Macrophyte Controls on Urban Stream Microbial Metabolic Activity

Abstract

Urban rivers worldwide are affected directly by macrophyte growth, causing reduced flow velocity and risks of flooding. Therefore, cutting macrophytes is a common management practice to ensure free drainage. The impacts of macrophyte removal on transient storage dynamics and microbial metabolic activity of wastewater-fed urban streams are unknown, preventing any assessment of the hydrodynamic and biogeochemical consequences of this management practice. Slug tracer injections were performed with the conservative tracer uranine and the reactive tracer resazurin to quantify the implications of macrophyte cutting on stream flow dynamics and metabolism. Macrophyte cutting reduced mean tracer arrival times in managed stream reaches but did not significantly decrease whole-stream microbial metabolic activity. In fact, transient storage indices were found to have increased after cutting, suggesting that macrophyte removal and the resulting increase in flow velocity may have enhanced hyporheic exchange flow through streambed sediments. Our results evidence that macrophyte cutting in nutrient-rich urban streams does not necessarily lead to lower in-stream storage and metabolism but that the gain in hyporheic exchange and streambed microbial metabolic activity can compensate for reduced in-stream storage. Increased stream flow resulting from macrophyte removal may therefore even enhance nutrient and pollutant attenuation capacity of streambed sediments.

Time Series of Electrical Conductivity Fluctuations Give Insights Into Long‐Term Solute Transport Dynamics of an Urban Stream

“…This observation agrees with the independent analysis of the tracer injections in river Erpe originally described by Romeijn et al. (2021). Comparing several slug tracer injection before and after mowing along Section 1, transient storage indices were found to have increased.…”

“…Irrespective of the reduction in t adv , higher median A S occurring after mowing indicates also an absolute increase in transient storage. This observation agrees with the independent analysis of the tracer injections in river Erpe originally described by Romeijn et al (2021). Comparing several slug tracer injection before and after mowing along Section 1, transient storage indices were found to have increased.…”

“…A reason could be the truncation of the breakthrough curves below 0.1 μg L −1 , which was the detection limit of the fluorometers. Especially for the curves at Station B, this led to a low representation of the tailing in the analyzed data (Romeijn et al., 2021). To test whether model parameter estimates based on the 48‐hr EC curves are suitable to model theoretical concentration curves, the parameters of all 48‐hr windows that contain the 8‐hr recording of the tracer tests were used to model theoretical concentration curves at Station B (Figure 6).…”

“…In June 2016 several slug injections of a fluorescent tracer were performed in River Erpe along Section 1. The detailed methods and analysis of the tracer tests are described in Romeijn et al (2021). For the purpose of the present study, we aimed to use two of the slug tests (Injection 1 starting at hour 1,640 [before mowing] and Injection 2 starting at hour 1,760 [after mowing]) to compare the resulting parameter estimates to the results of the time series analysis of EC curves.…”