In this presentation we explore river-scale patterns in spatial and temporal distributions of coarse carbon resources (CPOM)… but that wasn’t our original intention.
In 2021 we established a long-term field experiment to increase CPOM densities at four sites in the lower Goulburn River. We adapted methods established for small streams to increase densities of CPOM and dependent organisms (invertebrates, small fishes). However, the manipulation proved ineffective in this much larger system. Within sites, manipulated areas showed no significant increase in CPOM (vs Controls) after 24 months.
We shifted focus to the river scale, comparing CPOM densities among sites (n = 4) and times (n = 6). We predicted spatial patterns: (H1) sites upstream (below Goulburn Weir) have lower overall densities of CPOM – because Goulburn Weir blocks supply of drifting CPOM to upstream sites and tributary inflows supplement supply of CPOM to downstream sites. We also predicted temporal patterns: (H2) densities of CPOM at low flows (800-1500 ML / day) are a function of time since the last major flow event (8000-15000 ML / day).
Results strongly support our hypotheses. Spatial data showed lowest densities of CPOM directly below Goulburn Weir, and highest densities downstream of tributary junctions (H1). Amounts of retentive structure (wood, vegetation) were equivalent among sites, so we attribute these differences to supply. Temporal data showed strong effects of antecedent flows (H2). Size of the maximum daily flow during the preceding 60 days gave the strongest prediction of CPOM density (R2 > 0.9 at the downstream site).
Observed low supply and frequent flushing of CPOM may explain, in large part, why our efforts to boost CPOM densities failed in this system. More importantly, however, these patterns highlight that food-webs below weirs may be starved of carbon unless there is influx from riparian vegetation or tributary inflows.