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Read Phosphorus Cycle (p. 79) Nutrient Limitation: Primary Productivity – the rate at which organic matter is created by producers Is controlled by: Limiting nutrient – single nutrient that either is scarce or cycles very slowly, limiting the growth of organisms in an ecosystem Eutrophication: Process whereby bodies of water, such as lakes, estuaries, or slow-moving streams receive excess nutrients that stimulate excessive plant growth (algae, weeds) This enhanced plant growth, often called an algal bloom, reduces dissolved oxygen in the water when dead plant material decomposes and can cause other organisms to die. HOW? Decomposers need oxygen and a food source to live. More food (dead organic material from algal bloom or seaweeds)= More oxygen utilized More oxygen utilized by decomposers means less oxygen for other organisms living in the aquatic system Eventually other organisms (fish, crayfish, insects, other vegetation) die off due to a lack of oxygen Hypoxia – “Hypoxia occurs … when oxygen concentrations fall below the level necessary to sustain most animal life. Hypoxia results when oxygen consumption, primarily through decomposing organic material, exceeds oxygen production through photosynthesis and replenishment from the atmosphere.” - Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, 2004 Figure 1: Hungabee Lake (left), in the Canadian Rockies, is a crystal clear blue lake. In contrast, Lake Taihu (right) in China is considered a highly eutrophic lake; note its bright green color. -http://www.lakescientist.com/learn-about-lakes/waterquality/eutrophication.html