PAIRED WATERSHED INVESTIGATION OF SUSPENDED SEDIMENT AND PARTICULATE ORGANIC CARBON IN CENTRAL MASSACHUSETTS; IMPLICATIONS FOR STORM RUNOFF AND EXPORT OF ORGANIC CARBON

Differences in landuse and hydrologic flowpaths contribute to differences in suspended sediment concentrations (SSC) and fluvial particulate organic carbon (POC) in two otherwise similar central Massachusetts headwater channels. We compared the hydrology, SSC, POC, and total fluvial carbon export in Bigelow Brook, located within the protected and forested Harvard Forest Long Term Ecological Research site, and Beaver Brook, located entirely within the city limits of Worcester, Massachusetts. In the 3.5 km² urban watershed (16% forested, 37% impervious), the hydrologic response to precipitation was relatively large and fast. From May 2009 to March 2010, Beaver Brook had a basin lag of < 1 to 5.75 hours and 65% of total discharge occurred as stormflow. SSC increased with discharge, ranging from 0.42 to 223 mg/L. POC ranged from nondetectable to 41 mg/L, and increased with SSC. These results indicate that runoff over impervious surfaces wash both inorganic and organic sediment from hillslopes and streets into the channel during storm events, with potential environmental and geomorphic implications. Conversely, basin lag at the forested 0.65 km² Bigelow Brook watershed (79% forested, < 2% impervious) was 3 to 11 hours, and only 35% of total discharge occurred as stormflow. SSC was < 25 mg/L even during storm events, and were frequently < 5 mg/L. In this forested watershed with thick forest duff and organic soils, POC ranged from 0.23 to 4 mg/L, with low values at high discharge. Assuming that most POC in the stream is terrestrially derived, these results suggest that little surface runoff occurs in the forested Bigelow Brook watershed and very little organic matter is washed in from the surrounding hillslopes, even during significant precipitation events. Finally, we compared the relative importance of POC in the carbon budget for Harvard Forest using data from the LTER program. Using our average POC concentration of 1.8 mg/L, we estimate that ~ 1.5 tons/km² of carbon left the forested watershed as POC during the study period, compared to ~6.4 tons/km² that left as dissolved organic carbon (DOC). In comparison, ~2.5 tons/km² of carbon left the urban Beaver Brook watershed as POC during the study period, suggesting that urban storm runoff reduces a landscape’s efficiency as a carbon sink.