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LAKE HOPATCONG , New Jersey — Around Lake Hopatcong, New Jersey’s largest lake, workers have been laid off, sailing lessons canceled and summers ruined. The reason: clouds of blue-green algae in the water, blooming in quantities never before recorded .

State warnings that the water is unsafe — which began in June and remain in effect for all but one small branch of the lake — have come during a summer of unusually intense algae blooms in many parts of the country. Fueled by heavy rains and hot, sunny days, the blooms have caused high-season swimming bans from lakes in the Pacific Northwest to the entire Mississippi seacoast.

Climate change is a likely factor, scientists say, in an increase in blooms of cyanobacteria — single-cell organisms that, when they grow densely, can produce toxic substances.

More frequent, more intense rainstorms that drive nutrients like sewage and fertilizer into waterways — coupled with more hot days to warm the water — create ideal conditions for the blooms, which in recent years have appeared in more places, earlier in the summer.