A PRELIMINARY INVESTIGATION OF ORGANIC CARBON STORAGE IN URBAN HEADWATER STREAM SEDIMENTS

Few studies have examined the storage of organic carbon in headwater stream sediments as part of the global carbon budget. Headwater streams, however, are important biogeochemical “hotspots” and represent as much as 70% of the total catchment area. Because first order streams constitute the majority of stream length in a watershed, they may represent a significant sink of terrestrial carbon. The purpose of this study is to estimate the total organic carbon (TOC) content in the sediment of the first 300m of two urbanized headwater streams in the Rocky Creek watershed of upstate South Carolina. Land cover in Rocky Creek is 50.2% urban and 21.9 % forested. The sediment volume of each stream was estimated by measuring the bankfull width, length, and sediment depth of reaches characterized as riffles, runs, pools, and wetlands. The headwaters of Rocky Creek 27 (RC27) consisted of 64% runs, 17% riffles, 16% pools, and 3% wetlands. The headwaters of Rocky Creek 33 (RC 33) consisted of 60% runs, 32% riffles, and 8% pools. Sediment samples were collected from every third geomorphic feature in each headwater. In RC27, samples were collected from 2 wetlands, 6 pools, 10 riffles, and 12 runs. In RC33, samples were collected from 4 pools, 6 riffles, and 8 runs. Organic matter content was estimated by loss on ignition (LOI) and converted to total organic carbon (TOC) using the ‘Van Bemmelen’ factor of 0.58 g C/g organic matter. In RC27 the TOC decreased downstream from 6% to 0%. In RC33 the TOC varied between 0% and 3%, but showed no trend. In the first 300 m of each stream, an estimated 2.5 metric tons of organic carbon are stored in RC27 (1.28 MT in runs, 0.98 MT in pools, 0.22 MT in wetlands, and 0.06 MT in riffles), and an estimated 5.1 metric tons of organic carbon are stored in RC33 (3.73 MT in runs, 0.71 MT in pools, and 0.64 MT in riffles). Runs are the most significant store of organic matter as the dominant geomorphic feature, even though wetlands and pools contained greater concentrations of organic matter. The results suggest that headwater streams may represent an important sink of terrestrial carbon, even in urban watersheds.