BOD is high in a lake when

Presence of large amounts of nutrients in waters also causes excessive growth of planktonic (free-floating) algae, called an algal bloom which imparts a distinct colour to the water bodies. Algal blooms cause deterioration of the water quality and fish mortality. BOD is high and DO is low in such waters. Also, it is not potable (drinkable). 

Caddisflies, water pennies, stoneflies, and mayflies cannot tolerate pollution and live only in clean waters.