부산대학교 도서관

Availability of iron in an aquatic system is governed by the concentrations present within the catchment and prevailing environmental conditions. Iron is characterised by low solubility at circumneutral pH. Conversion of ferric iron, into soluble ferrous iron, requires a reduction in pH and redox potential. Research to date has tended to focus upon acid mine drainage, where anthropogenic activity initiates extreme acidity and the liberation of mobile iron. In instances of naturally occurring deposition, the acidity is derived from ecological processes, such as organic decay, respiration and natural acids. Once in the soluble form, iron is vulnerable to rainfall induced transport into receiving waterbodies. Precipitation of iron, from the water onto the streambed, takes place when pH increases downstream and conditions become increasingly oxidised. Interaction between iron-water and deposit concentrations was explored across the Antrim Plateau and Sperrin Mountains, in Northern Ireland. Basalt and brown earths dominate the Antrim Plateau, whereas the Sperrin Mountain soils are composed of peat underlain by schist. Significant relationships existed between water and deposit concentrations across both localities. Concentrations of deposit iron were consistently less in the Antrim Plateau, due to lower levels of iron in the water, as a consequence of variance in soil and rock type. In the Sperrin Mountains, eight streams were sampled monthly for one year, over a range of high to low iron sites. Seasonal variation in rainfall influenced iron concentrations in the water. The solubility of iron within streams in the Sperrin Mountains is predominantly controlled by pH. Dissolved oxygen was less influential owing to the hydrodynamics of upland stream networks, which are generally fast flowing and well oxygenated. The occurrence of iron deposition is therefore dependent upon the concentration of iron in the water and the pH of the waterbody. Composition of deposit material present on stone and tile substrates was analysed. Metal concentrations increased with increasing deposit density. A strong association also existed between deposit iron and organic matter concentration. Chlorophyll a concentrations increased with increasing deposit density, up to a critical point, after which they began to decline as deposit density continued to rise. Autotrophic index values behaved negatively with chlorophyll a concentration and positively with deposit organic matter and iron. The density of deposit material present within streams has a strong influence upon both the abundance of algae and invertebrate community structure. Deposit iron concentrations in excess of 1.0 mg cm-2 had a deleterious effect upon chlorophyll a concentration. Invertebrate numbers were also detrimentally affected by elevated concentrations of iron. Species composition was principally controlled by the density of deposit iron, which influenced algal abundance and controlled community structure in relation to feeding habit. The accumulation of deposit iron alters stream ecosystem functioning and facilitates the transition from autotrophy to heterotrophy. Key attributes of stream ecology are, therefore, altered by iron deposition as a result of food web simplification.