Freshwater algae include a wide range of organisms that float or are attached to submerged surfaces and have the ability to photosynthesize. They include members of a number of different groups including green algae, cyanobacteria (blue-green algae) and diatoms. There are over 5000 recorded species of freshwater and terrestrial algae (excluding cyanobacteria and diatoms) in the British Isles which exist in a wide range of habitats where there is sufficient moisture. They are morphologically diverse and exist as single cells, multicellular filaments or colonies and can be attached to plants, animals or various surfaces (periphyton) or free‐floating (phytoplankton) or be part of the benthos (flora and fauna found in the bottom sediments).
Freshwater algae belong to numerous evolutionary lineages and the flora of any aquatic system has many species from some or all of these lineages. Consequently their taxonomy is complex and, in the light of biochemical and molecular biological research, subject to change.
To simplify the organisation of this site, the cyanobacteria, desmids and diatoms have been placed in separate sections, although they occur in freshwater.
All images are by Chris Johnson, unless annotated otherwise.