Lipid composition of suspended particulate matter (SPM) in a southeastern blackwater stream

Suspended particulate matter (SPM) was collected seasonally for 1 yr at third- and fifth-order sites in a blackwater stream on the coastal plain in South Carolina. Fatty acids with carbon chain lengths from C12–C32 were the most abundant component among the lipid classes examined with total concentrations varying from 8.5 to 60.2 (mu)g L1. Unsaturated fatty acids predominated while significant concentrations of the even-chained saturated components C24–C30 derived from cuticular plant waxes were also found. Concentrations of aliphatic alcohols, with carbon chain lengths between C16 and C30, ranged from 0.52 to 2.73 (mu)g L1 and was dominated by the higher molecular weight compounds (C22C30) derived primarily from cuticular plant waxes. Total hydrocarbon concentrations ranged from 0.35 to 5.66 (mu)g L1 and showed no discernible trends with time or consistent difference between sites. The hydrocarbon assemblage observed indicates that these components are entirely of biogenic origin with no detectable anthropogenic contribution. The ratios of unsaturated to saturated fatty acids and cuticular to noncuticular fatty acids and alcohols at both stream locations indicate that the organic detritus associated with SPM collected during November and January is of more recent origin and less processed than that collected in the spring and summer months. Lipid concentrations at the third-order site generally had higher and more variable concentrations of the lipid classes compared with the fifth-order site. The ratio of saturated to unsaturated fatty acids was higher at the third-order site indicating that the organic fraction of SPM at this site was of more recent origin and less decomposed. The ratios of cuticular to noncuticular fatty acids and alcohols support this conclusion. These results indicate an export of particulate lipids of higher carbon resource quality from upstream to lower stream reaches.