Agardh's blue-green alga
Planktothrix agardhii
Planktothrix agardhii is a filamentous cyanobacterium belonging to the order Oscillatoriales. It forms long, unbranched trichomes and lacks specialized cells such as heterocysts or akinetes. The species is well-known for forming persistent water blooms in shallow, eutrophic to hypertrophic lakes. Due to its adaptation to low light intensities, it can dominate even in highly turbid waters.
Details
Oxygen production
Produces oxygen during the light phase; can lead to oxygen depletion at night or when the bloom dies off.
Habitat function
Serves as the basis of the food web in moderate densities, but acts as a habitat disruptor and is harmful during mass developments.
Nutrient uptake
Very efficient uptake of phosphate and ammonium, even at low concentrations due to high affinity.
Food source for
Partially zooplankton and herbivorous fish, although usability is limited by toxicity.
Human use
No positive use; considered a problem species for drinking water treatment and bathing water quality.
Ecology
Ecological role
Primary producer; however, it can harm zooplankton and disrupt the food chain through the production of hepatotoxins (microcystins).
Natural predators
Certain types of zooplankton (e.g., Daphnia, though they are often inhibited by toxins) and specialized protozoa or viruses (cyanophages).
Competitor species
Other cyanobacteria (e.g., Microcystis spp.) and green algae competing for light and nutrients.
Ecosystem service
Oxygen production through photosynthesis, though often offset by subsequent depletion processes during mass die-offs.
Threats
Improvement of water quality (oligotrophication) and reduction of phosphorus input.
Scientific profile
Profile
Distinguishing features
Possesses aerotopes (gas vacuoles) for buoyancy regulation. Lacks heterocysts and akinetes (resting cells). Apical cells often tapered or equipped with a calyptra.
Reproduction
Asexual by fragmentation of trichomes into shorter segments (hormogonia).
Protection & threats
Status not on standard scale
Main threats
Nutrient reduction (oligotrophication) and increase in water transparency.
Conservation measures
No conservation measures required; management usually aims at reducing the species (lake restoration).