| Summary: | The present study was a part of a larger research project entitled ForStreams. Changes in catchment vegetation can have large impacts on stream ecosystems, especially through transport of terrestrial organic matter into them. The objectives of the present study were to compare non-forested and forested catchments in Iceland in terms of litter transport into streams and litter decomposition within them. This was the first study made in Iceland on the terrestrial-aquatic interactions in forested ecosystems. The main study consisted of nine run-off fed headwater streams in Fljótsdalshérað in eastern Iceland (N 65°01’–N 65°10’ and W 14°28’–W 14°48’). The streams either ran through treeless heathlands, birch forests (Betula pubescens Ehrh.) or conifer plantations (mainly Larix sibirica Ledeb.). The transport of terrestrial organic matter was measured with litter traps inserted into the stream banks. Decomposition of litter was measured with the litter bag method, using fine and coarse mesh litter bags to evaluate the effects on microorganism and invertebrate activity in the decomposition processes. The invertebrate fauna found within the coarse bags was also studied. Litter transport into the birch and conifer forest streams was 30–33 times higher than into the heathland streams. Hence, the litter transport increased the amount of energy available in the forest streams. Decomposition rate of birch leaves, larch needles and grass litter was 0.0033, 0.0040 and 0.0044g g-1 DM day-1, respectively. This was low compared to measured decomposition rates in comparable studies elsewhere. Grass litter had the fastest decomposition rate in the forested streams, whereas there was no difference in decomposition rate of different litter types in the heathland streams. The decomposition rate was positively correlated with the concentration of phosphorous in the water across all catchment types. It was also negatively correlated with the number of Plecoptera (shredders) found in the litter bags. This could indicate a different ...
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