Animals can dies from drinking water from ponds and streams during the 80 – 90 degree days of summer. Fertilizers, rain, and hot, windy weather contribute to growth of blue-green algae, which produces the deadly toxin microcystin.
The main sources of the nitrogen and phosphorous (the ingredients of fertilizers) in ponds and streams are leaky septic systems, runoff from adjacent manure stockpiles, and runoff from the lawn fertilizers. Grass clippings raked onto streets and swept into storm water systems are carried to streams and ponds where their stored nitrogen and phosphorous are released.
A day or two of rain to enhance runoff and raise water levels, followed by hot days and gentle breezes rapidly increases new algae growth. Duckweed proliferates rapidly, providing a brilliant green mat on the pond’s surface. Underneath, blue-green algae grow quickly, imparting a brackish color to the water while releasing deadly toxins into the pond. The water remains toxic for several days and then returns to normal with a change to cooler weather. A high dose of microcystin causes neurological impairment to breathing, followed by death in minutes to hours, depending on the amount of poison consumed by the dog or cat. A lower dose of the poison will cause photosensitization and liver necrosis and death in a few days. There is no antidote for this toxin, and treatment by a veterinarian is typically unsuccessful.