Primary productivity refers to the rate, which the primary producers produce organic matter (food). The primary productivity will depend on the amount of nutrients available in the ecosystem.
If an ecosystem contains a nutrient, which affects the primary productivity, it slows the circulation, called limiting nutrient. Crops need nitrogen, for its growth and high yield. Absence or scarcity of such nutrients reduces the productivity of the soil. To compensate this, farmers use Nitrogen-Phosphorous- Potassium fertilizers.
If an ecosystem receives excess of limiting nutrient from other source, such excessive nutrients can have supportive effect on one species, whereas can have adverse effect on the other. A typical example of such effect is an algal bloom. Here, excessive nitrogen can cause matt growth of the algae on the water body and can hinder the propagation of other marine life.
Thus, the situation is a result of hypoxic condition developed in the water body due to the rapid decaying of dead algae.