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Background information

Recognising cyanobacterial blooms

The term „cyanobacterial bloom“ refers to the rapid and massive proliferation of planktic cyanobacteria in a water body. The main reason for a cyanobacterial bloom is usually an excessive fertilization, especially with phosphorus, of a water body. A bloom can lead to an olive-greenish, blue-greenish or reddish discoloration of the water, and eventually streaks or scums may appear at the water surface. In case of the development of Planktothrix rubescens, however, the water surface may be clear, since in summer this species usually inhabits the metalimnion (i.e. the zone between the warm surface water and the deeper colder water), and appears only in autumn at the surface with scum formation under suitable conditions.

Streaks at the water surface during a mass development of MicrocystisSurface scum of Microcystis aeruginosaDense bloom of Microcystis at the shoreline. Bathing can be hazardous! (© Martens) Also for animals cyanobacteria can be hazardous!Surface bloom of Planktothrix rubescens (© ARPA, Sicilia)Cyanobacteria appear truly bluish primarily when cells are lysing

Some cyanobacteria form aggregates or colonies visible to the naked eye. Microcystis can form irregular spherical colonies, while aggregates of Aphanizomenon flos-aquae look like grass cuttings or larch needles.

Microcystis bloom with visible coloniesVisible colonies of Microcystis aeruginosaBloom of Aphanizomenon flos-aquae with visible colonies (© K. Teubner)Visible colonies of Aphanizomenon flos-aquae (© K. Teubner)

In case of the over-fertilization of a water body other water plants can also grow. The duck weed (Lemna minor) can, similar to cyanobacteria, cover the whole surface of a water body and may be misidentified as a cyanobacterial bloom at first glance. However, L. minor is a true plant with roots and leafs (see picture below) and is thus easy to distinguish from cyanobacteria.

Mass development of Common Duckweed (Lemna minor)Common Duckweed (Lemna minor)

An increased turbidity of a water body, however, may also be of mineral origin caused by sediment particles suspended after e.g. a storm. A thorough investigation, including microscopy, can readily indicate whether the increased turbidity is due to sediment or cyanobacteria.