Zooplanktons are planktonic animals that range in size from microscopic rotifers to macroscopic jellyfish. The distribution of zooplankton is governed by salinity, temperature and food availability in the environment. The smallest zooplankton can be characterized as recyclers of water-column nutrients and often are closely tied to measures of nutrient enrichment.

Larger Zooplanktons are important food for forage fish species and larval stages of all fish. They also link the primary producers (phytoplankton) with larger or higher trophic-level organisms. The zooplankton community is composed of both primary consumers, which eat phytoplankton, and secondary consumers, which feed on other zooplankton.

Zooplankton can be classified into three size classes:

  • Microzooplankton–(protozoans and rotifers) are usually less than 200 microns in size.
  • Mesozooplankton-(including copepods and invertebrate larvae) are between 200 microns and 2 millimeters in size.
  • Macrozooplankton-(including amphipods, shrimp, fish larvae and gelatinous zooplankton or jelly fish) are greater than 2 millimeters in size.

Zooplankton, like phytoplankton, makes excellent indicators of environmental conditions within the Bay, because they are sensitive to changes in water quality. They respond to low dissolved oxygen, high nutrient levels, toxic contaminants, poor food quality or abundance and predation. A good picture of the current conditions in the Bay can be derived by looking at zooplankton indicators such as their biomass, abundance and species diversity.


Like land plants, phytoplankton fix carbon through photosynthesis making it available for higher trophic levels. The major environmental factors influencing phytoplankton growth are temperature, light and nutrient availability. Phytoplankton growth is usually limited to the photic zone, or the depth to which sunlight penetrates the water. Other Plankton as Indicator of Water Quality limitations to growth are nutrients such as nitrogen and phosphorous.

When water temperatures rise in the presence of excess nutrients in the water body, the Phytoplankton population can undergo rapid growth or algal blooms. When the phytoplankton numbers exceed the level leading to blooms causes the damage to the organism and the overall health of the water body. When there is competition of space and due to the limited life cycle the phytoplankton will eventually die and sink to the bottom for decomposition.

This process depletes the dissolved oxygen concentrations in the subsurface and bottom of the water body with resultant effect on the organisms present in that water column.

Phytoplankton is being used as indicators of environmental conditions because their populations are especially sensitive to changes in nutrient levels and other water quality conditions. A good picture of the conditions in the ecosystem can be derived by looking at phytoplankton indicators such as chlorophyll, primary production rates, biomass and species composition.

Source: Aquaculture Times


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