Fertilizers cause algae to grow. Algae are among the oldest living life forms on earth. More than 15,000 species of these multi-celled, complex photosynthesizing organisms live in fresh and salt water, ranging from tiny phytoplankton to enormous strands of seaweed. Intentional, limited applications of fertilizer can cause temporary algal blooms, which feed fish in aquariums, ponds or in commercial aquaculture. Unintentional or uncontrolled fertilizer flow into aquatic environments can cause excessive algal blooms, which lead to oxygen depletion and fish kills.
The nutrients in fertilizer, especially nitrogen and phosphorous, cause algae to grow in much the same way they cause land-based plants to grow. According to the North Dakota Extension Service report on the Unintended Impacts of Fertilizer and Manure Mismanagement on Natural Resources, the addition of even a small amount of an otherwise limiting nutrient into an aquatic system can cause a substantial increase in plant production.
Natural aquatic environments thrive in a fragile balance between nutrient demands and oxygen production. Introduction of fertilizers upsets this balance. A rapid growth, or bloom, of algae due to fertilizer initially releases additional oxygen into the aquatic system during daylight hours when the algae is engaged in photosynthesis. But this, in turn, causes a rapid growth of the organisms that eat the algae, and their oxygen demands can swiftly strip the dissolved oxygen available in the water body. When the algal bloom dies, its decomposition also rapidly consumes oxygen. The algal growth caused by fertilizers can lead to oxygen depletion, fish kills and the eventual death of the aquatic ecosystem.
Algal blooms caused by excess nutrients are an important component of many commercial fish production operations, especially those in the southern United States that raise catfish and other food fish in static ponds, where water is not replaced in the growing pond through the life of the fish. According to the Southern Regional Aquaculture Center, excess fish food, as well as fish waste, is usually taken in as nutrients by algae, which then produces oxygen during the daylight hours that supports the growing fish. If this cycle is disrupted by weather events or a limitation of some nutrient needed and the algae die back, fish farmers add organic fertilizer to re-initiate algal bloom. Without algae growth from fertilizer, commercial static pond fish farming would not be possible.
British researchers published a study in the May 2008 issue of Geothermal Transactions journal indicated that icebergs in the Southern Ocean and melting glaciers on Antarctica are releasing iron particulates into the water. The iron fertilization results in rapid algal growth. This algae then absorbs CO2 from the atmosphere, then sinks to the ocean floor with the CO2 locked away. Dr. Raiswell of the Earth and Biosphere Institute at the University of Leeds posits that this natural fertilizer released from melting glaciers may counter the effects of global warming caused by excess atmospheric CO2.
Writing about the massive fish kills in the Neuse River in 2009, the North Dakota Department of Water Quality notes that most fertilizer in aquatic systems is the unintentional by-product of human activity, including failed septic systems, domestic and farm animal waste, and excessive fertilizer run-off from farms, gardens and lawns. Improved sewage treatment processes and better farm and lawn management are recommended to reduce fertilizer inputs to natural aquatic systems.