Green algae. Man holding a piece of sea lettuce (green algae, Ulva sp.) picked up from a beach in Brittany, France. Many beaches in Brittany 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. The algae pose a health hazard as they produce poisonous fumes as they decay.