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Fjords provide the unique opportunity to look at historical sedimentation, including high resolution records of human induced environmental change. For this purpose sediment cores are taken in 2007 from the inner part of the marine fjord system of the Sogndalsfjord (140km distance to the coast), Western Norway, consisting of the inner fjord system, i.e. the anoxic (66-80m deep) Barsnesfjord, which connects 5km to the SW with the outer fjord system, i.e. the oxic (260m deep) Sogndalsfjord. Both are tributaries of the >1000m deep Sognefjord. The sediments are correlated with dated cores taken in 1988 from the same area. Although located closely together, both fjords show distinct differences in the sedimentation of signals from environmental change throughout the last 25 years.
The sediment record of the anoxic Barsnesfjord reveals a significant decrease in the number of the high productivity diatoms Sceletonema costatum and Chaetoceros sp., and the freshwater diatom Tabellaria flocculosa. An increase is observed in terrestrial organic material concentrations. The amount of mineral matter decreases. The inner fjord system is located close to the main river inlet, the Årøy-River, which receives its water from the Lake Hafslovatnet, 169m a.s.l. The timing of the sediment change coincides with the building of a hydro power plant in 1983 when the surface water runoff was moved from the Årøy-River and funnelled into a water tunnel. Since then water intake occurs at 10m water depth in the Lake Hafslovatnet. The water enters the lower Årøy-River about 1km N of its inlet to the Barsnesfjord.
No sign of the hydro power impact is seen in the outer fjord system, i.e. the oxic Sogndalsfjord, which is separated from the inner fjord system by a 7.5m deep sill. Sediments are mostly influenced by material supplied from the unchanged water course of the Sogndalselv-River. In addition, the treatment of local sewage seems to have left traces in the sediment. The diatoms Sceletonema costatum and Chaetoceros sp. behave antagonistic, while the number Tabellaria flocculosa increases. Terrestrial organic material concentration increases while the amount of mineral matter remains stable. Hydrographical measurements of water temperature, salinity and current activity confirm interpretations regarding the sewage supply.
Three main conclusions can be drawn from this investigation: (a) The primary concentration of nutrients in waters of inner fjord basins depends mostly on the supply of mineral nutrients from land. The effect of marine nutrients seems to be minor. (b) The sediment signals can thus be used to interpret directly the influence of environmental change on the terrestrial system including historical glacier variation as e.g. from the Little Ice Age. (c) The interpretation of sediments from inner marine fjord basins is heavily influenced by regional and even local events. Sediment processes vary a great deal between locations of only a few 100m distance to each other.
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