"URBAN STREAM SYNDROME" IN THE HEADWATERS OF THE BLACKSTONE RIVER

Urbanization has contributed to differences in suspended sediment concentrations (SSC), fluvial particulate organic matter (POM), and habitat health in two otherwise similar central Massachusetts headwater channels. We compared the stream hydrology, soil infiltration rates, SSC, POM, macroinvertebrates, and total fluvial carbon export in Bigelow Brook, a protected and forested watershed within the Harvard Forest Long Term Ecological Research site in Petersham, MA; and Beaver Brook, a headwater tributary of the Blackstone River located in Worcester, MA. Several engineering and restoration projects have been initiated in the headwaters of the Blackstone River to improve water quality, yet the river continues to display hydrologic, chemical, and biological attributes that fit the recently described “urban stream syndrome”. In the 3.5 km² urban watershed, the hydrologic response to precipitation was relatively flashy. In storm events, 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. POM 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. Beaver Brook also has high concentrations of nitrate and phosphate, poor coarse POM retention, and low stream macroinvertebrate diversity. Conversely, basin lag at the forested 0.65 km² Bigelow Brook watershed was 3 to 11 hours, and < 40% of total discharge occurred as stormflow. SSC was < 25 mg/L. In the forested watershed, POM ranged from 0.23 to only 12 mg/L. These results suggest that little surface runoff occurs in the forested watershed and very little organic matter is washed in from the surrounding hillslopes. These results are consistent with infiltration rates measured in the two watersheds, ~0.9 cm/min at Harvard Forest compared to ~0.4 cm/min in Worcester lawns. Finally, ~ 1.5 tons/km² of carbon left the forested watershed as POC, 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 POM during the study period, suggesting that urban storm runoff reduces a landscape’s efficiency as a carbon sink.