Trophic levels in the sea. Computer illustration showing the trophic levels of organisms in the sea. The trophic level of an organism is the position it occupies in a food chain. Here, the bottom level, at the base of the pyramid, is occupied by phytoplankton such as algae. The phytoplankton are eaten by zooplankton - small invertebrates, e.g.. crustaceans, which are in turn eaten by small fish, e.g.. herring. The small fish are eaten by predatory fish, e.g.. mackerel, which are eaten by larger predatory fish, e.g.. tuna.