Algae covered beach. Aerial view of a beach with green algae blooming on the sand. Many beaches in northern France are regularly affected by huge blooms of algae as a result of the leaching of fertilisers, primarily nitrates, from farms into the rivers. This causes an increase in the concentration of chemical nutrients in the ecosystem (eutrophication), which increases plant growth. As the algae decays, dangerous levels of toxic gases can be released. Photographed in Normandy, France.