The Arctic Cod (Boreogadus saida) is a small, cold-adapted fish that plays a large role in the Arctic marine food web. Phytoplankton are microscopic, plant-like organisms forming the base of nearly all oceanic food chains. Arctic Cod do not consume phytoplankton directly; they are predators that feed at a higher trophic level. Instead, the cod relies on organisms that graze on phytoplankton to acquire its energy.
The Arctic Cod’s Primary Consumers
The Arctic Cod is an abundant, mid-level predator that primarily feeds on zooplankton and small crustaceans, acting as a crucial link in the Arctic’s food web. Its diet centers on organisms like copepods and amphipods. Specific plankton species, such as hyperiid and gammarid amphipods, form the bulk of the cod’s caloric intake.
Diet varies based on the fish’s size, location, and prey availability; larger cod sometimes engage in piscivory, or eating smaller fish. Immature Arctic Cod often consume sympagic and pelagic species, including appendicularians and euphausiids. The fish is an opportunistic feeder, switching to any available prey when preferred food sources are scarce.
Arctic Cod are considered a keystone species because they funnel energy from lower trophic levels to the top predators. They have high lipid concentrations, making them an energy-rich food source for marine mammals and seabirds. Predators include ringed seals, beluga whales, narwhals, and seabirds such as thick-billed murres and glaucous gulls.
Phytoplankton’s Position in the Trophic Structure
Phytoplankton are the autotrophic organisms that form the first trophic level in the marine food web. These microscopic organisms harness solar energy through photosynthesis, converting carbon dioxide and water into organic nutrients. They are the foundational primary producers, supplying the initial energy for the entire marine ecosystem.
In the Arctic Ocean, these producers initiate the food chain, with spring and early summer often triggering massive blooms of phytoplankton and ice algae. Primary consumers, specifically microscopic zooplankton grazers such as copepods and krill, directly consume phytoplankton. These zooplankton are the herbivores of the Arctic seas, relying on the phytoplankton and sea ice algae for sustenance.
The Role of Energy Transfer in the Arctic Food Web
The Arctic food web is characterized by a relatively short food chain, which highlights the close connection between the base and the higher-level consumers like the Arctic Cod. Energy flows from the phytoplankton (Producers) to the zooplankton (Primary Consumers) and then to the Arctic Cod (Secondary Consumer). This short path means energy is transferred efficiently, but it also makes the higher trophic levels highly dependent on the stability of the lower levels.
Energy transfer is inherently inefficient, with approximately 90% of the energy lost as heat or used for metabolic processes between each trophic level. Only about 10% of the energy from one level is successfully passed on to the next. The Arctic Cod acts as an intermediary, effectively transferring the stored energy from zooplankton—which is about 70% of the zooplankton’s energy—to the larger marine mammals and birds.
This short, direct pathway is influenced by unique Arctic processes, such as the production of algae that grows within and under the sea ice. This ice-associated production provides an early, high-quality food source for zooplankton, which rapidly fuels the growth of the Arctic Cod. The efficiency and short length of this food web mean changes at the phytoplankton level can quickly have cascading effects on the entire ecosystem, including top predators.