What occurs when an aquatic ecosystem receives a large input of a limited nutrient?

When an aquatic ecosystem receives a significant influx of a limited nutrient, such as nitrogen or phosphorus, it can lead to an algal bloom. Algal blooms are rapid increases in the population of algae in aquatic environments, often resulting from nutrient enrichment. This phenomenon occurs because the abundant nutrients provide an ideal environment for algae to grow and reproduce quickly.

As algae proliferate, they can cover the surface of the water, blocking sunlight from reaching other aquatic plants. This disruption of photosynthesis can have a cascading effect on the entire ecosystem, leading to decreased oxygen levels when the algae die and decompose, which can harm fish and other aquatic organisms. Therefore, algal blooms not only serve as a clear indicator of nutrient overload but also reflect broader ecological consequences.

The other choices do not adequately capture the immediate outcome of nutrient input. While increased fish activity could potentially happen as a result of increased food availability, it is not a direct consequence of the nutrient input itself. Nutrient cycling refers to the movement and exchange of organic and inorganic matter back into the production of living matter but does not directly describe the immediate effect of a nutrient influx. Oxygen depletion can occur as a downstream effect of algal blooms, but it is not the initial response to a nutrient surge.