A Decade of Ecosystem-Scale Research at an Aridland Constructed Treatment Wetland

Daniel L Childers

doi:10.3389/fenvs.2020.576936

Abstract

Cities are increasingly pursuing more resilient and sustainable futures. One way to do so is by the increased use of Urban Ecological Infrastructure (UEI), including constructed treatment wetlands (CTW). This strategy is particularly important for aridland cities with scarce water resources. In this paper I synthesize nearly ten years of systems-level research at the Tres Rios CTW in Phoenix, AZ USA. Since July 2011, a research team that includes dozens of student volunteers has been sampling monthly for herbaceous biomass and productivity, water quality, transpiration rates, and aquatic metabolism. We also quantify belowground biomass and plant tissue nutrient content annually, and measured greenhouse gas fluxes from 2012 - 2014. Our peak summer biomass values are among the highest reported in the literature, and high rates of transpiration are associated with this biomass. Using our whole-system water budgets and tracer studies we have documented a slow movement of surface water into the marsh from adjacent open water areas that is driven by transpirational losses and that we refer to as the “biological tide”. With our nitrogen (N) budgets for the whole system and the vegetated marsh we showed that roughly 50% of the annual N uptake by the vegetated marsh is driven by new water entering via this biological tide. Our aquatic metabolism sampling suggested that the N uptake associated with the autotrophic water column was roughly 27% of the average annual N uptake by the vegetated marsh. The marsh is a source of CH4 and N2O across the air-water interface and the plants are a net source of CH4 but a net sink for N2O. Our combined flux estimates suggest that the Tres Rios marshes are a net sink for greenhouse gas equivalents because of this plant-mediated net uptake of N2O. Finally, over the years our Tres Rios CTW project has provided a platform for dozens of students and young people to experience ecological research, both in the field and in the lab.

Highlights

The global human population is becoming increasingly urban, with over 50% of all people currently living in cities (Wigginton et al, 2016) and increasing to a likely 80% by 2050 (Grimm et al, 2008)
The plant community was relatively diverse when the system was new, in 2011 and 2012, but by 2014 Schoenoplectus americanus and Schoenoplectus maritimus had been effectively extirpated from the marsh (Figure 2)
Constructed treatment wetlands have been an important component of critical Urban Ecological Infrastructure (UEI) for decades, and their use continues to increase as cities search for more adaptive, flexible, and economical ways to manage wastewater and stormwater challenges (Greenway, 2005; Nivala et al, 2013; others)

Summary

Introduction

The global human population is becoming increasingly urban, with over 50% of all people currently living in cities (Wigginton et al, 2016) and increasing to a likely 80% by 2050 (Grimm et al, 2008). Cities have transformed into “sanitary cities” that rely on highly centralized, engineered, and expensive infrastructure to keep inhabitants healthy (Melosi, 2000; Grove, 2009). These “gray” infrastructure strategies are largely able to meet short-term demands. Urban Ecological Infrastructure (UEI) is all infrastructure in a city that supports ecological structure and function and provides ecosystem services to urban residents. This broad, all-encompassing concept for nature in cities includes obvious forms, such as parks, residential yards, community gardens, lakes and rivers, and street trees. My focus here is on wetland UEI; on a type of wetland UEI that is being increasingly used to adaptively manage urban water resources: Constructed Treatment Wetlands

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