Leaf-shredding Insects as a Source of Dissolved Organic Carbon in Headwater Streams

Abstract

This study investigated the importance of leaf-shredding insects as generators of dissolved organic carbon (DOC) in a headwater stream. Three common southern Appalachian shredder species (Peltoperla maria, Pteronarcys scotti and Tipula spp.) were allowed to feed on naturally conditioned leaves, and the rates at which DOC was generated were measured. DOC generation rates of actively feeding shredders ranged from 0. 2-160 ,ug C mg shredder AFDWI day-', and 62 % of this variation could be explained by organism weight. Weight-specific rates increased as body size decreased, presumably in response to an increase in weight-specific metabolic rate. DOC generation rate per individual increased as animal size increased. DOC generation increased as generation of particulate matter increased, with a mean of 60 Ag DOC produced per mg AFDW particles generated. Less than 10% of DOC generated was produced by mechanical breaking up of the leaves. The DOC generated by shredders was predominantly in the high (> 10,000 daltons) molecular weight fraction. The rates presented are underestimates of actual rates; experiments with sterilized leaves demonstrated that microbial activity during experiments with natural leaves reduced the amount of DOC accumulating in the chambers. DOC released by actively feeding shredders is a potentially significant source of DOC in a small headwater southern Appalachian stream during low flow conditions. Shredder feeding is potentially as important a source of DOC in small streams as leaching of leaf litter. INTRODUCTION Dissolved organic carbon (DOC) is an important source of organic carbon in forested headwater streams; e.g., in Bear Brook, New Hampshire, 47 % of total organic carbon inputs was in the dissolved fraction (Fisher and Likens, 1973). DOC is taken up by microbial and chemical processes in the benthos (Cummins et al., 1972; Wetzel and Manny, 1972; Lush and Hynes, 1978; Dahm, 1981), and some is exported from the system as respired CO2 (McDowell and Fisher, 1976). Previous studies of DOC in streams have dealt primarily with sources and fates of allochthonous DOC (Fisher and Likens, 1973; McDowell and Fisher, 1976; Dahm, 1981). In this study we evaluate the potential importance of DOC released by the feeding activities of leaf-shredding aquatic insects. In headwater forested streams, where leaf litter inputs are high, leaf-shredding insects are one of the dominant functional groups in the benthos (Vannote et al., 1980). In southern Appalachian streams, shredder feeding accelerates leaf decomposition, and insect feces are an important source of seston (Wallace et al., 1982a). Our goal in this study was to determine if insect feeding could also contribute significant quantities of DOC in a stream. Release of DOC by invertebrates has been documented in a number of marine Uohannes and Satomi, 1967; Johannes and Webb, 1970) and freshwater groups Uohannes and Webb, 1970; Hargrave, 1971). Rates of release vary as a function of animal size and feeding habits: Hyallela azteca released only 5% of ingested calories (Hargrave, 1971) while Palaemonetes pugio released 33% of ingested organic carbon