Microcystis

Microcystis spp. · Phytoplankton

Under the microscope · Photo: Thomas Gyselinck / CC BY

NativePlanktonic cyanobacteria (in the water)Can produce toxins

Also called Blue green algae, Pond scum

Can produce toxins

Only a lab test shows whether a bloom is toxic. Keep people, pets and livestock out of discolored or scummy water until it is tested.

How to identify Microcystis

Under the microscope: tiny round cells packed by the hundreds or thousands into irregular, jelly coated colonies
Cells look dark or sparkly from gas vesicles, with no filaments and no heterocysts
In the water: green flecks or grains like ground pepper or grass clippings that float to the top when the water is still
Heavy blooms form thick pea soup or spilled paint scums along downwind shores
Rotting scums smell like sewage or septic and can turn bluish white

Similar species

Aphanocapsa also makes jelly colonies but its cells are smaller, pale and loosely spaced; Woronichinia and Snowella colonies are hollow balls with cells around the edge.

Profile

Season	Shows up in early summer, peaks July through September, and can linger into October; colonies overwinter on the bottom sediment.
Starts	About 65 to 70 F and warmer, calm sunny weather
Light	Full sun; floats up to the light with gas vesicles
Depth	Surface and upper few feet; scums on top, resting colonies on the bottom
How it spreads or reproduces	Colony breakup and cell division; overwinters on sediment and rises again in spring
Habitat	Nutrient rich ponds, lakes and reservoirs, especially calm, warm water with high phosphorus; very common in irrigation reservoirs and golf course ponds.

Why it matters in a lake

One of the most common toxic bloom formers in the region and the best known producer of microcystin, a liver toxin that can kill dogs and livestock. Not every bloom is toxic, so only testing shows the toxin level. A Microcystis bloom points to excess phosphorus and warm, stagnant water.