Practice question

Phosphorus frequently limits primary productivity in freshwater because photosynthetic organisms require phosphate for ATP, nucleic acids, phospholipids, and many metabolic reactions, yet biologically available phosphate is often scarce. Unlike carbon and nitrogen, phosphorus has no large gaseous reservoir that rapidly replenishes most ecosystems. It enters lakes mainly through rock weathering, soil erosion, sewage, detergents, and fertilizer runoff, and it can become adsorbed to sediments or precipitated with minerals. When the supply of phosphate is below biological demand, adding more light or another nutrient produces little growth, whereas adding phosphorus can stimulate phytoplankton and aquatic plants. Excess input may therefore initiate eutrophication, algal blooms, decomposition, and oxygen depletion. The wording “aquatic systems” is broad: nitrogen often limits marine production, and nitrogen-phosphorus co-limitation is common. Nevertheless, among the substances listed, phosphorus is the standard limiting nutrient emphasized for many lakes and other fresh waters; potassium and sulfur are rarely the principal controls, while inorganic carbon is usually replenished through dissolved carbon and atmospheric exchange.