Answer: Algal blooms
Step-by-step explanation:
The fertilizers are the chemicals which are used in the agricultural field so as improve the quality of nutrient deficient soil.
According to the given situation, if the fertilizer runoff enters into the ocean it will increase the levels of nitrogen in the oceanic water. The nitrogen being an important nutrient for plant growth will likely to cause algal bloom in the ocean water.
The algal bloom in the ocean water will cut the supply of atmospheric oxygen in the water. The depletion of oxygen in the water will result in decline in the population of the marine organisms. Hence, this will negatively affect the ecosystem.