Phosphorus and aquatic plants

Aquatic systems receive the bulk of their nutrient supply from stream inflow. In stream communities, and also in lakes with a stream outflow, the export of nutrients in outgoing stream water is a major factor in nutrient budgets of aquatic communities. By contrast, in lakes without an outflow, nutrient accumulation in permanent sediments is often the major export pathway. Only a small fraction of available nutrients is incorporated into the biological interactions of stream communities (Winterbourn and Townsend 1991). In streams and rivers, the majority of nutrients flow on, as particles or dissolved in the water, to be discharged into a lake or the sea. Nevertheless, some nutrients do cycle from inorganic forms in freshwater, to inorganic forms in animals or plants, to inorganic forms in water, and so on. Because of the transport downstream, the displacement of nutrients may be best represented as a spiral (Elwood et al. 1983), where rapid phases of inorganic nutrient displacement alternate with periods when the nutrients are locked in biomass (e.g. in aquatic plants). Aquatic plants may obtain nitrogen (N) and phosphorus (P) from the sediment and then release these elements into the water. These plants function as a source for nutrients, by trapping fine organic and inorganic particles, enhancing mineralization of organic matter through oxidation of the sediments, and altering the localized environment, thus enabling P release through reducing conditions and increased pH and temperature. Oxygen translocation to the roots of plants has the effect of oxidizing the immediate sediment environment, and this may limit P availability (Moore et al. 1994; Wigand et al. 1997). Aquatic plants can also have a significant impact on a system’s light environment and nutrient budget (Reckhow and Chapra 1999).

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