Identifying Blue-Green Algae


Blue green algae contain single-celled species as well as cells arranged in filaments and colonies. It is extremely difficult to make out the individual filaments, colonies and cells they usually form into clumps. These clumps can be easily seen and will often look like brown dots, greenish bundles or brown or green flakes.

In fresh waters, they’re suspended within the water or attached to rocks and other surfaces.  It’s only when they rapidly increase and form an algal bloom that they are more apparent, to such an extent that they can discolour the water appearing as a variety of greens, blues and sometimes browns.  Sometimes a scum may form on the surface, usually at the water’s edge.


They are natural inhabitants of many inland waters, estuaries and the sea. In fresh waters, they are found in suspension and attached to rocks and other surfaces at the bottom of shallow waterbodies and along the edges of lakes, rivers and canals. 

In natural quantities they make a big contribution to the natural biology of the waterway.  It’s only when  there is an over-abundance of nutrients (nitrates and phosphates), a prolonged hot weather or when waters have been polluted by agricultural, domestic or industrial discharges that the algae multiplies rapidly, creating algal blooms covering the water surface.

Reporting Invasives

You should record the presence of invasive species on a national database. You can record online at the Non-native Species Secretariat website. You should also alert the appropriate land owner, council and or management body to the presence and location of this plant.