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Marine Food Webs

Without microscopic plankton, the marine food chain would collapse!

All organisms in an ecosystem are linked together by their feeding relationship. The sequence of steps that represent the feeding relationships in the ecosystem are the food chains. In a food chain diagram, arrows show the direction of energy flow in the form of nutrients being passed from one organism to another, progressing from the simplest of these to the most complex.

In the marine environment food chains begin with microscopic algae (called phytoplankton). The algae are eaten by tiny sea animals (called zooplankton), which are in turn eaten by small fish, crustaceans, and a variety of other sea animals. They, in turn are eaten by larger carnivores.

Above: The marine food chain

The main source of energy in the marine environment food chains is the sun. As food is passed along the food chain, only about 10% of the energy is transferred to the next level. For example, only 10% of the energy phytoplankton received from the sun can be used by zooplankton at the next level. From one level to the next about 90% of the energy consumed by the previous level is lost. Consequently there have to be a lot more organisms at the lower level than at the upper levels.

Food chains show only simple feeding relationships, and we therefore need to look at food webs if we are to understand ecosystems better. Food webs are made up of all of the different interconnected food chains that exist in the ecosystem.

Right: Example of a marine food web

Photosynthesis in the marine environment

Plants have special adaptations for producing food. One of these adaptations is a chemical called chlorophyll. Chlorophyll gives plants a green colour and enables them to accomplish photosynthesis, the process by which plants turn sunlight, water and carbon dioxide into sugar and oxygen. The sugar is the plants’ food and the oxygen is given off as a waste product.

carbon dioxide + water + sunlight = sugar + oxygen

6CO₂ + 6H₂O + sunlight = C₆H₁₂O₆ + 6O₂

Phytoplankton can only exist in the top layers of the ocean (euphotic zone) since it needs sunlight to accomplish photosynthesis.

Plankton

Plankton is the term used to describe tiny, floating animals and plants that live in the sea. Most are incapable of free swimming and drift around on the ocean currents. There are two main types of plankton: Phytoplankton are the plants and zooplankton are the animals. Both are important sources of food for fish and other animals.

Plankton attains its greatest density in the upper, lighted zone, the phytoplankton is able to do photosynthesis in this area and zooplankton also stay in this area to graze on the phytoplankton.

Phytoplankton comprise an enormous number of diatoms and other microscopic algae. They have a large surface in relation to their mass in order to help them keep afloat in the water column. Many form chains of cells or have large but light weight surfaces. Diatoms, for example, are single celled plants that constitute more than half of the plankton in the ocean. Others have branches or arms. Dinoflagellates have a whip like flagella which they flick to help maintain their position.

Zooplankton includes representatives from virtually every group of animals. Some zooplankton are the larval stages of other marine animals. Fish, crabs, starfish, lobsters and barnacles all exist in the plankton for the first stages of their lives. The most common type of zooplankton are copepods (right); they have long antennae and feet which are used for rapid swimming and the antennae also help with buoyancy. Some marine animals use gas or fluid filled floats; others have developed spines, flagella or mucous nets to help them stay afloat.