Seasonal and diurnal fluctuations of coarse particulate organic matter transport in a snowmelt‐dominated stream

Abstract

We measured coarse particulate organic matter (CPOM) transport along a wood-rich, pool-riffle mountain stream in the Southern Rockies of Colorado, USA, to examine how spatial variations in storage features and temporal variations in discharge influence the transport of CPOM. Ecologists have found that the majority of annual CPOM export occurs during periods of high discharge. More recently, geomorphologists have begun to examine the transport of CPOM as bedload. There has been, however, little direct sampling of CPOM to evaluate how shorter (diurnal) and longer (seasonal peak flow) variations in discharge affect CPOM transport, and no examination of where CPOM is transported in the water column (primarily in suspension or as bedload). We collected CPOM moving as bedload, in suspension (at 0.6 of the flow depth) and at the surface to evaluate CPOM transport processes. Samples were collected at three sites: (1) in the backwater pool upstream from a channel-spanning logjam; (2) immediately downstream from the logjam; and (3) in a riffle about 10 bankfull-channel-widths downstream from any channel-spanning logjams. During sample sets, we collected samples over 15-min increments at approximately 4-hr intervals over a 24-hr period. Seven sample sets were distributed over a period of 2 months that spanned the rise, peak, and recession of the annual snowmelt flood.We found that the majority of CPOM is transported in suspension following a clockwise hysteresis loop in which CPOM peaks prior to discharge during the seasonal hydrograph.

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

“…We did not sample CPOM transport rate overnight in either stream because after assessing discharge trends at each site, we found relatively little 24-h variability in flow. In addition, other studies have found that the diurnal cycle does not significantly impact CPOM transport (Marshall et al, 2021).…”

“…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).…”

“…However, we lack understanding of the magnitude of and processes influencing CPOM export in headwater streams, particularly across differing hydrologic regimes (e.g., intermittent or ephemeral systems compared to perennial streams) (Bunte et al, 2016;Turowski et al, 2016;Shumilova et al, 2019). In addition, the role of geomorphic complexity, including variations in valley geometry and the frequency of instream retentive features (e.g., wood), in mediating CPOM transport has only rarely been studied [see Jochner et al (2015), Marshall et al (2021)]. Here, we investigate how geomorphic and hydrologic setting influence CPOM transport and storage in headwater streams.…”

“…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).…”

“…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). In addition, the influence of broader-scale geomorphic context (e.g., valley bottom geometry) on CPOM transport has not yet been investigated.…”