Marine trophic levels (diagram credit: University of Waikato).
Trophic levels are a defined by the position of organisms in a food chain or food web. The different levels help define the flow of energy within an ecosystem. The lowest trophic level is primary producers, such as algae and phytoplankton, which generate their own energy from the sun via photosynthesis. Primary consumers, such as herbivorous zooplankton, must eat primary producers as their source of energy. As a general rule, each trophic level up equates to capturing 10% of the energy from the trophic level below it (e.g., a juvenile fish uses 10% of the energy it gets from eating zooplankton and only 1% of the energy the zooplankton gets from eating phytoplankton).
Many food webs contain organisms, such as fish that eat other fish and zooplankton, that do not fall into simple trophic level constructs. Stable isotope analyses can be used to infer diet and trophic level for these complex diets by comparing isotopic signatures in tissue from the organisms of interest to those from samples of known origin. Commonly used isotopes for aquatic ecosystem food web analyses include 13C, 15N and 34S.