How does nitrogen enter the ocean?

Nitrogen enters the ocean primarily through a process known as runoff. This occurs when nitrogen compounds, often from fertilizers, animal waste, or sewage, are washed from land into rivers and eventually reach the ocean. The runoff from agricultural fields contains nitrates and ammonium, which are forms of nitrogen easily utilized by aquatic plants and phytoplankton. This influx of nitrogen can enhance productivity in marine ecosystems, although excessive nitrogen from runoff can lead to algal blooms and eutrophication, negatively impacting water quality and marine life.

While atmospheric deposition occurs and contributes to nitrogen levels in the ocean, it is generally a smaller source compared to runoff. Volcanic eruptions release various gases, but nitrogen is less prominent in this context compared to other elements. Photosynthesis, on the other hand, utilizes carbon dioxide and does not directly involve nitrogen entering the ocean; instead, it's a process primarily concerned with the conversion of light energy into chemical energy in the presence of carbon compounds. Thus, runoff represents the most significant pathway through which nitrogen naturally enters marine environments.