SEASONAL DYNAMICS OF DETRITUS AND ASSOCIATED MACROINVERTEBRATES IN A CHANNELIZED BOREAL STREAM

This study documents various components of detritus dynamics in a chan nelized third order stream in Central Finland. Total annual litterfall entering the stream was 310 g AFDM m(-2). Leaf fall, which comprised 86.7% of total input, exhibited a distinct seasonal peak in September- October, with a maximum daily input rate of 13 g m(-2). The contributi on of woody detritus to total input was only 5%. The mean annual stand ing stocks of coarse (23.0 g AFDM m(-2)) and fine (1.76 g AFDM m(-2)) benthic organic matter are among the lowest reported for woodland stre ams. Macroinvertebrate standing stock peaked in February (1205 mg DW m (-2)), and the annual minimum (276 mg DW m(-2)) occurred in August. Sh redding invertebrates did not reach their maximum biomass until March, although the availability of benthic leaf litter peaked already in Oc tober. In contrast, FPOM-collectors and predatory invertebrates tracke d closely the seasonal availability of their food resources. Leaf rele ase experiments showed that the stream had a low retention capacity: 1 3-40% of 500 artificial leaves were retained within a 50 m experimenta l section at discharges ranging from 0.46 to 1.52 m(3) s(-1). The stre am is poorly retentive because it lacks effective retention devices (e .g. large woody debris). Due to the low retentivity of the stream bed and fast decomposition rates of the primary litter sources (birch and alder leaves), shredders probably face a severe springtime food limita tion in this stream. We suggest that adding retentive devices to strea m bed is a viable management strategy for restoration of channelized w oodland streams.