It’s been a bad summer for Madison-area lakes. The Yahara River between lakes Mendota and Monona last month became a dead zone after green scum covered the surface. Heavy and frequent rain washed huge amounts of soil, manure and other phosphorus-laden material into the water, triggering the largest bloom of blue-green algae on Lake Mendota since 1994, according to the UW-Madison Center for Limnology. And when the algae dies and decomposes, it reduces oxygen in the water, killing fish.