Phytoplankton â€“ They are the autotrophs or producers of a marine food web. They occupy the base of the trophic level. They are unicellular organisms that float in water. They can prepare their food either by photosynthesis or chemosynthesis.
The upper surface of the ocean that receives light can perform photosynthesis. They trap the solar energy and combine it with inorganic molecules CO2 (from the atmosphere) and H2O and dissolved nutrients. With the help of chlorophyll (the green pigment in plants), it converts them into organic molecules carbohydrates(sugar) and releases O2 into water. Different algae like diatoms, dinoflagellate perform photosynthesis.
Beyond a depth in the ocean, sunlight cannot penetrate. So in this area phytoplanktons perform chemosynthesis. The source of energy is the inorganic chemical compounds, rather than sunlight. They use the energy stored in the chemical bonds of H2S and CH4. This energy is combined with CO2 (dissolved in water)and H2O. It produces carbohydrate and sulfur compounds. Some bacteria and other microorganisms perform chemosynthesis.
Zooplankton and Herbivores â€“ They are the primary consumers or herbivores of a marine food web. They feed on the phytoplankton. They can be small unicellular or large multicellular organisms. Again small zooplanktons can be eaten by the large zooplanktonsâ€”for example â€“ Copepod, Amoeboid or ciliates, Shrimp, etc. Some large herbivores like a mussel, oyster, green sea turtle, parrotfish, surgeonfish, manatees also exist.
Carnivores â€“ The secondary consumers or carnivores feed on the zooplanktons or herbivores. It includes jellyfish, squid, sea star, octopus, small fishes, and even large whales. They can be eaten by some tertiary consumers of marine nature.
Top Predators â€“ The apex of the trophic level is occupied by the predators. Some have no natural predator and are usually large and fast. It includes large fishes, marine mammals, birds, and even human beings.
Trophic levels in Marine food web
All organisms in a food web are grouped into different trophic levels. It is based on their feeding pattern. Different trophic levels are arranged in the form of an ecological pyramid. The base or first trophic level is occupied by the phytoplankton’s. The second trophic level is occupied by the zooplanktons and herbivores and so on. There is a flow of nutrients and energy from one trophic level to another. At each trophic level, some amount of energy is used by the body, and rest is lost in different activities and also as heat energy. So only a portion of the energy is transferred to the next trophic level.
Again the biomass at each trophic level is different. Biomass is the total dry weight of all the organisms at a trophic level. Each successive trophic level has less number of organisms. Hence the biomass is less. This means that the biomass of the phytoplankton is much larger compared to the top predators. However, this is not always the case. In some, the successive trophic level supports a larger number of organisms and has larger biomass.