The nitrogenous nutrient in the modern (and most oxygenated) ocean is dominated by nitrate. However, nitrate can be quickly removed during water column deoxygenation. Oceanic nitrate inventory can be greatly reduced during ocean anoxic events. This occurred during the onset of the end-Permian mass extinction and was followed by a shift in oceanic nutrient-N inventory from nitrated dominated to ammonium dominated state. The consequences included a boom of diazotrophs and potentially ammonium toxicity affecting marine animals.