Long-Term Dynamics of Coarse Particulate Organic Matter in Three Appalachian Mountain Streams

Abstract

Export of coarse particulate organic matter (CPOM) from three headwater streams was studied continuously over a period of 8 (one stream) to 9.5 years (two streams) at the Coweeta Hydrologic Laboratory, North Carolina. Annual CPOM export among years varied by factors of 9.2 to > 16 ×, much greater than the 3.5 to 5.0 × variation in annual stream discharge. Annual export was poorly correlated with annual discharge and was more a factor of storm frequency and magnitude. Over 63 to >77% of all CPOM export occurred during the largest 20 storms. Export during individual sampling intervals was strongly related to maximum discharge during the interval (r² 0.43-0.54, p<0.001) for all streams throughout the study. Total leaf export and leaf export per unit maximum discharge were greatest during the autumn and lowest during the spring and summer months in all streams. Export of woody debris was more seasonally variable than that of leaves in two of the three streams. Although allochthonous CPOM is the greatest source of organic matter inputs to these streams (>86%), CPOM represented only 1.8-3.8% of total organic matter export, indicating that CPOM retention is high in these small, high-gradient streams. Most export occurs in the form of fine particulate organic matter (FPOM) and dissolved organic matter (DOM), or respired CO₂. Stream retentiveness was further illustrated by movement of marked artificial "leaves" and "sticks", which displayed maximum downstream movement of ∼42 m/yr for "leaves" and 10 m/yr for "sticks". Surprisingly, export as a percent of inputs was greatest for woody debris. This is probably attributable to both underestimates of woody litter inputs and slower decomposition rates for woody debris, resulting in greater standing crops and availability of woody debris for transport during storms. For three of the eight years of study, one stream (C 54) received seasonal treatments with an insecticide that drastically reduced invertebrate populations. Large increases in leaf litter standing crop were observed following three years of treatment compared with untreated streams. However, CPOM export from the treated stream did not increase significantly compared with untreated streams. During treatment the CPOM:FPOM ratio of exported particulate organic matter was over twice that of untreated streams, or C 54 during periods when the stream was not treated. This increase was primarily a result of massive reduction in FPOM export, rather than large increases in CPOM export. Although storms removed large amounts of CPOM from the wetted perimeter of all streams during 1989 and 1990, most removed material was retained within the narrow riparian zone, and not exported to downstream reaches. Movement and exchange of CPOM between the wetted perimeter and adjacent riparian zones in these high-gradient streams demonstrates similarities with streams draining lowland regions with extensive floodplains.