Final answer:
Algae are most linked to eutrophication because they rapidly grow due to excess nutrients in water, causing oxygen depletion and other environmental issues. Therefore, the correct answer is option 3) algae.
Step-by-step explanation:
Eutrophication is a process where water bodies receive an excess of nutrients, such as nitrates and phosphates, which often leads to a rapid increase in algae growth. This phenomenon typically results in oxygen depletion and negatively impacts water quality and aquatic life. In a given food web, such as the Lake Ontario ecosystem, primary producers, which include photosynthetic organisms like algae and phytoplankton, form the base of the food chain. These organisms are consumed by primary consumers, which are herbivores.
Secondary consumers eat the primary consumers, and these are usually carnivorous. The tertiary consumers feed on other carnivores. However, the excessive growth of algae due to the surplus of nutrients is the prime agent of eutrophication. Legumes, while they can contribute to nutrient levels due to their nitrogen-fixing abilities, are not typically associated with aquatic ecosystems and eutrophication. Top predators, or apex consumers, are at the top of the food chain and do not directly contribute to eutrophication. Therefore, the group of organisms that are most linked to eutrophication are the algae, making option 3 the correct answer to the student's question.