The most obnoxious forms are buoyant during the day and can form thick surface scums, especially on a calm sunny afternoon. This scum may be blown into shallow water making the shoreline appear slick with blue-green paint. Sometimes surface algae are literally shocked by sunlight and die. As they decompose, the odor can be unpleasant and the oxygen in the water is depleted. If enough algae die and the oxygen in the water is consumed faster than the atmosphere can recharge it, hydrogen sulfide, which smells like rotten eggs and is very toxic to fish, builds up in the bottom water.
Unionized ammonia can also build up to concentrations toxic to fish and othe aquatic organisms.