Structure and grazing of stone surface organic layers in some acid streams of southern England

Abstract

SUMMARY. Stone surface organic layers were investigated at five sites on small, acid streams in the Ashdown Forest, southern England. Sites differed in stream water pH (means 4.3–6.6) and some other physicochemical features. Organic layers at the stream bed surface differed between sites in structure and in the amount of organic carbon present. Algae were abundant at the sites with higher conductivity and pH, iron bacteria (mainly Leptothrix sp.) predominated in the iron‐rich Broadstone stream, whereas the surface layer at the most acid site was predominantly a rather structure less organic film with few living organisms. Amounts of organic carbon on stones buried within the stream bed for 3 months were almost identical at all sites. When viewed with the scanning electron microscope (SEM), organic layers on buried stones were rather structure less with some amorphous, floceulent material. The paucity of microflora suggests that these layers may have been formed primarily by abiotic mechanisms. X‐ray microanalysis of organic layers indicated that complexes of organic matter with the metals iron, aluminium and manganese were present. Laboratory experiments with seven species of invertebrates showed that all were able to remove and at least partially ingest organic layers and underlying sandstone. The weight of material removed was highest on microbially impoverished layers from the most acid site, and was substantially lower where algae were abundant.6. The role of stone surface organic layers in stream systems is discussed. They may be important sites of dissolved organic matter (DOM) uptake and transfer to the benthos. A major impact of acidity on running water could be through its effect on the structure and function of organic layers.