The formation, structure and utilization of stone surface organic layers in two New Zealand streams

SUMMARY.1  The development of stone surface organic layers was investigated in dark and light experimental channels at two field sites. Layer formation was monitored by measuring organic carbon, chlorophyll‐ a , ATP and rates of oxygen consumption, and using scanning electron microscopy. 2  In the darkened forest stream channel an organic layer consisting of slime, fine particles, bacteria and fungi developed and attained maximum biomass (=0.08 mg cm ‐2 ) in about 2 months. At the second site, channels were fed by spring water low in dissolved and particulate organic matter (DOC < 0.5 g m ‐3 ) and no organic layer developed on stones in the dark. Organic layers grown in channels subject to natural light intensities and photoperiods were dominated by diatoms and/or filamentous algae at both sites. 3  Laboratory experiments carried out in enclosed, recirculating stream channels demonstrated the importance of dissolved organic matter (DOM) as a prerequisite for layer formation. Also. DOM additions in the form of leaf leachates stimulated oxygen consumption by preformed layers. Uptake by microorganisms accounted for most of the reduction in water‐column DOM. 4  Radiotracer experiments ( 14 C and 144 Ce) showed that several common stream invertebrates could feed on ‘heterotrophic’ layers. Calculated assimilation efficiencies ranged from 18% to 74% and imply that nonautotrophic components of stone surface organic layers are likely to play a significant role in carbon transfer to the benthos, particularly in small, shaded streams.