Explain how a reduction in the nutrients in the upper layers of the ocean could lead to a decrease in the numbers of forage fish such as anchoveta.

The explanation can be found below:

Explanation:

In an aquatic ecosystem, organisms feed on one another as food source to acquire energy. This feeding pattern is termed as FOOD CHAIN. Food Chain of an aquatic ecosystem begins with producers like phytoplanktons and progresses. These phytoplanktons e.g algae, which are mostly microscopic require certain nutrients e.g phosphate for growth. They tend to bloom in the presence of abundant nutrients in the upper layer of the water body.

However, in a case where the nutrients reduces, this means that the rate at which these phytoplanktons grow will also decrease. Fishes like Anchoveta feeds directly on phytoplanktons in the aquatic food chain. A decrease in the amount of phytoplanktons in the water body means there would be less to feed on by the forage fishes, hence, gradually reducing their population.

Hence, in a nutshell, a reduction in available water nutrients will lead to a reduction in the growth rate of phytoplanktons, which will utterly result in a decrease in the numbers of forage fishes.