Published on: 6.6.2022

Information produced by Finnish Environment Institute

Periphyton indicate the status of a water body

Pier posts covered in grey fluff, treacherous shoreline rocks, a slimy fishing net. You can find periphyton on a wide range of underwater surfaces.

Periphyton grab all available substrates: rocks, wooden surfaces, fish traps or even the leaves and stems of aquatic plants. In favourable conditions they spread across the substrate quickly. A fishing net may get slimy in a few days.

Similarly to plankton or aquatic plants, periphytic algae have their natural role in aquatic environments. They may play  an important role in the food web, especially in small and shallow lakes or those with clear waters. Gastropods and many other organisms graze on periphyton.

The species composition of periphyton varies depending on the substrate and environmental conditions. A thin, slimy surface is created when single-cell algae, mostly diatoms, join together to form multi-species mats along surfaces, while larger filamentous algae can be seen swaying to the rhythm of the waves.

As a lake is affected by eutrophication, periphyton growth gathers momentum, and the species composition changes. These changes can be used to assess the ecological status of water bodies. Periphyton are collected from rock surfaces, and diatom species are identified in the samples. Species are given points based on the types of water bodies in which they usually occur. The index derived from these points is used as an indicator of eutrophication.

In a lake affected by eutrophication, periphyton may end up smothering aquatic plants. In the Baltic Sea, filamentous algae mats that have become detached from their substrate and sunk to the bottom have been observed to use up oxygen across large areas of the seabed.

Image: © Jari Kurvinen, Vastavalo