Transport and Export of Coarse Particulate Organic Matter (CPOM) in Steep Streams
Abstract:Coarse particulate organic material >1 mm (CPOM) comprising mainly leaves, needles, cones, twigs, and decaying wood pieces is an energy source in headwater ecosystems, and CPOM transport of is one form of watershed carbon export. However, sampling methods for instantaneous CPOM transport rates and annual loads are not well developed, and hence not much is known about annual CPOM export from high energy, forested mountain streams.
CPOM transport samples were collected over one-month snowmelt highflow seasons in two high-elevation, subalpine, Rocky Mountains streams with snowmelt regimes as well as in a montane torrent in the Swiss Pre-Alps with a pluvio-nival regime. Basin areas ranged from 1 to 13 km2. Bedload traps and automated hanging baskets installed at an overfall, developed for sampling gravel bedload, proved suitable for sampling CPOM transport.
Bedload traps with 0.3 by 0.2 m openings are typically deployed for an hour at a time during wadeable flows, collect samples back-to-back, and may trap volumes of up to 20 liters in the attached 1.4 m long nets. At the Swiss site, hanging baskets with a 1 m3 volume are deployed at an overfall in unwadeable flows, and bedload traps in wadeable flows. When flow overtopped bedload traps, CPOM transport rates were adjusted to estimate and add the unsampled CPOM portion traveling higher in the water column; when nets overfilled, CPOM transport rate were adjusted for the reduction in captured transport.
CPOM transport rates in consecutive samples were well related to flow but showed pronounced hysteresis over diurnal fluctuations of flow, between consecutive days, and over the rising and falling limbs of the high-flow season. Hysteresis effects require intensive sampling and use of separate rating curves for all rising and falling limbs in order to compute annual CPOM load. Annual CPOM export for the two Rocky Mountain streams was 2.7 and 4 kg/ha/year, but both streams exported 6.5 and 6.6 kg/ha/year per forested portion of the watershed.
Instantaneous CPOM transport rates are moderately well related to discharge, unit discharge, and unit stream power within individual streams. However, all three streams follow a similar transport relation of CPOM only with unit discharge, suggesting that unit discharge could be suitable to predict CPOM transport rates in steep streams.