Changes in stream benthic organic matter following watershed disturbance

Abstract

Benthic organic matter was collected quarterly from streams draining a 9‐yr‐old clearcut, an 18‐yr‐old “old‐field”, a 25‐yr‐old successional forest, and two reference watersheds at Coweeta Hydrologic Laboratory in the Appalachian Mountains of North Carolina. USA. Samples were separated into large benthic organic matter (LBOM >1 mm) and fine benthic organic matter (FBOM <1 mm). An additional survey of large (>5 em diam.) and small (1–5 cm diam.) wood was conducted. Standing stocks of LBOM ranged from 124 to 235 g AFDM m−2 (ash tree dry mass) and were significantly higher in streams draining reference watersheds and the successional forest than in either the recent clearcut or old‐field. Reference sites exhibited LBOM peaks in late autumn and spring. No seasonal patterns were observed in disturbed streams. Standing stocks of FBOM averaged 113 to 387 g AFDM m−2, and the stream draining the successional forest had significantly higher FBOM levels than the other sites. In reference streams, FBOM abundance peaked in spring. In disturbed streams, FBOM standing stocks were highest in summer or late autumn. Standing stocks of large wood ranged from 0 to 3956 g AFDM m−2 and were significantly higher in the reference streams than in streams draining the old‐field or successional forest. Small wood averaged 11 to 342 g AFDM m−2 and was significantly lower in the stream draining the old‐field than at the other sites. Comparisons of organic matter inputs with standing stocks indicated that disturbed streams at Coweeta receive less material and process it faster than reference streams. Disturbed streams also appear to be less retentive than reference streams and exhibit a gradual decline in FBOM during the winter when large, long‐duration storms combined with low particle generation rates deplete accumulated FBOM.