Linking stormwater control performance to stream ecosystem outcomes: incorporating a performance metric into effective imperviousness

Stormwater control measures, such as raingardens, tanks, or wetlands, are often employed to mitigate the deleterious effects of urban stormwater drainage on stream ecosystems. However, performance metrics for control measures, most commonly pollutant-load reduction, have not permitted prediction of how they will change stream ecosystems downstream. Stream ecosystem responses have more commonly been predicted by catchment-scale measures such as effective imperviousness (percentage of catchment with impervious cover draining to sealed drains). We adapt effective imperviousness, weighting it by a performance metric for stormwater control measures aimed at stream protection, the stream stormwater impact metric. Weighted effective imperviousness can serve as a predictor of stream response to stormwater control. We demonstrate its application in a before-after-control-reference-impact experiment aiming to test if stream health is improved by dispersed stormwater control measures. Trends in weighted effective imperviousness showed wide variation in degree of stormwater control achieved in the six experimental sub-catchments, despite similar effort in implementing control measures across the sub-catchments. Greater reductions in weighted effective imperviousness (on a log-scale, on which stream response is predicted) per unit effort were observed in smaller catchments with lower starting effective imperviousness. While implementation of control measures was sufficient to expect a stream response in at least two of the experimental sub-catchments, we did not achieve the reduction in effective imperviousness that we were aiming for. Primary limitations to success were the lack of available space in these established suburbs, particularly for final control measures near pipe outlets into streams, and a lack of demand for harvested stormwater. The use of the continuous variable, weighted effective imperviousness, to measure impact on streams, and the protracted period of SCM implementation that varied among catchments, required a new approach to modelling “before-after-control-impact” experiments, which has potentially broader application.

Streambed pollution: A comprehensive review of its sources, eco-hydro-geo-chemical impacts, assessment, and mitigation strategies

Streambeds are among the important components of stream ecosystems and support several critical ecosystem services such as transformation of organic matter and nutrients and provide habitat for aquatic organisms. Increasing anthropogenic influence introduces multiple stressors to the stream networks resulting in pollution of streambeds, which in turn, could have detrimental effects on overall stream ecosystem health. However, there are gaps in the current understanding of the impacts of streambed pollution and the mitigation strategies lack holistic approach. In this review, we first present a global inventory to highlight the status of streambed pollution around the globe. Next, we synthesize the state-of-art knowledge of conventional and emerging forms of contaminants, their overall impacts on stream ecosystem functions, and finally present future directions to comprehend the problem of streambed pollution. We highlight that fine sediments and plastics (found especially in urban streambeds) are among the major physical pollutants of streambed pollution and the chemical pollutants generally comprise of hydrophobic compounds including various legacy contaminants such as polychlorinated biphenyl (PCB), dichlorodiphenyltrichloroethane (DDT), a wide range of pesticides and a variety of heavy metals. Further, in recent years, highly polar and hydrophilic emerging contaminants such as micro-plastics, pharmaceutical waste and personal care products have been identified in rivers around the world. We stress that the impacts of streambed pollution have been largely studied with discipline-driven perspectives amongst which the ecological impacts have received a lot of attention in the past. To present a comprehensive outlook, this review also synthesizes the hydrological, geomorphological and biochemical impacts of different forms of streambed pollutants. In the end, we endorse the positive and negative aspects of the current impact assessment methodologies and also highlight various physical, chemical and biological remediation measures that could be applied to alleviate streambed pollution.

Ecological Responses of Macroinvertebrates to an In-Stream Ecosystem Restoration Technique in a Tropical Stream in Eastern Uganda

A field experiment was conducted to examine the ecological responses of macroinvertebrates to an in-stream ecosystem restoration technique called woody debris introduced in a stream in different arrangements to show how they (woody debris) affected the macroinvertebrate ecology, specifically assemblage composition and biometrics in River Nabongo. The experiment was carried out in two heterogeneous stream environments i.e., i) in a riffle found in the middle reaches of the river at a higher altitude and ii) a pool in the lower altitude and reaches of the river. Each of these two treatments had a control plot for comparison purposes. Four macroinvertebrate sampling campaigns were launched in experimental sites from September 2019 to April 2021. All restoration structures had more macroinvertebrates than control and pre-treatment sites. The introduction of simple structures at the riffle site led to an increase in collector-filterers from 9-128 individuals, while at the pool site all the structures increased macroinvertebrates by 1151 individuals. Taxon richness was highest in the complex plot with 14±0.41 which significantly differed from the rest of the sampling plots at P<0.05. The relative abundance of taxa at the pool site varied significantly from one sampling plot to another at P<0.05 with the highest mean abundance registered in complex and simple structures having 61.3±0.10 and 23.5±0.11 respectively. It was concluded that complex woody debris structures increase the diversity, abundance and richness of aquatic macroinvertebrates by providing hard substrates for colonization by algae and microorganisms on which macroinvertebrates feed. We recommended that other researchers should study the impact of other in-stream ecosystem restoration techniques such as floating islands, constructed wetlands, D-deflectors, a comparison of which with restored woody debris will enable ecologists to choose the most suitable technique to apply at different stream points.