Many small aquatic organisms use coarse plant detritus (CPOM) as a base resource of food and habitat, but to become available to organisms CPOM must first be retained within the benthos. Retention occurs when drifting CPOM is trapped by hydraulic features (e.g. deposition in slackwaters) or structural features (e.g. wood) but, in human-altered landscapes, channels have often lost retentiveness via channel simplification, removal of large wood and clearing of riparian vegetation that contributes wood and CPOM to the channel.
Recent field experiments suggest that sites with predominantly structural retention can better retain CPOM resources through time than sites with predominantly hydraulic retention, because CPOM deposited in slack-water zones may periodically be flushed away by elevated flows. We apply this logic to establish a new field experiment in the Lower Goulburn River, a large (~ 40 m wide) river with a simplified channel. We predict that remnant slackwater areas provide important habitat for small aquatic organisms in this system, but CPOM resources are scarce in slackwaters which are periodically flushed by high natural and regulated flows, including environmental flows.
To buffer these high-value habitats, we added stakes to 24 slackwaters in the lower Goulburn River (4 sites, 6 slackwaters / site) to test the simple hypothesis that added structure will increase long-term retention of resources within these areas. Within each slackwater, stakes were installed in two randomly located plots, with two random Control plots of equal size.
The experiment was initiated in June 2021 and remains ongoing, but results from the first 12 months are consistent with our predictions. Densities of CPOM decreased in Control plots after high flow periods (winter, spring), but were maintained by added structure in staked plots. These outcomes suggest a simple method to increase the resilience of slackwater habitats in a river with regulated flows.