Diatoms

Diatoms as indicators of water quality

Diatom growth on a stone in the river bed

Live specimens of diatoms (with cell content)

Preserved specimens of diatom shells (without cell content) Photos: AWEL, Canton ZH

Diatoms are single-celled algae with a two-part siliceous cell wall, which is rigid and chemically stable. Microscopic in size, diatoms are identified by species-specific shell characteristics. Thousands of different species are found worldwide, in both fresh and salt water and even at dry sites.

Diatoms are well suited to serve as bioindicators, as they occur throughout the year in flowing waters, their response to changes in environmental factors is well known, and reliable results can be obtained with the aid of routine methods. In a waterbody, the composition of diatom species depends largely on water quality, while the role of ecomorphological factors is much less significant than for aquatic invertebrates.

The assessment elaborated as part of the Modular Stepwise Procedure is based on a comparison of data obtained from surveys of diatoms in Swiss watercourses between 1985 and 1999 with findings for chemical samples from the same period.
The evaluation of chemical parameters provides information on levels of pollution at the sites investigated. The same type of assessment is to be yielded by studies of diatom growth at the same sites.

The new Swiss diatom index is intended to make it possible in the future to draw conclusions concerning chemical pollution of waterbodies on the basis of diatom growth. The composition of diatom species in a given stretch of water changes in response to chemical pollution, with certain species disappearing and being replaced by less sensitive ones. Studies of the frequency of diatom species provide evidence of pollution levels in the days and weeks prior to sampling.

The Diatoms module is currently being piloted in cantonal studies.

Draft version module "Diatoms Level I"