Abstract
Abstract. 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 remote-open-ocean biogeochemistry. We find that at a steady state the biogeochemical feedbacks in the marine nitrogen cycle, nitrogen input from biological N2 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.
Highlights
Nitrogen plays a key role in marine biogeochemistry in coastal and open oceans as it is one of the major limiting nutrients for algal photosynthesis
We find that at a steady state the biogeochemical feedbacks in the marine nitrogen cycle, nitrogen input from biological N2 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 %
Marine productivity generally increases in proximity to the nitrogen input, but we find a decline in productivity in the modeled Bay of Bengal and near the mouth of the Amazon River
Summary
Nitrogen plays a key role in marine biogeochemistry in coastal and open oceans as it is one of the major limiting nutrients for algal photosynthesis. Da Cunha et al (2007) used an ocean biogeochemistry model to analyze the impact of river nutrient fluxes (N, Si, Fe, and carbon) on the global and coastal ocean primary production but concentrated on a short time period of a few decades, likely not long enough to study the feedbacks of the N cycle in the open ocean considering that the mean residence time of fixed nitrogen in the ocean has been estimated to be a few thousand years (Gruber, 2004). In order to disentangle the effects of the different sources of N, we are using UVic without atmospheric N deposition to focus on the marine biogeochemical response to riverine N inputs to the coastal ocean To do this we make use of modeled estimates of riverine dissolved inorganic nitrogen (DIN) export from watersheds (Mayorga et al, 2010). We performed a series of experiments where we studied the responses of the ocean to riverine nutrient supply to individual regions in order to find out if N impacts the global ocean differently, depending on the region where river supply takes place
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
