Window on the Lake: What is it about blue-green algae?
By Jim Steele ?
Blue-green algae are not plants but instead a type of bacteria called cyanobacteria, which can photosynthesize their own food from sunlight.
Many (there are thousands of species) have another characteristic that can make them particularly successful; they can obtain nitrogen from air.
Plants and other algae cannot use nitrogen directly from the air but must depend on “nitrogen fixing” bacteria located in soil or roots (e.g. legumes) or high nitrogen levels naturally in rain.
Low available nitrogen can often limit algae and aquatic plants but not the specialized blue-greens such as our recent visitor, lyngbya.
Many blue-greens have specialized sheaths that make it hard for animals to digest. The sheath is often sticky and can foul filtering and digestive systems. So, zooplankton and other feeders of algae will select against this kind of blue-green.
This provides them a competitive advantage to other algae as the summer wears on. The zebra and quagga mussels for example will filter feed on microscopic food but bypass sticky sheathed blue-greens. Expect more blue-green algae blooms if we get these mussels.
The sheath can contain buoyant gas vesicles allowing some species to float toward the surface to increase sunlight availability.
This is important when turbidity prevents light penetration for other species.
Blue-green algae are noted by their ability to exist in harsh conditions such as in hot springs, thermal vents and high acid conditions and are found most everywhere else as well. When conditions are both highly variable and extreme, blue-greens are the noted performer. Along with these survival skills comes some unpleasant attributes, such as foul odors and water taste. Our water agencies drawing lake water must deal with this during the summer.
There are a variety of toxins attributed to blue-greens that can cause health issues, but only below-harmful levels have been detected in Clear Lake. In the worst case, nervous system and liver toxicities have been reported at severely infested lakes elsewhere. This is something to keep in mind as water bodies have been closed during blooms to protect the public.
Most lakes are low in phosphorus. The one nutrient that keeps blue-greens in check is the high requirement for phosphorus compared to other aquatic plants.
When a lake”s phosphorus levels are increased through erosion, septic systems, fertilizers and street runoff, a blue-green bloom is more likely.
Water routed through wetlands and lake-margin plants settle sediments and use phosphorus before it enters the lake.
But other limiting nutrients can also play a roll such as iron needed by both plants and algae.
Blue-greens require less iron than other plants and do well when others cannot. The iron cycle in the lake can play an important part but needs more study.
Clear Lake water is becoming clearer compared to the past and a scenario might be suggested to explain the recent robust blue-green outbreaks in the summer.
As efforts to curb high nitrogen inputs are becoming effective such as routing waste water to The Geysers, the phosphorus inputs from land runoff are steady.
The high populations of algae using available nitrogen begin to wane in the warm late summer and the nitrogen fixing blue-green have less competition for the remaining phosphorus and really begin to increase.
They can float to the surface increasingly blocking their competitors from light until they dominate the lake.
Efforts to curb phosphorus will be very effective, but is not the total story. A process that stores nutrients and recycles them called “eutrophication” will limit the effect of reducing run-off and needs to be considered. Next-up eutrification.
Jim Steele is a retired Cal Fish and Game scientist, registered professional forester, part-time consultant and full-time Lake County resident-volunteer.
