What's behind the algae at Lake Anna


Since the middle of July, the Virginia Department of Health has kept the upper part of Lake Anna, north of the Route 208 bridge, under a Harmful Algae Bloom (HAB) Advisory. This area includes the swimming beaches on the west side of Lake Anna State Park.

Water samples in that part of the lake have shown high levels of cyanobacteria, making the water unsafe for swimming and recreation. The VDH will issue another update on the lake during the week after Labor Day.

How does this happen?



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Algae blooms are coloring the water green in the upper part of Lake Anna. Efforts are underway to manage the blooms.




Algae are simple microscopic plants and an important part of the food web. But blooms like these occur when colonies of algae grow rapidly out of control. Although some blooms occur naturally, their numbers have increased nationwide over the past 40 years. Specifically at Lake Anna, the blooms have been increasing over the past five years.

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Like any organism, algae need food. But when too many nutrients collect and pool in a certain location, a critical threshold is reached, and algae expand quickly and dramatically. In this case, the nutrients are phosphorus, nitrogen and carbon, which are common in fertilizers. Rain washes these nutrients from lawns and farmlands, into tributaries, and ultimately into lakes. Sluggish water circulation can keep the nutrients from dispersing into lower concentrations, allowing the algae to multiply rapidly.



Freshwater Blooms

(Climate Central)


In some ways, this is analogous to thunderstorms on a hot, humid day, when there just needs to be a trigger to get the process going. In the atmosphere, a clear morning sky can erupt into an array of thunderstorms in a matter of hours once a trigger is reached. Algae blooms can expand almost as rapidly.

The conditions that trigger the rapid onset of a bloom are not especially well understood, but there are certain conditions that increase the risk.

Development around the lake and its tributaries is a big contributor, but higher water temperatures contribute to blooms, and heavy rain can bring more pollutants into the waters. As the planet warms, water temperatures climb and rain gets heavier, meaning the bloom risk increases.



CC - Algae Bloom Impacts

(Climate Central)


Strictly speaking, the blue-green algae at Lake Anna are cyanobacteria, but they have the same chlorophyll in their cells as algae, so in effect, that act like algae. These cyanobacteria blooms cause water to become discolored and murky, often with a surface film and a distinct odor.

Most common in summer and early fall, blooms are not just unsightly, but they can produce airborne toxins. In addition to smelling bad and causing breathing problems, they can trigger asthma attacks.

Not surprisingly, swallowing the water or swimming in it also causes problems. Skin irritation is the most obvious external effect, but in high enough doses, ingesting the contaminated water can cause damage to the liver and the neurological system.

One type of algae found in the water, cyanobacteria, can cause skin rashes, stomach illness, vomiting and diarrhea. 

Blooms are also increasing in brackish water and coastal locations, so the economic impacts extend beyond inland recreational lakes. The shellfish from the impacted waters are not safe to eat, so recreational and commercial fisheries suffer. This cascades into losses for the restaurant and tourism industries.

Increased water flow with cooling fall temperatures and more regular breezes will begin to break up the blooms, but reducing the nutrient pollution at the source is the first step in keeping the water cleaner.

Additional efforts are underway to manage the blooms. The Lake Anna Civic Association is working with an outside firm to treat the water in the upper part of the lake. The active ingredient in the treatment is sodium percarbonate, which is also the active ingredient in some powdered detergents.

Preliminary testing of the applications started earlier this summer. Follow the progress at the Lake Anna Civic Association website.

The harmful algae blooms are growing at Lake Anna.

One type of algae found in the water, cyanobacteria, can cause skin rashes, stomach illness, vomiting and diarrhea. 

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