Increased nitrogen level in ocean water can lead to excessive growth of aquatic plants in the ocean which in turn leads to high oxygen (dissolved oxygen) uptake by these plants when they decompose. The overgrowth of these plants also prevent light penetration in the ocean water. All these can affect other aquatic organisms particularly those that depend on these dissolved oxygen and/or light penetration (such as fish and crustaceans).
The answer is the forcing mechanism. It is the process that would alter the balance of the climate system that includes solar irradiance,the volcanic eruptions as well as the enhancement of the greenhouse effect through emission of the carbon dioxide.
