The one that does not look like seaweed is not leafy. It is not branching. It is not even a plant. It is a slick of green on the water, a scum that looks like someone spilled paint. From a distance, it looks like a patch of green floating on the surface. Up close, it is a film of tiny, single-celled organisms, each one invisible to the naked eye, but together forming a visible layer. It is the organism of the warning sign, the one that tells you something is wrong. What makes it special is the bloom. When conditions are right, the blue-green film multiplies rapidly, forming a dense, visible bloom on the surface of the water. The bloom can cover hectares of lake or estuary, turning the water green and oily. It is a sign that something is wrong, too many nutrients, too much warmth, too little flow. It is the organism of the imbalance, the one that thrives when the system is broken. The blue-green film is toxic. Many species of cyanobacteria produce toxins that can harm animals and humans. The toxins can cause skin rashes, nausea, liver damage, and even death in extreme cases. Dogs are particularly vulnerable. They can die after drinking water from a bloom. The blue-green film is not a friend. It is a warning. Biologically, the blue-green film is a cyanobacterium, one of the oldest life forms on Earth. Cyanobacteria were the first organisms to evolve photosynthesis, more than two billion years ago. They changed the atmosphere, producing the oxygen that made complex life possible. The blue-green film is a living fossil, a reminder of the deep time of life on Earth. To find a blue-green film is to find a sign of imbalance. The green slick on the water is not beautiful. It is not interesting. It is a symptom of too much nutrient pollution, too much warmth, too little movement. It is the organism that reminds us that we are not separate from the environment, that what we put on the land ends up in the water, that the warning signs are there if we choose to see them.