By Patrick Goodwin, Certified Lake Manager and Aquatic Research Scientist
Nutrient over-enrichment of waters by urban, agricultural, and industrial development has promoted the growth of cyanobacteria as harmful algal blooms (HABs). These blooms increase the turbidity of aquatic ecosystems, smothering aquatic plants and thereby suppressing important invertebrate and fish habitats. The die-off of blooms may deplete oxygen, killing fish. Some cyanobacteria produce toxins, which can cause serious and occasionally fatal human liver, digestive, neurological, and skin diseases. Cyanobacteria blooms threaten aquatic ecosystems including many Florida waterways, lakes and ponds so it’s important to identify the type of algae you are experiencing.
When lakes become stagnant and stratify, cyanobacteria can begin to proliferate causing HABs to occur. Cyanobacteria have a competitive advantage in stratified lakes. Many can move themselves using gas vesicles up and down in the water column allowing them more access to light (photosynthesis). Cyanobacteria out-compete other groups of algae, especially diatoms, which tend to sink more rapidly from their heavy outer shells (called frustules). Breaking stratification by circulating and mixing the water essentially “levels the playing field” for other more desirable groups of algae to become established. Mixing the water column can prevent cyanobacteria from reaching harmful bloom levels.
One commonly used lake management technique to reduce the occurrence of HABs is to pump compressed air to the bottom of a water body, in a process called aeration . Aeration increases water circulation throughout the water column. The circulation prevents stratification (layering of water due to temperature and density differences) from occurring.
The process of circulating water also induces another process called oxygenation (adding oxygen). Circulating or mixing water will increase the contact time of atmospheric oxygen with water, allowing for increased oxygen concentrations throughout the water column. Also, to a lesser extent, oxygenation will occur from exchanges with bubbles as they rise upwards to the surface. Maintaining aerobic (oxygenated) conditions in a lake is the other crucial mechanism for which aeration can reduce HABs. Cyanobacteria evolved to prefer reduced (low oxygen) environments, while green and diatom algae evolved to prefer oxidized (high oxygen) environments. Aeration will alter a lake’s water chemistry to become more oxidized making it less favorable for cyanobacteria to proliferate.
Another important phenomenon that occurs when lakes lose oxygen is the release of nutrients from lake sediments (mainly during periods of stratification). When oxygen becomes absent at the sediment-water interface, phosphorous and nitrogen will begin releasing upwards to the surface waters. These surface nutrients can feed and stimulate HABs. The oxygenation process that is induced by aeration can moderate nutrients being released from lake sediments.
While aeration may seem simple on paper, the process of designing a proper aeration system that will meet your expectations and be cost-effective is often easier said than done. Under-aerating a lake can cause more problems than solve. Undersized aeration systems can bring nutrient-laden and sometimes toxic water upwards, which can further stimulate an HAB and/or cause a fish kill. At the same time, over-aerating a lake is costly, competing for funding with other restoration strategies . Consulting with an experienced aeration vendor can help ensure a proper aeration design.
Patrick earned a Bachelor’s degree from the University of North Florida and a Master of Science in Lake Management from the University of New York, SUNY – Oneonta. As an elite NALMS Certified Lake Manager, he is a lead researcher and a recognized national authority on sustainable lake restoration technology including nutrient reduction, water quality improvement, lake aeration design and cyanobacteria management. He has been on the Florida Lake Management Society Board (FLMS) and a member since 2015. In addition, he is on the Student Committee for the Midwest Aquatic Plant Management Society (MAPMS), as well as nine other prestigious aquatic plant and lake management organizations in the country. Patrick has presented research or conducted training relating to sustainable lake management technology for multiple organizations at their annual conferences.