Seasonal dynamics of detritus and associated macroinvertebrates in a channelized boreal stream

Abstract

This study documents various components of detritus dynamics in a channelized third order stream in Central Finland. Total annual litterfall entering the stream was 310g 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 contribution of woody detritus to total input was only 5 %. The mean annual standing 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 streams. Macroinvertebrate standing stock peaked in February (1205 mg DW m -2 ), and the annual minimum (276 mg DW m -2 ) occurred in August. Shredding invertebrates did not reach their maximum biomass until March, although the availability of benthic leaf litter peaked already in October. In contrast, FPOM-collectors and predatory invertebrates tracked closely the seasonal availability of their food resources. Leaf release experiments showed that the stream had a low retention capacity: 13-40 % of 500 artificial leaves were retained within a 50m experimental section at discharges ranging from 0.46 to 1.52 m 3 s -1 . The stream 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 limitation in this stream. We suggest that adding retentive devices to stream bed is a viable management strategy for restoration of channelized woodland streams.