The link between erosion, phosphorus and water quality

Abstract

In lakes, the solubility of phosphorus (P) in water, and therefore its availability to aquatic plants (macrophytes and algae) for growth, is controlled by oxygen and pH. The supply of dissolved reactive P (DRP) to the lake water column is mostly from sediment release from decomposing plant material and iron oxides during periods of low oxygen, although in Lake Rotorua and most of the other lakes on the central volcanic plateau, spring-fed streams carry relatively high concentrations of DRP into the lakes. The mechanism for releasing iron-bound DRP from the sediment focusses around the oxidation state iron (Fe) and manganese (Mn). In well oxygenated conditions iron exists in the oxidised state as ferric ions which form ferric oxides that are insoluble in water. These oxides sequester DRP as they precipitate and remove it from the water column. When the oxygen concentration falls to zero (anoxic conditions) iron exists in the reduced state as ferrous ions, which are soluble in water. As the ferric form reduces to the ferrous form it dissolves and the DRP bound to the iron is released into the water column where it is used by algae for growth. The algae eventually senesce and die, returning the P to the sediment as particulate P, where it can be recycled during the next period of anoxia. Apart from recycling of algal biomass, P also comes from the catchment, bound to the iron oxides in soil particles. The P content of the soil is greatest in the finest soil particles, which are the first to be eroded by rainfall and do not settle until they reach the calm waters of a lake. There they augment the P load from the senescing algal biomass, thereby increasing the amount of DRP that can be released during the next anoxic event. Because of the high background concentration of dissolved inorganic nitrogen in the lake water, the addition of any DRP will stimulate algal growth and thus result in a deterioration of lake water quality. Land management strategies to reduce soil erosion include changes to the way land is farmed and the interception and retention of fine sediment using detention bunds.