Late summer is the time when people really visit lakes. The chilly waters of early summer have warmed to the perfect temperature for diving in on a hot humid day. Swimming in the cool waters of a lake is one of the great pleasures of August in Vermont. Late summer is also often the time when cyanobacteria are most noticeable on our northern lakes.
Cyanobacteria can be found year-round in the northern climates; however, they have evolved several unique strategies that allow them to thrive in the water conditions of late summer. At this point in the year, the nutrients raised from the bottom of the lake during spring turnover have been used up. There is typically less wind and many lakes are fully stratified. Finally, the water itself can be quite warm, particularly at the surface where the sun is most intense. Other algae find these conditions difficult to grow in and may disappear while cyanobacteria growth increases. Let’s take a look at some of the ways cyanobacteria have evolved to thrive in late summer waters.
In general, the small cells of cyanobacteria have more surface area to volume than the other algae, which allows more opportunity overall for nutrient uptake. Some cyanobacteria are able to collect and store phosphorus (P) in their cells to use a later time, a process called luxury uptake. Others have special enzymes that allow them to break down and use forms of tightly bound organic P that other algae cannot. Nitrogen fixation is an important adaptation that allows some cyanobacteria to use atmospheric nitrogen (N) to support their growth. Some species can regulate their buoyancy through the use of gas vesicles and drop down to the sediments to obtain scarce N and P. All of these can help cyanobacteria find nutrients when other species find it more difficult.
There is typically less wind in summer and phytoplankton have to work hard to stay near the surface where they can get light. Cyanobacteria do not have flagella or cilia to help them stay afloat. In addition to gas vacuoles used by some cyanobacteria to actively maintain their position near the surface, they may form large flat colonies that don’t sink easily or collect into bundles of filaments that trap oxygen released by photosynthesis to help them float. Filamentous forms living in soft sediment have the ability to move and avoid burial as sediment shifts.
Overall, cyanobacteria are more tolerant of warmer waters. They thrive in hot springs and volcanic areas around the world. Summer surface water temperatures are pretty warm even in Vermont but easily tolerated by cyanobacteria. Temperatures are yet higher in the scums that develop as abundant cyanobacteria gather at the surface and soak up sunshine. Research indicates that cyanobacteria are especially adapted for this with special compounds that protect them against the harsh conditions that develop as the sun cooks all that biomass.
Lastly, cyanobacteria have adapted to life under a variety of light regimes. Like all phytoplankton, they have chlorophyll a. They also have phycocyanin, which gives them their characteristic bluish coloration, phycoerythrin, and chlorophyll f. This set of pigments allows them to capture light not only at the surface, but deep in the water column or when piled into a surface scum. New research into chlorophyll f suggests that it enables photosynthesis in the ultralow light conditions found inside benthic cyanobacteria mats and lake scums.
Cyanobacteria can produce many different potent toxins harmful to people and animals called cyanotoxins. We don’t fully understand why these compounds are produced and cyanobacteria don’t always produce them. In general, they remain inside the cell until it is damaged or decays. Cyanotoxins are thought to discourage predation by grazers, help scavenge iron from the water for use in nitrogen fixation, help cyanobacteria communicate with one another, or protect them from the harsh conditions at the water surface. Understanding cyanotoxin production is an active area of research.
There are lots of resources to help you learn more about cyanobacteria and how to recognize when they may be cause for concern. An online document compiled by the Interstate Technology and Regulatory Council (ITRC) summarizes our current understanding of these complex organisms. A companion Visual Guide to Common Cyanobacteria shares images of planktonic cyanobacteria as they appear on lakes and ponds as well as under the microscope. A second document focused on benthic and attached cyanobacteria is scheduled for release in Spring 2022.