Extreme Algal Bloom
An algal bloom is a rapid increase or accumulation in the population of algae (typically microscopic) in an aquatic system. Algal blooms may occur in freshwater as well as marine environments. Typically, only one or a small number of phytoplankton species are involved, and some blooms may be recognized by discoloration of the water resulting from the high density of pigmented cells. Algal bloom concentrations may reach millions of cells per milliliter. Algal blooms are often green, but they can also be other colors such as yellow-brown or red, depending on the species of algae. A 2011 record-breaking algae bloom in Lake Erie was triggered by long-term agricultural practices coupled with extreme precipitation, followed by weak lake circulation and warm temperatures, scientists have discovered. The Carnegie researchers also predict that, unless agricultural policies change, the lake will continue to experience extreme blooms.
There was a major algal bloom in Lake Erie in 2011. This was the result of several factors such as excess nutrient availability and contributing weather factors.
"The factors that led to this explosion of algal blooms are all related to humans and our interaction with the environment," says Bruce Hamilton, program director at the National Science Foundation (NSF), which funded the research through its Water, Sustainability and Climate (WSC) Program.
"Population growth, changes in agricultural practices and climate change are all part of the equation," says Hamilton. "These findings show us where we need to focus our attention in the future."
Results of the research are published in this week's online early edition of the journal Proceedings of the National Academy of Sciences.
"The perfect storm of weather events and agricultural practices that occurred in 2011 is unfortunately consistent with ongoing trends," says Anna Michalak, the paper's lead author and a scientist at the Carnegie Institution for Science's Department of Global Ecology, located at Stanford University.
"That means that more huge algal blooms can be expected in the future, unless a scientifically-guided management plan is implemented for the region."
Freshwater algal blooms are the result of an excess of nutrients, particularly some phosphates. The excess of nutrients may originate from fertilizers that are applied to land for agricultural or recreational purposes. They may also originate from household cleaning products containing phosphorus. These nutrients can then enter watersheds through water runoff. Excess carbon and nitrogen have also been suspected as causes.
When the plants and algae die, decomposers in the water that feed on them use up oxygen, which can drop to levels too low for aquatic life to thrive.
At first the Lake Erie algae were almost entirely Microcystis, an organism that produces a liver toxin and can cause skin irritation.
The scientists combined sampling and satellite-based observations of the lake with computer simulations to track the bloom.
It began in the lake's Western region in mid-July and covered an area of 230 square miles.
At its peak in October, the bloom had expanded to more than 1,930 square miles. Its peak intensity was more than three times greater than any other bloom on record.
They found that three agriculture management practices in the area can lead to increased nutrient runoff: autumn fertilization, broadcast fertilization (uniform distribution of fertilizer over the whole cropped field), and reduced tillage.
Conditions in the fall of 2010 were ideal for harvesting and preparing fields and increasing fertilizer application for spring planting.
A series of strong storms the following spring caused large amounts of phosphorus to flow into the lake.
This onslaught resulted in one of the largest spring phosphorus loads since 1975, when intensive monitoring began.
Lake Erie was not unusually calm and warm before the bloom. But after the bloom began, warmer water and weaker currents encouraged a more productive bloom than in prior years.
The researchers' data did not support the idea that land-use and crop choices contributed to the increase in nutrient run-off that fueled the bloom.
They found that severe storms become more likely in the future, with a 50 percent increase in the frequency of precipitation events of .80 inch or more of rain.
Stronger storms, with greater than 1.2 inch of rain, could be twice as frequent.
The researchers believe that future calm conditions with weak lake circulation after a bloom's onset are also likely to continue, since current trends show decreasing wind speeds across the United States.
That would result in longer-lasting blooms and decreased mixing in the water column.
For further information see Algal Bloom.
Fish in Bloom image by Tom Archer via National Science Foundation.