It looks like gunk or slime. It’s thick, green and it is blocking vast swaths of waterway outside of Boston.This scum, filamentous green algae, is nontoxic but takes oxygen away from fish and other wildlife. Experts say it is a situation driven by climate change. "Hotter summers, droughts, and when you have very little flow in the river, that makes the problem worse," said Alison Field-Juma, executive director at OARS, a nonprofit organization dedicated to the Assabet, Sudbury and Concord rivers. "It’s wreaking havoc with nature’s HVAC system. It’s turning up the thermostat, it’s turning off the sprinklers," said Lisa Vernegaard, executive director of Sudbury Valley Trustees.Strong winds from Tropical Storm Isaias pushed the blooms a bit to the side the earlier this week, but environmentalists want to push them out of the way for good. They say that making the change will take a concerted effort from people who live in the towns around the Assabet, Sudbury and Concord rivers. "If collectively we reduce or even stop fertilizing our lawns, that’s not a hard thing to do," Vernegaard said."Every water body needs to have some nutrients because you want it to be able to feed the aquatic life, right? But this has just got way too much," Field-Juma said. "And we will get fish kills if the water temperature is high and the oxygen is depleted by all of this vegetation."Vernegaard and Field-Juma point out that the Clean Water Act has helped to make significant progress in recent years, but they say there's still a long way to go.