It is easier to understand food chains and food webs by considering the number of steps, or trophic levels, that the energy has travelled. The first step in the flow of energy, and therefore the first trophic level, is occupied by the primary producers that originally capture the energy and store it in organic compounds. Consumers that feed directly on the producers are called primary consumers and occupy the next trophic level. At the next level are the secondary consumers, predators that eat the primary consumers. Feeding on the secondary consumers are the tertiary consumers and so on. Each trophic level relies on the level below for sustenance.
Trophic level 1
The first level in the ocean consists of marine plants which include phytoplankton, seaweeds and some seagrasses. These are the primary producers which fix energy from the sun and make it available to life forms in the other trophic levels.
- Seaweeds are types of algae which grow on the sea bottom in areas of shallow water where there is enough light to photosynthesise. These include red, green and brown algae. The colours relate to their light-capturing pigments which vary according to the depths at which they are found.
- Phytoplankton are not as conspicuous as the seaweeds because they are mainly microscopic single-celled plants. Many are also types of algae, including the dinoflagellates which are a vital part of life on coral reefs. The group also includes photosynthetic bacteria. Together, they drift with the ocean currents and so are available as a primary food source throughout the marine ecosystem.
- Sea grasses are actually flowering plants which live in shallow coastal waters. They harbour very specific communities which are dependent on them.
Trophic level 2
At this level, the organisms involved are hugely diverse and have an equally diverse way of making use of the food sources from the first trophic level. These include the browsers and grazers, filter feeders and deposit feeders.
- Zooplankton is animal plankton, which drifts in the ocean like the phytoplankton. It includes single-celled animals, the juvenile stages of many marine and shore animals and larger animals such as jellyfish. Zooplankton feed in many ways, mainly on phytoplankton, but as this group includes many larger forms of larvae and juveniles it is inevitable that zooplankton will be feeding on other zooplankton as well.
- Browsers and grazers include numerous species including molluscs, such as gastropod snails, which have a toothed tongue or radula which rasps away at algae. Urchins also belong to this group, and in large numbers can have a major effect on the numbers of larger algae such as kelp. Some fish graze specifically on algae including the farmer fish (a type of damselfish) which actively ‘gardens’ a crop of algae to feed on.
- Filter feeders eat both phytoplankton and zooplankton. They do this by straining the incoming sea water using various structural traps to harvest the plankton. It is mostly the smaller filter feeders, including many types of worms, sponges and bivalves which consume phytoplankton.
- Deposit feeders concentrate on the bacteria-rich slime which coats rocks and muddy shores. This group includes many worms and crabs.
Trophic level 3
This level is comprised of carnivores, which actively hunt down and eat the herbivores from level 2. Many animal groups are included, but this is where the fish come into their own. Fish are a highly diverse group and although some exist in level 2 as grazers, the majority are in the third and higher levels.
If zooplankton have fed on phytoplankton, this puts some filter feeders in the next trophic level. A lot of the larger filter feeders in fact get most of their nutrition from zooplankton. These include whale sharks and quite a few types of whale which feed on a vast scale. In doing this, they skip the energy lost in going through more trophic levels. Food webs are complicated things, and this is just one example of how grouping animals with very different life cycles can cause problems. Filter feeders can belong to trophic levels 2 and 3 (see the infoburst on food chains and energy pyramids).
Trophic level 4
This is another level of carnivores, which hunt down and eat lower-level carnivores and herbivores. They are usually swift, voracious hunters because they have to capture a lot of prey to gain the energy they require.
Trophic level 5
There can be a whole chain of fish preying on one another and lengthening the chain considerably, but at the top of the pyramid is the ultimate predator. In the case of the coral reefs this honour usually belongs to a shark.