Still water gives harmful algae the
upper hand
Algae are tiny, often microscopic, water plants that play an
important part in a healthy ecosystem. They are at the bottom
of the food chain and produce most of the oxygen that we breathe.
There are more than 25,000 different species, including brown
diatoms that dominate in the spring, green species that dominate
in summer, and blue-green species that can dominate in stagnant
warm water in late summer.
Like all plants and animals, algae require nutrients,
especially nitrogen and phosphorus, to grow. Moving water allows
the "good" brown and green microscopic algae to outcompete
blue-green algae for nutrients by gliding up and down the water
currents. In stagnant water, brown and green algae sink to the
bottom while blue-green algae adjust their buoyancy to move
between water levels and consume a disproportionate share of
nutrients.
Under these conditions, the blue-green algae explode
into large blooms that can form a slimy mat on the water surface
and give off foul odors. These blooms may contain toxins and
taste- and odor-producing compounds that diminish the water’s
aesthetic value and make the treatment of drinking water difficult
and expensive. Also, as the algae bloom dies and decays, the
bacteria digesting it can use up virtually all of the oxygen
in the water. Without life-supporting oxygen, fish and other
oxygen-consuming organisms either migrate or die off while anaerobic
microbes flourish.
Circulation, the mechanical mixing of a water
body’s layers, mimics natural stream flow that enables
other types of algae to successfully compete and keeps the blue-green
variety in check. As lakes and reservoirs absorb increasing
levels of nutrient-laden runoff from lawns, farms and cities,
circulation has proven itself a valuable tool to preserve and
improve water quality.