What happens to allochthonous material that falls into streams? A synthesis of new and published information from Coweeta

1. One of two things can happen to allochthonous material once it enters a stream: it can be broken down or it can be transported downstream. The efficiency with which allochthonous material is used is the result of these two opposing factors: breakdown and transport. 2. The present synthesis of new and published studies at Coweeta Hydrologic Laboratory compares biological use versus transport for four categories of particulate organic material: (1) large wood (logs); (2) small wood (sticks); (3) leaves; and (4) fine particulate organic matter (FPOM). 3. Over 8 years, logs showed no breakdown or movement. 4. The breakdown rate of sticks (< 3 cm diameter) ranged from 0.00017 to 0.00103 day, while their rate of transport, although varying considerably with discharge, ranged from 0 to 0.1 m day. 5. Based on 40 published measurements, the average rate of leaf breakdown was 0.0098 day. The leaf transport rate depended on stream size and discharge. 6. The average respiration rate of FPOM was 1.4 mg O2 g AFDM ±1 day over a temperature range of 6±22 °C, which implies a decomposition rate of 0.00104 day. Transport distances of both corn pollen and glass beads, surrogates of natural FPOM, were short (< 10 m) except during high discharge. 7. Estimates of transport rate were substantially larger than the breakdown rates for sticks, leaves and FPOM. Thus, an organic particle on the stream bottom is more likely to be transported than broken down by biological processes, although estimates of turnover length suggest that sticks and leaves do not travel far. However, once these larger particles are converted to refractory FPOM, either by physical or biological processes, they may be transported long distances before being metabolized.