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 |