Biodiversity and performance of constructed wetlands; a comparison with natural wetlands

Abstract

Natural wetlands (NWs) are one of the most economically and ecologically important habitats on Earth, currently comprising about 6% of the world’s surface area. They provide many ecosystem services including the supply of fresh water, food and building materials, water quality improvement, biodiversity support and flood mitigation. The loss of NWs (> 50%) globally over the past two centuries has, however, greatly reduced their facilitation in water purification and wastewater treatment. For this reason, various types of artificial wetlands (constructed wetlands; CWs) have been designed to assist in the removal of a range of pollutants from wastewaters, and accordingly, improve water quality.

Constructed wetlands have several advantages in comparison to conventional wastewater treatment systems. They are a sustainable, green system requiring lower operation and maintenance costs. In addition, the vegetation in CWs assists in many important pollutant removal mechanisms including sedimentation, filtration and plant uptake of metals and nutrients. However, a paucity of information exists on metal and nutrient accumulations within vegetation in CWs, with many studies focusing on concentrations only. As a result, best practices for the harvesting of CW vegetation as a means of metal / nutrient removal, is lacking. This study addresses this significant knowledge gap in CW performance via biomass harvesting of CW vegetation. The additional benefits to biodiversity provided by CWs have received comparatively less attention than their capacity in wastewater treatment. This provides the incentive for the second aim of this study where the biodiversity value of CWs in comparison to that of NWs is assessed. For the first time, the suitability of terrestrial habitats surrounding CWs and NWs for the protected smooth newt is compared, with a view to recommending newt-friendly changes to existing and future CW design. Similarly, marsh flies (Diptera: Sciomyzidae) which are recognised bioindicators of wetlands, are used to quantify, for the first time, the value of CWs to aerial invertebrate diversity. This study also assesses the impacts of water quality and the habitats surrounding CWs on marsh flies since no systematic study has examined this to date. The results of the study show that the concentrations and accumulations of metals and nutrients in CW vegetation follow contrasting seasonal patterns. Some metals and nutrients measured in the belowground (BG) biomass were greater than 80% of the more commonly measured aboveground (AG) biomass suggesting that analysis of emergent shoots only may significantly underestimate the metal and nutrient uptake capacity of CW vegetation. Based on the results of the study, it is important to schedule harvesting at specific times of the year to coincide with maximum accumulations of specific metals and nutrients in CW plants. The study also shows that CWs present an opportunity to compliment biodiversity in the locations in which they are placed. The results of a Habitat Suitability Index (HSI) whereby each CW and NW received a score, concluded that appropriate management of the areas immediately surrounding CWs can provide habitat for the protected smooth newt and recommendations to improve new and existing CWs as newt-friendly habitat were crafted. Marsh fly assemblages are similarly influenced by habitats surrounding CWs. In addition, the potential value of CWs to marsh fly conservation is evidenced by over one third of the Irish sciomyzid fauna being represented in the eight CWs in this study, including four species listed as scarce or threatened in the UK. In conclusion, the results of this study have reinforced the notion which suggests that CW treatment performance is better when plants are present, due to the uptake capacity and accumulation of metals and nutrients into the CW vegetation. The results have elucidated the seasonal patterns of metals and nutrients in AG and BG biomass in a temperate oceanic climate, and provide recommendations on removal via vegetation harvesting, which could prevent potential pollution events in receiving waters. In addition, CWs can now be viewed as crucial in providing habitat to species of conservation concern such as the smooth newt, and scarce and threatened sciomyzid flies, that may be otherwise absent in the surrounding landscape in which CWs are placed. Extensive recommendations to include minor modifications to the future design and management of CWs for smooth newts and marsh flies are provided, which can also be applied to enhance CWs for other wildlife groups and species of conservation concern.