This picture shows a bloom of algae that occurred in Utah Lake, Utah last summer. The lake covers an area of 385km2, and nearly all of it was covered by the bloom. The proliferation of algae was probably caused by warm weather, low water levels and run-off of pollutants from the neighbouring land. Some samples taken from the lake and surrounding waterways contained high levels of cyanobacteria, also known as blue-green algae, which can be toxic.