After the last few weeks of hot days, lack of rain and calm winds, I have had several calls from people who are concerned about scums or slicks on ponds that are a result of algae die off. There are a number of species of algae that naturally occur in ponds, some are harmless, but others, can be toxic to mammals if consumed in high enough concentrations. Problems with blue-green algae occur only under specific environmental conditions and are dose related, that is they are only seen when the animals consume water containing large quantities of organisms. Hot, dry, calm days stimulate reproduction of the algae which, under normal conditions, are not in high enough concentrations to cause a problem. If, however, large numbers of algae start to die, gas is produced inside the cells of the algae, and the colonies tend to float to the pond surface. After this, even a gentle wind will concentrate the organisms downwind to form a “scum” on or just below the pond surface. Animals drinking from this concentrated scum can be sickened and, in some cases, die from ingesting to much of the toxin. Rain or any disturbance of the water tends to break up the scum and make poisoning less likely, but caution should be your watch word if you see “slicks” on the downwind side of your ponds.