DISSOLVED CARBOHYDRATE CONCENTRATION, COMPOSITION, AND BIOAVAILABILITY TO MICROBIAL HETEROTROPHS IN STREAM WATER

Dissolved total carbohydrates (DTCH), dissolved free monosaccharides ( DFMS), dissolved organic carbon (DOC), biodegradable DOC (BDOC), and h umic substances (HS) were measured in White Clay Creek (WCC), a stream in southeastern Pennsylvania Piedmont, USA. Samples were collected ov er different seasons and under baseflow and stormflow conditions. DOC concentrations ranged from 1.0 to 12.8 mg/L C with the highest concent rations associated with stormflows. Carbohydrates ranged from 0.42 to 12.4 mu M and accounted for 2.9 to 12.7 % of the DOC. Humic substances represented the major DOC fraction, accounting for 55 to 72% of the D OC pool under all flow conditions. The humic fraction had a lower carb ohydrate content (4.4 %) than the non-humic fraction of DOC (7.2 %). S tormflow DOC was enriched in carbohydrates relative to baseflow DOG, b ut the percentage of humic-C changed little. Carbohydrates were primar ily present as dissolved polysaccharides (55 %), but a significant fra ction was bound to humic substances (40 %), while a small proportion w as present as monosaccharides (5 %). The major monosaccharides in stre am water, listed in order of decreasing concentration, included glucos e, galactose, rhamnose, xylose, arabinose, mannose, and fucose. On ave rage (30.6 +/- 7.4)% (n = 44) of the stream water DOC was biodegradabl e, and carbohydrates accounted for 9.9 to 17.7 % of the BDOC.