There is not an exact water temperature where the bluegreens will crash. Temperature thresholds tend to be species, variety or even strain specific. It even gets more compicated and sometimes other things besides temperature cause densities to decrease or even crash. Some species have narrow tolerances whereas others have wide ranges of tolerance. Nothing is usually real simple in natural systems.
Generally cooler waters than approx 78F-82F, will start slowing their metabolism and then other species are better able to compete. Nutrient balances or ratios and other environmental factors also play a big part in growth cycles.
