Biomass (ecology) - Ocean Biomass

Ocean Biomass

Ocean biomass, in a reversal of terrestrial biomass, can increase at higher trophic levels. In the ocean, the food chain typically starts with phytoplankton, and follows the course:

Phytoplankton → zooplankton → predatory zooplankton → filter feeders → predatory fish

Phytoplankton are the main primary producers at the bottom of the marine food chain. Phytoplankton use photosynthesis to convert inorganic carbon into protoplasm. They are then consumed by microscopic animals called zooplankton.

Zooplankton comprise the second level in the food chain, and include the larva of fish, squid, lobsters and crabs, small crustaceans such as copepods and krill, and many other types.

In turn, smaller zooplankton are consumed both by larger predatory zooplankters, such as krill, and by forage fish, which are small schooling filter feeding fish. This makes up the third level in the food chain.

The fourth trophic level consists of predatory fish, marine mammals, and seabirds which consume forage fish. Examples are swordfish, seals and gannets.

Apex predators, such as orcas which can consume seals and shortfin mako sharks which can consume swordfish, make up the fifth trophic level. Baleen whales can consume zooplankton and krill directly, leading to a food chain with only three or four trophic levels.

Marine environments can have inverted biomass pyramids. In particular, the biomass of consumers (copepods, krill, shrimp, forage fish) is larger than the biomass of primary producers. This happens because the ocean primary producers are tiny phytoplankton which grow and reproduce rapidly, so a small mass can have a fast rate of primary production. In contrast, terrestrial primary producers grow and reproduce slowly.

There is an exception with cyanobacteria. Marine cyanobacteria are the smallest known photosynthetic organisms; the smallest of all, Prochlorococcus, is just 0.5 to 0.8 micrometres across. Prochlorococcus is possibly the most plentiful species on Earth: a single millilitre of surface seawater may contain 100,000 cells or more. Worldwide, there are estimated to be several octillion (~1027) individuals. Prochlorococcus is ubiquitous between 40°N and 40°S and dominates in the oligotrophic (nutrient poor) regions of the oceans. The bacterium accounts for an estimated 20% of the oxygen in the Earth's atmosphere, and forms part of the base of the ocean food chain.