Gulf of Mexico Ecosystem Information

Photosynthesis by algae critical to all life in Gulf

Plant life has traditionally been described as being near the bottom of the food chain. "Oh, bottom," many students inferred, "as in least important. And top-the powerful lions and tigers and humans definitely most important."

Well, not exactly. When we powerful carnivores die our bodies are subdued by bacteria, and, through decomposition, become plant food and remain part of the cycle of life.

Organisms at the bottom of the food chain are actually primary producers, which means they capture the sun's energy, making it available to other organisms in the system.

Today, the term food web is used to more accurately describe the complex interdependence among organisms in an ecosystem. The image of a mob removes much of the temptation to rank life forms above and below one another. It also more clearly illustrates that a healthy ecosystem requires balance among the life forms that comprise it.

The Gulf of Mexico

When something disturbs the balance among the life forms that are part of the marine food web, that disturbance can have serious ramifications for the ecosystem.

For example, a sudden decline in phytoplankton would starve the small animals (zooplankton) that feed on phytoplankton. Without enough zooplankton, squid, jellyfish, baleen whales and other animals that feed on zooplankton would be in trouble. Even the scavengers along the sea floor, which might feast for awhile on all of the dying animals, would eventually be affected.

Algae and other floating plants photosynthesize, capturing the sun's energy for subsequent use by other organisms in the food chain. Without algae, other sea life would die.

Algae blooms

Drawing from this information, a reasonable person might conclude that an explosion in the phytoplankton population would be good news for the ecosystem. But phytoplankton overgrowth, also known as an algae bloom, is the beginning of a chain of events that can deprive other sealife of oxygen.

The process occurs as follows:

Nutrients enter the marine system in a variety of ways. In an estuarine area, nutrient levels are high because river water carries nutrients from the river's watershed into the estuary. These nutrients are food for algae.

When nutrient concentrations are very high, the availability of so much food leads to an explosion in algae populations.

The zooplankton that feed on the algae can't eat fast enough to manage the prolific microorganisms. The billions of algae that are not eaten complete their short life cycles, dying and falling to the bottom of the sea, where they decompose.

Decomposition requires oxygen, and with so much organic material decomposing, dissolved oxygen supplies dwindle. When oxygen depletion is severe, sealife, especially benthic organisms like clams and crabs, can suffocate. Indeed, crabs and some other animals at the mercy of low oxygen levels (hypoxia) or lack of oxygen (anoxia) sometimes run or jump right out of the water in an attempt to escape. This is sometimes called a jubilee, not because it's fun for the crabs, but because of the feast in store for the people harvesting the stricken animals along the shoreline.

The article above appeared in a special Fall 1997 issue ofGulfwatch, a cooperative venture of the National Association fo Conservation Districts and the Gulf of Mexico Program. It is published bimonthly by the National Association of Conservation Districts in League City, Tx. Questions or comments or to subscribe should be directed to the editor at 281-332-3402.

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