Origins, Impacts and Mitigation of Algal Blooms in Freshwater Lakes

An algal bloom overtaking the western basin of Lake Erie in 2011.  Image: NOAA CoastWatch

In 2011, Lake Erie, one of the five Great Lakes in the United States experienced the largest algal bloom on record (1,920 square miles).  Lake Erie isn’t the only lake under attack-- freshwater lakes in Canada (Lake Winnipeg), Switzerland (Lake Zurich) and China (Lake Taihu) are experiencing increasing nutrient loads which stimulates the growth of cyanobacteria, or blue-green algae.  Algal blooms were the norm in the 1940-1950’s in Europe and the United States until the 1970’s when wastewater treatment plants were installed, which decreased nutrient loads in the water.  In the past two decades however, algal bloom intensity has been on the rise. 

Algal blooms are increasing locally and globally which threaten the quality of water, sport, wildlife, and tourism.  Large blooms have crippled tourism in regions where sport fishing and lake recreation are major industries, and many towns and cities are forced to filter or shut off drinking water.  The multibillion-dollar commercial fishing industry could be hit hard, as well.  Not only is the cyanobacteria visually unappealing to most, the decomposing algae consumes the oxygen in the lake’s deep center each summer, creating a dead zone where deep water fish cannot exist with the lack of oxygen.  Lake Erie fish like perch-- a favorite food of the walleye-- could suffer if hypoxic events continue to expand due to algal growth.  Also, some cyanobacteria produce toxins (hepatotoxins or neurotoxins) that are harmful to humans and other wildlife. 

With uncontrolled growth of algae, studies have been conducted to verify the origins of the nutrients entering the freshwater systems.  The National Science Foundation awarded a five-year grant to a team of researchers, including Anna M. Michalak from the Carnegie Institution for Science, who is the principal investigator of the project.  High-tech tools and sophisticated statistical analysis were used to assess whether the algal bloom in Lake Erie was driven by an unfortunate combination of circumstances or if it is a sign of things to come.  They concluded that trends in agricultural practices, increased intensity of precipitation, weak lake circulation, and calm water conditions acted together to yield the bloom.  With global climate change expected to increase the severity and frequency of precipitation events, algal blooms will most likely increase in intensity and extent in years to come.

Now with the primary causes identified, the most favorable target is to change application of fertilizers in agricultural practices.  Changes in the timing and method of applying agricultural fertilizer are the best options for reducing phosphorus load in freshwater lakes.  These options include: 1.) a shift toward more fall fertilizer applications instead of spring applications; 2.) the use of broadcast fertilizer applications that do not incorporate fertilizer into the soil; and 3.) an increase in no-till field management that leads to a build-up of phosphorus in the top layers of soil. 

With changes in agricultural application of fertilizers, it is hopeful that algal bloom intensity and extent will decrease.  Even with improved fertilizer application, the fate of the algal blooms relies also upon the amount of precipitation, which we cannot control.  If above-average precipitation in the watershed continues this spring in the United States, fertilizer can spread across the rich Midwest farmland in late fall and early winter.  The fertilizers will be carried into nearby tributaries and rivers which empty into Lake Erie and it will fuel higher levels of algal growth. 

To reduce algal bloom occurrence, the Ecological Forecasting Lake Erie project led by researchers at the University of Michigan suggest that phosphorus loads needs to be reduced 78 percent from the 2005-2011 average.  Reducing fertilizer run-off from agricultural land by changing practices is the best option for reducing algal blooms, as we can control this aspect of the problem.  Hopefully in the future, algal blooms will be reduced in size and extent to protect the wildlife, tourism, sport, and water quality from its negative effects.