Large spatiotemporal variability in metabolic regimes for an urban stream draining four wastewater treatment plants with implications for dissolved oxygen monitoring

Abstract

Urbanization and subsequent expansion of wastewater treatment plant (WWTP) capacity has the potential to alter stream metabolic regimes, but the magnitude of this change remains unknown. Indeed, our understanding of downstream WWTP effects on stream metabolism is spatially and temporally limited, and monitoring designs with upstream-downstream comparison sites are rare. Despite this, and despite observed spatiotemporal variability in stream metabolic regimes, regulators typically use snapshot monitoring to assess ecosystem function in receiving streams, potentially leading to biased conclusions about stream health. To address these important practical issues, we assessed the spatiotemporal variability in stream metabolism at nine sites upstream and downstream of four WWTPs in a suburban stream. We used one year (2017–2018) of high-frequency dissolved oxygen (DO) data to model daily gross primary productivity (GPP) and ecosystem respiration (ER). We found that GPP was 1.7–4.0 times higher and ER was 1.2–7.2 times higher downstream of the WWTPs, especially in spring when light was not limited by canopy shading. Critically, we observed that these effects were spatially limited to the kilometer or so just downstream of the plant. These effects were also temporally limited, and metabolic rates upstream of WWTPs were not different from sites downstream of the plant after leaf-out at some sites. Across sites, regardless of their relation to WWTPs, GPP was positively correlated with potential incident light suggesting that light is the dominant control on GPP in this system. Temporal windowing of DO to proposed regulatory monitoring lengths revealed that the violation frequency of water quality criteria depended on both the monitoring interval and start date. We conclude that spatiotemporal variability in metabolism and DO are crucial considerations when developing monitoring programs to assess ecosystem function, and that evidence of WWTP effects may only arise during high light conditions and at limited scales.

Stability in headwater chemical signatures across a dynamic flow regime in a highly urbanized Piedmont catchment

“…Our synoptic sampling campaign aimed to characterize the effects of urbanization on the temporal and spatial stability of stream ion chemistry, as well as to understand the dominant spatial scales (i.e., the size and distribution of source/sink landscape patches) which control the export of major ions in a highly developed watershed. Our results show pronounced stability of SO 4 2− , Ca 2+ , K + , NO 3 − within the USRW over time (Figure 2a and 4), contrary to the idea of temporal heterogeneity that is sometimes thought to be characteristic of urban systems (Ledford et al, 2021; Ledford & Lautz, 2015). Our finding of pronounced spatial stability over time despite expected heterogeneity fit with the pattern observed at 56 headwaters in France over a 12‐year period (Abbott et al, 2018).…”

River metabolic fingerprints and regimes reveal ecosystem responses to enhanced wastewater treatment

“…Thus, the cycling of nutrients can both depend upon, and influence, the metabolic regimes of rivers. Increasing availability of multiyear high temporal resolution DO timeseries are now providing new insights into drivers that shape metabolic regimes and fingerprints in different climatic regions and land use settings (Diamond et al, 2021;Ledford et al, 2021;Pearce et al, 2020). Accordingly, there is growing interest in the insights that river metabolism can provide into river ecosystem health and function (Aristi et al, 2015;…”

“…Thus, the cycling of nutrients can both depend upon, and influence, the metabolic regimes of rivers. Increasing availability of multiyear high temporal resolution DO timeseries are now providing new insights into drivers that shape metabolic regimes and fingerprints in different climatic regions and land use settings (Diamond et al., 2021; Ledford et al., 2021; Pearce et al., 2020). Accordingly, there is growing interest in the insights that river metabolism can provide into river ecosystem health and function (Aristi et al., 2015; Halliday et al, 2016; Koenig et al., 2019; Rode et al., 2016; Sudduth et al., 2011; Yates et al., 2018), including nutrient cycling (Jarvie et al., 2018a, 2020; O'Donnell & Hotchkiss, 2019; Plont et al., 2020), the effects of climate and land use change (Cross et al., 2022; Griffiths et al., 2013; Gucker et al., 2009; Ledford et al., 2017; Yates et al., 2013), and as a tool in environmental management (Chowanski et al, 2020; Cohen et al, 2013; Halliday et al., 2015; Jankowski et al., 2021).…”

Environmental injustice and Escherichia coli in urban streams: Potential for community‐led response

“…Technological advances in wastewater treatment have increased our ability to remove solid material from the waste stream and to filter, disinfect, and even use tertiary treatments like denitrification to remove nutrients, improving downstream water quality (Carey & Migliaccio, 2009; Ledford, Diamond, & Toran, 2021; Ledford, Kurz, & Toran, 2021; Ledford & Toran, 2020). The requirement that wastewater is treated before discharging to waterbodies in the United States was driven by the development of multiple laws, including the Clean Water Act (CWA, Box 1).…”