Longitudinal and seasonal changes in the origin and quality of transported particulate organic matter along a gravel-bed river

Abstract

Particulate organic matter (POM) plays an important role in nutrient dynamics in river ecosystems, but little is known about changes in the origin and quality of POM in relation to physical and seasonal changes along rivers. Using stable isotope and stoichiometric analyses, we investigated the changes in origin and quality of POM of three different size fractions (fine [FPOM], 1.2-100 lm; medium [MPOM], 100-250 lm; and coarse [CPOM], 250-1,000 lm) at 14 sites along a gravel-bed river over four seasons. FPOM and MPOM accounted for 90% of all POM at all study sites. At each site, the d 13 C level was lower for FPOM (range: -29.0 to -21.1%) than for MPOM (-26.9 to -17.2%) and CPOM (-27.5 to -16.3%). The C:N ratio was lower for FPOM (6.9-15.6) than for MPOM (6.3-17.4) and CPOM (5.7-27.1). The contribution of periphyton to POM of all size fractions had a tendency to increase downstream, though the trend was less clear and varied seasonally for MPOM and CPOM between sites in middle and downstream reaches. Contrastively, the C:N ratio in all size fractions of POM consistently decreased downstream. The downstream decrease in the C:N ratio of POM can be partly explained by the increase in the contribution of periphyton, which seems to be associated with increased discharge and enhanced periphyton dislodgement, especially in winter. In addition, an increase in bacterial biomass associated with the greater nutrient availability in pool areas is another possible reason for the decrease in the C:N ratio of POM downstream.

The quality and availability of fine particulate organic matter for collector species in headwater streams

“…The values obtained (0.3-25.9 g AFDM/m 2 ) were within the range of values reported for low-order streams in other studies: 0.8 g/m 2 [24], 5.32 g/ m 2 (annual mean; [9]); 750 g m 2 [25] and higher than the obtained for CPOM in a larger local stream [26]. The values obtained (0.3-25.9 g AFDM/m 2 ) were within the range of values reported for low-order streams in other studies: 0.8 g/m 2 [24], 5.32 g/ m 2 (annual mean; [9]); 750 g m 2 [25] and higher than the obtained for CPOM in a larger local stream [26].…”

“…Other small invertebrates sampled from the FPOM deposits included Leuctra sp., whose early instars have been reported to feed on fine particles of organic matter [15]. For comparative purposes, the C: N range obtained here (21-30 and one value of 51) was within the range reported in the literature: 7-39 [8], 17-28 [5], and 10-34 [12] but greater than the value of 9.3 reported for headwaters in Japan [9]. This result could reflect two non-exclusive causes.…”

“…High quality was observed during summer, as reported by Akamatsu et al [9]. High quality was observed during summer, as reported by Akamatsu et al [9].…”

“…This value is much less than the maximum of 50,000 reported by Romito et al [17] for Appalachian headwater streams. For example, a higher abundance of FPOM was reported for autumn than for summer in a Japanese study [9]. The total abundance of invertebrates in depositional areas of the benthos (c. 50% of total streams length) was correlated with FPOM availability across streams.…”

“…However, the opposite pattern was reported by Bonin et al [5] for C:N. The higher quality of summer samples could be related to a presumably higher residence time of particles and therefore to a longer period allowing higher microbial colonization and perhaps a higher abundance of algae during summer. Differences between studies may reflect differences in forest cover [5], the location along the longitudinal gradient in the stream [9], the size of particles and their origin (e.g. Differences between studies may reflect differences in forest cover [5], the location along the longitudinal gradient in the stream [9], the size of particles and their origin (e.g.…”