What are they?
Blue-green algae are a diverse group of primitive aquatic organisms that are common in lakes, reservoirs, and ponds throughout the world. More correctly, scientists refer to them as cyanobacteria. They have many of the characteristics of other bacteria, but use the same photosynthetic pigment (chlorophyll-a) all advanced plant species have.
Why are they in our waters?
Blue-green algae are a natural component of all aquatic ecosystems. Even in a very pristine setting, it may be common for blue-green algae to be the dominant algal type during the summer months, although they will be present in fairly small numbers.
What is not “natural” are the very large or frequent blooms of blue-green algae that
impair recreational uses or water supply. An algae “bloom” is the name given to the result of rapid population growth, which many species of algae can undergo if conditions are right. Most blue-green algae blooms appear as surface scums, clumps (colonies) floating in the water (which can look like grass clippings or curds of green cottage cheese), or simply a strong green color in the water. Very often, they are also accompanied by foul odors (septic, fishy, or petroleum-like odors are the most common).
Blue-green algae are adapted to a given environment, just as other organisms are. The conditions that most blue-green algae seem to flourish in include standing water (such as a pond or lake), warm water temperatures (as seen in the summer), and nutrient rich conditions (which increase when urban lands and agricultural activities are present within the watershed). The nutrient of most concern in fueling blue-green algae blooms isphosphorus, but nitrogen can be a secondary concern.
Blue-green algae, when these basic growth conditions are met, can be very effective in out-competing other types of algae. Many species can control their buoyancy, which allows them to “shade out” other types of algae, preventing them from getting enough sunlight for photosynthesis. This adaptation is what allows blue-green algae to readily form surface covering blooms, which can then concentrate along the windward shores of lakes.