Watersheds acidity

Consider a lake with a smaU watershed in a forest ecosystem. The forest and vegetation can be considered as an acid concentrator. SO2, NO2, and acid aerosol are deposited on vegetation surfaces during dry periods and rainfalls they are washed to the soil floor by low-pH rainwater. Much of the acidity is neutralized by dissolving and mobilizing minerals in the soil. Aluminum, calcium, magnesium, sodium, and potassium are leached from the soil into surface waters. The ability of soils to tolerate acidic deposition is very dependent on the alkalinity of the soil. The soil structure in the... 

In 1968, reports from Sweden, subsequently confirmed in other industrial countries, noted that shallow lakes with low concentrations of divalent cations were becoming more acidic with consequent decreases in aquatic plants and animals. In severely affected lakes and ponds, only acidophilic algae survived. Increased acidity and the runoff of solubilized aluminum and other metal ions from surrounding watersheds are now known to be primarily responsible for formation of these almost sterile bodies of water. 

Schwesig D, Ilgen G, Matzner E. 1999. Mercury and methylmercury in upland and wetland acid forest soils of a watershed in NE-Bavaria, Germany. Water Air Soil Pollut 113 141-154. 

Acidification of acid-sensitive waters is accompanied by severe changes in biological communities. Effects range from reductions in diversity without changes in total biomass to elimination of all organisms. In many cases the immediate cause of the changes is unknown. Some effects are the result of H" toxicity itself or of the toxicity of metals mobilized from the watershed, others have more indirect causes such as changes in predator-prey interactions or in physical conditions of lakes (ex. transparency). [14]... 

Base cations are mobilized by weathering and cation exchange reactions that neutralize acids in the watershed. They respond therefore indirectly to changes in sulphate and nitrate concentrations. In fact, if acid anion concentrations (mainly sulphate) decrease, base cations are also expected to decrease. However, in the last few years an increase of the occurrence of alkaline rain episodes (probably due to climatic effects) has been observed and it is likely that calcareous Saharan dust, rich in base cations, is responsible for it [27]. Accelerated weathering, resulting from recent climate warming may also contribute to higher base cation concentrations [28]. 

Karathanasis, A.D. Evangelou,V.P. Thomp-son,Y.L. (1988) Aluminum and iron equilibria in soil solutions and surface waters of acid mine watersheds. J. Environ. Qual. 17 534-543... 

Stanton, M. 1973. The role of weathering in trace metal distributions in subsurface samples from the Mayday Mine Dump near Silverton, Colorado. In Church, S. E. (ed) The USGS Preliminary Release of Scientific Reports on the Acidic Drainage in the Animas River watershed San Juan County, Colorado, US Geological Survey, Denver, CO, USA, 77-85. 

Nitrification. Nitrification, the oxidation of NH4+ to N03 , is mediated by bacteria and fungi in both the terrestrial and aquatic portions of watersheds. It is an important process in controlling the form of N released to surface waters by watersheds, as well as in controlling the acid-base status of surface waters (Figure 1). Nitrification is a strongly acidifying process, producing 2 moles of H for each mole of N (NH4+) nitrified. 

Eutrophication. Thus far N has been discussed in terms of its prominence as an acidic anion (i.e., as N03 ). As in terrestrial ecosystems, inorganic forms of N also act as nutrients in aquatic systems, and a possible consequence of chronic N loss from watersheds is the fertilization of lakes and streams. Establishing a link between N deposition and the eutrophication of aquatic systems depends on a determination that the productivity of the system is limited by N availability and that N deposition is a major source of N to the system. In many cases the supply of N from deposition is minor when compared to other anthropogenic sources, such as pollution from either point or nonpoint sources. 

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