DIGITALNA ARHIVA ŠUMARSKOG LISTA
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ŠUMARSKI LIST 11-12/2013 str. 41     <-- 41 -->        PDF

1. The drift densities of benthic animals are expected to be lower downstream than upstream of the pond. The amount of drifting invertebrates decreases with decreasing water velocity (Elliott 1967; Everest & Chapman 1972). Reservoirs effectively block the drift of benthic species which cannot survive in a lentic (still water) environment (Ward & Stanford 1980). Gönczi et al. (1986) found that the drift of benthic animals below hydro-power dams was reduced compared to rivers with no dams.
2. It is likely that the amount of pelagic invertebrates in the drift is higher downstream than upstream of the pond. When a stream is dammed, the lotic taxa of the benthos are expected to be replaced by organisms preferring a lentic environment (Baxter 1977). In beaver ponds lotic taxa may be replaced by lentic (Sprules 1940; Macdowell & Naiman 1986; Naiman et al.1988).
3. The drift densities of Plecoptera should be lower downstream of the pond. Plecoptera have been shown to decrease in the original stream bed after impoundment (Sprules 1940; Baxter 1977; Nummi 1989). In the site immediately downstream of the dam, Smith et. al. (1991) found lower densities of Plecoptera. Plecoptera are typically reduced or absent below dams (Ward and Stanford 1980).
4. It is likely that the functional feeding group ratio of filtering collectors to gathering collectors is higher downstream the dam. Clifford et al. (1993) found that the fauna of the wood and debris dams shows similarities to the fauna of lake outlets, and filter-feeding Simuliidae were abundant in high densities. Streams below reservoirs are characterised by a predominance of filter-feeding Trichoptera and Simuliidae (Ward & Stanford 1980). The pond accumulates large amount of fine particulate organic matter in the sediment so the amount of FPOM deposited in benthos (the food for gathering collectors) downstream of the dam probably is reduced (Naiman et. al 1986).
Study area
Područje istraživanja
The study was performed in central Sweden (63°N, 15,30–16°E), in the eastern parts of the county of Jämtland (Fig. 1). The beaver dams are located in the drainage basins of river Indalsälven and river Ljungan, at 270–380 m above sea level. The landscape is hilly with the highest peaks reaching an altitude of over 500 m. Mean temperature in January is –10 °C and in July +15 °C. The precipitation is 700 mm per year. The dominant tree species are Norway spruce (Picea abies), Scots pine (Pinus sylvestris) and birch (Betula spp). Along the streamside there is also alder (Alnus incana) and small numbers of rowan (Sorbus aucu­paria), bird-cherry (Prunus padus) and sallow (Salix caprea). There is intensive forestry in the region, and the amount of old-growth stands is thus very low. According to local hunters, the first beavers were observed in the area during the 1950s. Today beaver is very abundant in the area, and most of the permanently flowing streams have or have had beaver present since late 1970’s, i.e. 20 years before the study (Andreas Redin, personal observation). Fish fauna present in small streams in this area is generally brown trout (Salmo trutta) and minnow (Phoxinus