Optimising multi-pollutant phytoremediation strategies to sustainably improve raw water quality

Abstract

Surface waters are vital for supporting people and ecosystems; however, freshwater quantity and quality is under increasing pressure from multiple stressors such as diffuse pollution and climate change that can both impair water quality and reduce ecosystem-service provision. Diffuse pollutant impacted freshwaters can often contain ‘cocktails’ of multiple pollutants such as nitrogen (N), phosphorus (P), and heavy metals such as iron (Fe), copper (Cu) and Zinc (Zn). However, nature-based solutions (NbS), such as aquatic phytoremediation that capitalises on the ability of macrophytes (aquatic plants) to remove and sequester pollutants from freshwaters whilst simultaneously providing ecosystem services, have the potential to tackle the multi-faceted challenges that are associated with pollutant-impacted waters. The overarching aim of this study was to optimise a series of phytoremediation strategies that could improve water quality and freshwater ecosystems. The research was primarily framed around the plant community paradigm of the ‘mass ratio hypothesis’ and the plant strategies ‘Competitor-Stress tolerator-Ruderal’ (CSR) frameworks. A mixed-method approach including a field survey, a mesocosm experiment, and field trail was employed to develop a new plant community-based approach for optimising aquatic phytoremediation.

Firstly, the phytoremediation potential of different inorganic pollutants within wild stands of macrophytes and floating treatment wetlands was quantified. Standing stocks of macronutrient-type pollutants had strong positive significant correlations with sampled plant community biomass, while standing stocks of Cr, Cu and Mo micronutrient-type pollutants were positively correlated with biomass. Conversely, Fe, Mn, and Zn standing stocks were not correlated with biomass. Understanding wild macrophyte communities based on their overall CSR strategy representation with a simulated harvest regimes showed that optimal harvest strategies could be developed.

Secondly, mesocosm experiments demonstrated strong negative correlations between concentrations of Ca, K, P and Zn and maximum root length; although some pollutants were not effectively removed using phytoremediation e.g., Cr and Fe. On average, over 24% of Mn, TIN and P removal was by accounted for by plant uptake, which means it is an important factor for removal of these pollutants. However, plant uptake mechanisms only removed <4% of the total Zn, K, Cu, Ca, Mg and Na from experimental systems.

Finally, a field trial comprised of floating treatment wetlands (FTWs) planted with various plant communities was used to quantify those ecosystem services provided by phytoremediation systems beyond just pollutant removal. However, there was no significant difference in invertebrate community assembly between monoculture and polyculture FTWs indicating that this service may be equally provided by most community types. However, FTWs can be used to support macroinvertebrate communities suggesting these systems can also be utilised to increase habitat within freshwaters.

Plant community structure in phytoremediation systems was found to be a key determinant of success of phytoremediation and ecosystem service provision. Communities structured by plant height can moderate levels of ecosystem multi-functionality (i.e., number and quantity of ecosystem service provision). Small-emergent communities outperformed all other community types due to their increased provision of both regulation and maintenance, cultural and provisioning services. Conversely, large emergent communities that are more typical candidates for phytoremediation had the highest levels of multifunctionality only when function performance was lower. In terms of pollutant removal, increasing species diversity allowed more pollutants to be targeted overall. However, depending on the pollutant, the removal of single pollutants was more effective when single macrophyte species were used.

This research demonstrates the importance of plant community as a factor in designing phytoremediation strategies for improving water quality and providing ecosystem services. By considering the key outputs of this thesis, phytoremediation can be employed in a range of impacted freshwaters with different issues to remove pollutants and improve habitat quality. Therefore, affirming phytoremediation as a NbS that can be utilised to tackle interconnected challenges including diffuse pollution and resource depletion.