Phytoplankton and Periphyton Community Analyses

The phytoplankton community consists of free-living algae which are suspended in the water column. It is most important in standing water systems such as lakes and ponds. Because algae are primary producers and form the base of the food web, higher organisms depend on them for food and for the oxygen released during the process of photosynthesis. These tiny plants have an extremely high rate of reproduction; therefore, in the presence of sufficient light and ample nutrients, their populations can explode into blooms which can contribute to oxygen depletion, fish kills, and aesthetic problems that can interfere with recreational use. Problems in the phytoplankton populations are often the first indicators of anthropogenic stress on a system.

The periphyton community is made up of algae inhabiting the surfaces of underwater vegetation, rocks, and other substrates. Due to the sedentary nature of periphyton, the community composition, structure, and biomass are sensitive to changes in water quality and are often used as indicators of ambient conditions.

Changes in the phytoplankton (free-water) and periphyton (attached) algal communities can be particularly useful as assessment tools, due to their rapid response to environmental stress. Studies of algal, combined with macroinvertebrate communities, provide a valuable assessment of the overall health of aquatic systems.

Measures of Algal Community Health

Several different measurements of algal community health are routinely employed to determine whether populations are disturbed. These are:

• Taxa richness: the number of different types of organisms present in a system.
• Shannon-Weaver diversity: an index that measures the distribution of organisms present. Low diversities represent conditions where only a few organisms are abundant, to the exclusion of other taxa.
• Numbers of pollution sensitive taxa: certain taxa are labeled as sensitive or tolerant to pollution, based on the literature.
• Community structure: measurements of shifts in proportions of major groups of organisms, compared to reference conditions.
• Algal biomass: the amount of algal growth a body of water can support, measured as algal density or chlorophyll a.
• Habitat Assessment: quality of the local environment with respect to the needs of the organisms investigated.

The Standard Operating Procedures used in the collection and analysis of periphyton and phytoplankton samples can be viewed or downloaded from the Biology Section SOPs.