Why is lake green after rain?

The more phosphorus present, the more frequent algae occur and the greener the water becomes. Clear lakes with little algae and other organic material are often blue in colour (left), while lakes with high levels of nutrients and algae are green (centre) and lakes surrounded by heavily forested watersheds, wetlands or marshes can be brown in colour (right). In many lakes, algal abundance is determined by the amount of phosphorus dissolved in the water. If your pond or lake is overcrowded, you may find that the water quality deteriorates as excessive amounts of fish waste are excreted.