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Frequently Asked Questions

What causes Harmful Algal Blooms?

What causes an algal bloom? - There is no single factor which cause an algal bloom. A combination of optimum factors such as the presence of good nutrients, warm temperatures and lots of light all encourage the natural increase in numbers of blue-green algae in our waterways. Nature mostly takes care of the temperature and light, but the increased presence of nutrients such as phosphorous is largely due to poor farming practices such as high use of fertilizers and presence of livestock near water supplies, as well as effluent and run-off from towns and cities near waterways. The ponding of water and reducing river flow rates tends to improve the light and sometimes the nutrient environment for algal growth making water turbulence a major factor in bloom development. Pesticides and other chemicals may affect the natural grazers which would otherwise control algal growth and their presence increases the risk of blooms.

How do cyanobacterial blooms form? - Cyanobacterial blooms occur when algae that are normally present grow exuberantly. Within a few days, a bloom can cause clear water to become cloudy. The blooms usually float to the surface and can be many inches thick, especially near the shoreline. Cyanobacterial blooms can form in warm, slow-moving waters that are rich in nutrients such as fertilizer runoff or septic tank overflows. Blooms can occur at any time, but most often occur in late summer or early fall. They can occur in marine, estuarine, and fresh waters, but the blooms of greatest concern are the ones that occur in fresh water, such as drinking water reservoirs or recreational waters.

I've heard zebra mussels are causing the blooms. How does that work? - Zebra mussels have been implicates as a factor promoting the formation of harmful algal blooms in the Great Lakes region, particularly for low-phosphorus inland lakes. By removing natural competitors (green algae) and/or altering the chemical composition of the water, zebra mussels may promote HABs. Zebra mussels have been shown to be capable of selecting which algae they consume -- spitting out presumably toxic forms such as Microcystis. By filtering the water, zebra mussels also increase the amount of light reaching the bottom of the lake, which promotes the growth of large benthic forms of algae such as Ciadophora which may break free during storms or due to wave action to form floating mats or wash up on the beaches.

Can you get a blue-green algal bloom in winter? - Yes, however, this is less likely than in summer. Algal blooms can occur at any time of year as long as conditions such as temperature and nutrients are right for growing.