Plant structural diversity alters sediment retention on and underneath herbaceous vegetation in a flume experiment

Sediment retention is a key ecosystem function provided by floodplains to filter sediments and nutrients from the river water during floods. Floodplain vegetation is an important driver of fine sediment retention. We aim to understand which structural properties of the vegetation are most important for capturing sediments. In a hydraulic flume experiment, we investigated this by disentangling sedimentation on and underneath 96 vegetation patches (40 cm x 60 cm). We planted two grass and two herb species in each patch and conducted a full-factorial manipulation of 1) vegetation density, 2) vegetation height, 3) structural diversity (small-tall vs tall-tall species combinations) and 4) leaf pubescence (based on trait information). We inundated the vegetation patches for 21 h in a flume with silt- and clay-rich water and subsequently measured the amount of accumulated sediment on the vegetation and on a fleece as ground underneath it. We quantified the sediment by washing it off the biomass and off the fleece, drying the sediment and weighting it. Our results showed that all manipulated vegetation properties combined (vegetation density and height, and the interaction of structural diversity and leaf pubescence) explained sedimentation on the vegetation (total R2 = 0.34). The sedimentation underneath the vegetation was explained by the structural diversity and the leaf pubescence (total R2 = 0.11). We further found that vegetation biomass positively affected the sedimentation on and underneath the vegetation. These findings are crucial for floodplain management strategies with the aim to increase sediment retention. Based on our findings, we can identify management strategies and target plant communities that are able to maximize a floodplain’s ability to capture sediments.

Evaluating stream CO2 outgassing via Drifting and Anchored flux chambers in a controlled flume experiment

<p>Carbon dioxide (CO<sub>2</sub>) emissions from running waters represent a key component of the global carbon cycle. However, quantifying CO<sub>2</sub> fluxes across air-water boundaries remains challenging due to practical difficulties in the estimation of reach-scale standardized gas exchange velocities (k<sub>600</sub>) and water equilibrium concentrations. Whereas craft-made floating chambers supplied by internal CO<sub>2</sub> sensors represent a promising technique to estimate CO<sub>2</sub> fluxes from rivers, the existing literature lacks of  rigorous  comparisons  among  differently  designed chambers and deployment techniques. Moreover, as of now the uncertainty of k<sub>600</sub> estimates from chamber data has not been evaluated.  Here, these issues were addressed analyzing the results of a flume experiment carried out in the Summer of 2019 in the Lunzer:::Rinnen - Experimental Facility (Austria). During the experiment, 100 runs were performed  using two different chamber designs (namely, a Standard Chamber and a Flexible Foil chamber with an external floating system and a flexible sealing) and two different deployment modes (drifting and anchored). The runs were performed using various combinations of discharge and channel slope, leading to variable turbulent kinetic energy dissipation rates (1.5 10<sup>-3</sup>< ε < 1 10<sup>-1</sup> m<sup>2</sup> s<sup>-3</sup>). Estimates of gas exchange velocities were in line with the existing literature (4 < k<sub>600</sub> < 32 m d<sup>-1</sup>), with a general increase of k<sub>600</sub> for larger turbulent kinetic energy dissipation rates. The Flexible Foil chamber gave consistent k<sub>600</sub> patterns in response to changes in the slope and/or the flow rate. Moreover, Acoustic Doppler Velocimeter measurements indicated a limited increase of the turbulence induced by the Flexible Foil chamber on the flow field (22 % increase in ε, leading to a theoretical 5 % increase in k<sub>600</sub>).<br>The  uncertainty  in  the  estimate  of  gas  exchange  velocities  was  then estimated  using  a  Generalized Likelihood Uncertainty Estimation (GLUE) procedure. Overall, uncertainty in k<sub>600</sub> was moderate to high, with enhanced uncertainty in high-energy setups. For the anchored mode, the standard deviations of k<sub>600</sub> were between 1.6 and 8.2 m d<sup>-1</sup>, whereas significantly higher values were obtained in drifting mode. Interestingly, for the Standard Chamber the uncertainty was larger (+ 20 %) as compared to the Flexible Foil chamber.  Our study suggests that a Flexible Foil design and the anchored deployment might be useful techniques to enhance the robustness and the accuracy of CO<sub>2</sub> measurements in low-order streams. Furthermore, the study demonstrates the value of analytical and numerical tools in the identification of accurate estimations for gas exchange velocities.<br>These findings have important implications for improving estimates of greenhouse gas emissions and reaeration rates in running waters.</p>