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.