed yearly addition of 22.8 to 45.6 Tg of riverine nitrogen and limit the impact on global marine productivity to < 2 %. Global experiments that regionally isolate river nutrient input show that the sign and strength of the feedbacks depend on the location of the river discharge and the oxygen status of the receiving marine environment. Marine productivity generally increases in proximity to the nitrogen input, but we also find a decline in productivity in the modeled Bay of Bengal and near the mouth of the Amazon River. While most of the changes are located in shelf and near-coastal oceans, nitrogen supply from the rivers can impact the open ocean, due to feedbacks or knock-on effects.