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In the last 30 years Lake Winnipeg has experienced a steady surge of blue-green algae growth and although algae grows naturally in the lake, excessive blue-green algae blooms are caused by high ratio levels of nitrogen and phosphorus draining into the lake via rivers and surface runoff. Up to five and a half million people rely on the health of Lake Winnipeg. The lake is an economical powerhouse that supports a $100 million a year tourism industry and a $25 million a year fishing industry. Healthy algae populations play an important role in keeping lake Winnipeg's ecological systems balanced. Green algae provides food for zooplankton, which are then eaten by larger fish in the lake. However the toxins that blue-green algae release can destroy fresh water ecosystems and can be dangerous for humans and other species. Very high levels of the algae toxin microcystin closed Victoria Beach off from the public in the summer of 2003. Grand Beach and other surrounding settlements along the lake are often closed for a short time during summer months due to E. coli and algae-toxin related threats. Immense algae blooms have appeared in the northern part of Lake Winnipeg in the last decade with hundreds of square kilometers of the lake covered with a thick toxic layer of blue-green algae. Local residents in surrounding communities say the blue-green algae is well known for creating deadly water conditions in prairie dugouts and have been known to even kill livestock. Commercial and aboriginal fishermen on the lake often find their nets temporarily disabled during the summer months because of the thick algae conditions. In 2006, Lake Winnipeg's algae blooms was considered to be the worst algae problem of any large freshwater lake in the world, according to "Canadian Geographic".