Red Algae If your lake takes on the rich, deep red color of wine in autumn, don’t immediately run to the store for cheese and crackers. What you are witnessing is probably an algae bloom. Planktothrix rubescens (formerly known as Oscillatoria rubescens) is a species of blue-green algae (a.k.a. cyanobacteria) that can have a very distinctive red color. Additionally, the cells can attach end-to-end and may form filaments of several millimeters in length, making them easily visible to the naked eye. During winter this algae may even seep through fissures in the ice, creating colorful “flowers” across the lake’s surface.

For much of the year P. rubescens resides near the metalimnion, that transition area between the warm epilimnion and the colder hypolimnion (see Lake Stratification in this issue). P. rubescens blooms are frequently reported in fall when lake temperatures cool and the water column starts mixing deeper. This change in lake thermal structure doesn’t necessarily promote the bloom of P. rubescens, but rather moves the bloom from the metalimnion to the surface. The colorful algae become apparent once this metalimnion has mixed with the cooling surface layer of the lake.

Like many blue-greens, P. rubescens is capable of regulating its buoyancy, effectively moving itself up and down within the water column. When high in the water column they receive enough light to photosynthesize. During photosynthesis, they produce dense carbohydrates that eventually weigh the cells down, causing them to sink. Once in deeper water (sometimes the hypolimnion) where there isn’t enough light to photosynthesize, P. rubescens will respire, using the carbohydrates for energy, thus lightening their load. A by-product of their respiration is the expansion of their “gas vacuoles”. Eventually, with enough respiration, the gas vacuoles increase buoyancy enough to lift the algal cell back up in the water column.

NEXT

Back to the Water Line

Brought to you by the Lakes of Missouri Volunteer Program