Final answer:
In the provided food chain, the last organism is a tertiary consumer, represented by the Chinook salmon being the apex predator in the Lake Ontario ecosystem.
Step-by-step explanation:
In this food chain, the last organism represents a tertiary consumer. This is because a tertiary consumer is defined as a carnivore that preys on other carnivores. The hierarchy of a food chain typically starts with producers, which are photosynthetic organisms like plants or phytoplankton. These producers are eaten by primary consumers, which are herbivores. Next in line are the secondary consumers, which are typically carnivores that eat the primary consumers. In the provided information about a Lake Ontario food chain, the Chinook salmon is indicated as the apex consumer, which is generally a term synonymous with tertiary consumer when they feed on other carnivores. Apex consumers sit at the top of the food chain and are not preyed upon by any other species.