The scientist has a water sample from an unknown location in the lake. When bromothymol blue is added, the sample becomes blue. From where did the sample most likely come?

The aqueous bromothymol blue solution act as a pH indicator and turns blue only in case when the pH of the sample is above or equal to 7.6

Since the pH of the water at the surface of the lakes is higher and ranges between 7.5 to 8.5 the color of bromothymol turn blue.