Solution: In contaminated water, algal blooms can be discovered. Algae overgrowth, particularly blue-green algae, causes them (cyanobacteria). Their expansion pollutes the water supply. They breathe in carbon dioxide and breathe out oxygen.

Red tides are caused by the rapid expansion of red-pigmented dinoflagellates such as Gonaulax, which turns the sea, red. Toxins produced by these algae destroy fish and other aquatic species.