Plankton are made of the microscopic plants and animals suspended in the water column, such as oceans, seas, or bodies of fresh water. They are the foundation of the food chain in aquatic ecosystems. The plankton also represent one of the most direct and profound responses to pollution. In fact, the degree of eutrophication or nutrient enrichment is often gauged by the amount of plankton growth in an aquatic environment.

Classification

Plankton are primarily divided into broad functional groups:

• Phytoplankton (plant): autotrophic, prokaryotic or eukaryotic algae that live near the water surface where there is sufficient light to support photosynthesis.

• Zooplankton (animal): small protozoans or metazoans that feed on other plankton and telonemia.

• Bacterioplankton: bacteria and archaea, which play an important role in remineralising organic material down the water column.

Environmental Impact and Management

Because of plankton's critical position at the foundation of the food chain, the plankton response to pollution has many ramifications. For example, the increased growth of plankton in response to excessive nutrient additions initiates a chain of events that leads to the adverse symptoms of eutrophication, such as poor water clarity and low dissolved oxygen concentrations and consequently the loss of habitats for living resources.

Because of plankton's fundamental importance to the eutrophication process, limitation of their growth, or production, is often one of the direct targets of management actions. These actions are typically directed at reducing nutrient inputs as a means of limiting plankton growth. The limitation of plankton growth is in turn expected to improve some of the impacts that result from excessive growth. Thus, an assessment of water quality to guide and evaluate management actions logically includes the measurement of plankton communities and their growth rates.