One way to measure the health of a marine ecosystem is to examine the various food chains present. A food chain indicates the movement of energy through the system. It starts with the producers which are the phytoplankton and the macroalgae. These “plants” convert sunlight to usable sugars through the process of photosynthesis. They are then consumed by herbivores or animals that feed on plants and omnivores which obtain their nutrition from plants and animals. The carnivores feed on other animals and represent the top of the food chain. Each of these stages is called a trophic level and the largest biomass is at the bottom of the food chain. Energy is lost at each stage to heat and metabolic activities such as respiration. In fact only about 10% of the incoming energy is available at each stage.
However the concept of a food chain is misleading. Most organisms feed on more than one source, so it’s more accurate to refer to a food web which describes the interactions between several food chains. And so a food web may consist of dozens of species ranging from the microscopic diatoms to the enormous whales.
Feeding mechanisms among animals vary as well. There are those that consume or graze plants directly. Periwinkles (Littorina littorea) use a structure called a radula to scrape small algae and bacteria off rocks and seaweed. Purple sea urchins (Arbacia punctulata) use their five teeth to grind and scrape seaweeds such as kelp.
Other methods of feeding include the filter feeders such as clams and other bivalves which bring in water through a siphon and small food particles are sorted for consumption. Jellyfish use their tentacles to capture their prey immobilizing them with their stinging cells or nematocysts. Some polychaete worms remove food material from ingested sediments. Sea stars pull a mussel’s shell apart with their strong arms and then move their stomach into the prey for digestion. Many marine crabs have a very diverse diet feeding directly on both live and dead material.
The predators at the top of the food web are fewer in number and biomass. They have a significant ecological role in the food web. Not only do they prevent the overpopulation of smaller species, but they can increase biodiversity by making more available habitat for colonization by others. For example, mussels can form very dense populations making it difficult for other species to live and grow in that area. A group of sea stars will feed on the mussels and open up additional space for bryozoans, anemones, worms, and other mollusks. Having strong healthy links at each trophic level is needed to maintain a sustainable biodiversity.
Copyright Gil Newton 2009 Thanks to Chris Dumas for logo image.