A young boy plays on a barrier over a canal by the sea front. Waste from city inhabitants and industry is dumped into the river system every day. Excessive Algae growth caused by nitrates and phosphates from sewage and fertilizers draining into canals also contributes to their blocking. An excess of algae clogs up waterways, using up dissolved oxygen as it decomposes and blocking light to deeper waters. This is proven to be harmful to larger aquatic organisms such as fish, inhibiting their respiratory functions.