Marine food web

A marine food web is a food web of marine life. At the base of the ocean food web are single-celled algae and other plant-like organisms known as phytoplankton. The second trophic level (primary consumers) is occupied by zooplankton which feed off the phytoplankton. Higher order consumers complete the web. There has been increasing recognition in recent years concerning marine microorganisms. Habitats lead to variations in food webs. Networks of trophic interactions can also provide a lot of information about the functioning of marine ecosystems.

Compared to terrestrial environments, marine environments have biomass pyramids which are inverted at the base. In particular, the biomass of consumers (copepods, krill, shrimp, forage fish) is larger than the biomass of primary producers. This happens because the ocean's primary producers are tiny phytoplankton which grow and reproduce rapidly, so a small mass can have a fast rate of primary production. In contrast, many significant terrestrial primary producers, such as mature forests, grow and reproduce slowly, so a much larger mass is needed to achieve the same rate of primary production. Because of this inversion, it is the zooplankton that make up most of the marine animal biomass.

Food chains and trophic levels

Food webs are built from food chains. All forms of life in the sea have the potential to become food for another life form. In the ocean, a food chain typically starts with energy from the sun powering phytoplankton, and follows a course such as:

phytoplankton → herbivorous zooplankton → carnivorous zooplankton → filter feeder → predatory vertebrate

Phytoplankton don't need other organisms for food, because they have the ability to manufacture their own food directly from inorganic carbon, using sunlight as their energy source. This process is called photosynthesis, and results in the phytoplankton converting naturally occurring carbon into protoplasm. For this reason, phytoplankton are said to be the primary producers at the bottom or the first level of the marine food chain. Since they are at the first level they are said to have a trophic level of 1 (from the Greek trophē meaning food). Phytoplankton are then consumed at the next trophic level in the food chain by microscopic animals called zooplankton. Zooplankton constitute the second trophic level in the food chain, and include microscopic one-celled organisms called protozoa as well as small crustaceans, such as copepods and krill, and the larva of fish, squid, lobsters and crabs. Organisms at this level can be thought of as primary consumers. In turn, the smaller herbivorous zooplankton are consumed by larger carnivorous zooplankters, such as larger predatory protozoa and krill, and by forage fish, which are small, schooling, filter-feeding fish. This makes up the third trophic level in the food chain.

The fourth trophic level consists of predatory fish, marine mammals and seabirds that 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 a fifth trophic level. Baleen whales can consume zooplankton and krill directly, leading to a food chain with only three or four trophic levels. In practice, trophic levels are not usually simple integers because the same consumer species often feeds across more than one trophic level. For example, a large marine vertebrate may eat smaller predatory fish but may also eat filter feeders; the stingray eats crustaceans, but the hammerhead eats both crustaceans and stingrays. Animals can also eat each other; the cod eats smaller cod as well as crayfish, and crayfish eat cod larvae. The feeding habits of a juvenile animal, and, as a consequence, its trophic level, can change as it grows up. The fisheries scientist Daniel Pauly sets the values of trophic levels to one in primary producers and detritus, two in herbivores and detritivores (primary consumers), three in secondary consumers, and so on. The definition of the trophic level, TL, for any consumer species is

T L i = 1 + ∑ j ( T L j ⋅ D C i j ) , {\displaystyle TL_{i}=1+\sum _{j}(TL_{j}\cdot DC_{ij}),}

where T L j {\displaystyle TL_{j}} is the fractional trophic level of the prey j, and D C i j {\displaystyle DC_{ij}} represents the fraction of j in the diet of i. In the case of marine ecosystems, the trophic level of most fish and other marine consumers takes value between 2.0 and 5.0. The upper value, 5.0, is unusual, even for large fish, though it occurs in apex predators of marine mammals, such as polar bears and killer whales. As a point of contrast, humans have a mean trophic level of about 2.21, about the same as a pig or an anchovy.

By taxon

Primary producers

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