Coarse Particulate Organic matter (CPOM) provides essential energetic and habitat resources for many benthic organisms in rivers, but is not available unless held (retained) on the riverbed. We surveyed the retentiveness (capacity to retain CPOM) of shallow, slackwater areas in a large lowland river to test whether: (H1) slackwaters with more retentive features (wood, vegetation, depositional areas) retain more CPOM; (H2) depositional areas (the entire riverbed within slackwaters) retain the highest proportion of total CPOM per site; (H3) wood and vegetation are the most efficient features at retaining CPOM.
In 24 slackwaters (4 sites, six slackwaters / site) of the lower Goulburn River, southeast Australia, we conducted line-intercept surveys along random transects to quantify: channel retentiveness (average m of retentive features / m of transect); CPOM density (average m of CPOM / m of transect) and retention efficiency of different types of retentive features (average m of CPOM / m of retentive feature). Densities of invertebrates and fishes were also surveyed.
Slackwaters with higher retentiveness retained more CPOM (supporting H1). Depositional areas retained the most CPOM overall (supporting H2). Retention efficiency was high for wood but low for vegetation (opposing H3). Overall, stocks of CPOM in slackwaters of the lower Goulburn River were low compared to stocks previously reported for smaller tributaries. We note that some features (vegetation in particular) retained CPOM poorly in slackwaters compared to faster flowing tributary channels and posit that differences in hydrology between small and large rivers may influence the effectiveness of some features at retaining CPOM.
Outcomes of this study highlight the context dependence of retention processes, which appear to operate differently in large vs small channels, and point to wood as an effective retentive feature that might be added to slackwaters to enhance stocks of CPOM resources. Implications for benthic fauna are discussed.