Coarse and Fine Particulate Organic Matter Transport by a Fourth-Order Mountain Stream to Lake Bourget (France)

Abstract

Transport of coarse particulate organic matter (CPOM) derived from forest litterfall has been hardly studied in rivers, unlike fine particulate organic matter (FPOM) or dissolved organic matter (DOM). Yet, many rivers are dammed or run into lakes, and there is growing evidence that CPOM accumulation in river delta participates substantially in ecological processes such as greenhouse gas emissions of lakes and reservoirs. We investigated the transport of CPOM and FPOM by the Leysse River (discharge from 0.2 to 106 m3 s−1) to Lake Bourget (France) in relation to aerial litter deposition, river network length, and discharge. Over a 19-month study period, the volume-weighted mean CPOM and FPOM concentrations were 1.3 and 7.7 g m−3, respectively. Most CPOM and FPOM transport occurred during major flood events, and there were power relationships between maximum discharge and particulate organic matter (POM) transport during these events. The annual export of CPOM (190 t AFDM) was 85% of the litter accumulation in autumn on permanent sections of the riverbed (224 t AFDM), which suggests that export is a major process compared to breakdown. Export of CPOM was 1.25 t yr−1 km−2 of the forested catchment area. This study highlights the need to account for long-range CPOM transport to describe the fate of litter inputs to streams and to quantify the organic matter input and processing in lakes and reservoirs.

Geomorphic complexity influences coarse particulate organic matter transport and storage in headwater streams

“…Coarse particulate organic matter (CPOM) is an important component of stream ecosystems (Fisher and Likens, 1973;Vannote et al, 1980) and watershed carbon cycling (Turowski et al, 2016). The composition and origin of CPOM varies, with CPOM particles ranging from autochthonous algal and plant fragments to allochthonous woody material (e.g., very small wood pieces, twigs), leaves, fruits, and coniferous needles and cones (Bunte et al, 2016;Iroumé et al, 2020;Gaillard et al, 2021;Marshall et al, 2021). Here, we define CPOM as organic matter 1-100 mm in diameter that does not meet the criteria of small wood, as defined by Galia et al (2018).…”

“…The magnitude and frequency of high flow events can determine the likelihood that stored CPOM will be mobilized (Small et al, 2008), but how geomorphic complexity modifies CPOM transport rates during high flow events is unclear. The vast majority of CPOM transport occurs during periods of high flow, particularly on the rising limb of higher flows such as peak snowmelt discharge (Bunte et al, 2016;Gaillard et al, 2021;Marshall et al, 2021). Features associated with CPOM retention (e.g., channel-spanning logjams) may in some cases limit local CPOM transport (Jochner et al, 2015), but among the few studies that exist, some have found that the opposite is true, and that logjams do not influence local CPOM transport (Marshall et al, 2021).…”

Drivers of Coarse Particulate Organic Matter Transport and Storage in Headwater Streams

“…Coarse particulate organic matter (CPOM), generally defined as organic matter 1-100 mm in diameter that does not meet the criteria of small wood (> 1 m in length and 0.05-1 m in diameter or 0.5-1 m in length and > 0.1 m in diameter (Galia et al, 2018)), is an important component of stream ecosystems (Fisher & Likens, 1973;Vannote et al, 1980) and watershed carbon cycling (Turowski et al, 2016). The composition and origin of CPOM in rivers varies, with CPOM particles ranging from autochthonous algal and plant fragments to allochthonous woody material, leaves, fruits, and coniferous needles and cones (Bunte et al, 2016;Gaillard et al, 2021;Iroumé et al, 2020;Marshall et al, 2021). In many headwater streams, the majority of CPOM is terrestrial in origin; therefore, CPOM represents an important subsidy of organic matter and carbon to aquatic systems (Turowski et al, 2016;Wallace et al, 1995).…”