In 19 years of piloting his boat around Lake Superior, Jody Estain had never observed the water change as it has this summer. The lake has been unusually balmy and cloudy, with thick mats of algae blanketing the shoreline.

“I have never seen it that warm,” said Mr. Estain, a former Coast Guard member who guides fishing, cave and kayak tours year-round. “Everybody was talking about it.”

But it was not just recreational observers along the shores of the lake who noticed the changes with concern. Lake Superior, the largest of the Great Lakes with more than 2,700 miles of shoreline, is the latest body of water to come under increased scrutiny by scientists after the appearance this summer of the largest mass of green, oozing algae ever detected on the lake.

From the Gulf Coast to the northernmost shores of the United States, scientists and government officials are working to decipher algae blooms to help them interpret the causes of the blooms, changes to their climates, and the effects the blooms have on public health and regional environments.