The potential of wetlands for mitigation of eutrophication and brownification

Abstract

Increasing nutrient concentrations (eutrophication) and water color due to dissolved organic matter (brownification) pose continuous threats to freshwater ecosystems worldwide, threatening ecosystem services like drinking water supply. Changing the increasing trends of eutrophication and brownification on our aquatic ecosystems is challenging. However, constructed wetlands could be a potential local, or regional, scale mitigation measure. Wetlands are highly diverse ecosystems of high importance to humans. They provide several ecosystem services, such as nutrient retention, carbon sequestration and flood protection as well as increased biodiversity. They remove both nitrogen and phosphorus by several in-system processes. However, drought vulnerability can lead to that nutrients and organic matter are released upon rewetting, demanding careful site selection for construction. Additionally, wetlands may have a potential to mitigate eutrophication and brownification simultaneously, since shallow water and longer retention time enable in-system processes such as denitrification, photooxidation and microbial degradation of humic substances, suggesting unexplored multifunctional benefits. My thesis explores the multifunctional potential of wetlands for mitigation of eutrophication and brownification simultaneously. Specifically, I aimed at assessing the capacity of wetlands to reduce total organic carbon (TOC), water color, total nitrogen (TN) and total phosphorus (TP), as well as the potential to reduce algal growth potential (AGP), i.e. the potential for algal blooms. I also aimed at assessing if common primary producers differ in their efficiency to reduce nutrient concentrations, TOC and water color, and AGP. To fulfil the aims of this thesis I performed both fieldwork and laboratory experiments. Additionally, I used future scenarios to estimate the potential of wetlands to mitigate eutrophication and brownification of downstream lakes. Wetlands displayed a high variability in reducing TN, TP, TOC and water color, both temporally and spatially. Some wetlands reduced nutrient concentrations, TOC, and water color simultaneously, whereas others were less efficient. Efficiency peaked in summer and with continuous water flow. Wetlands also reduced the growth potential of phytoplankton in a downstream lake. I also show that there are both temporal within and between wetland variations. Wetlands in catchments dominated by agriculture or pastureland often experiences larger fluctuations in algal growth potential than wetlands in catchments dominated by forests. The common primary producers, Elodea, Myriophyllum, and filamentous algae, varied in their abilities to reduce TN, TP, TOC and water color. Elodea was the most efficient of the three macroflora in reducing all targeted parameters, whereas Myriophyllum and filamentous algae reduced nitrogen, phosphorus, and water color, but increased the concentration of TOC. This highlights the potential benefits of diverse plant communities in constructed wetlands, likely allowing for a higher overall efficiency of the system compared to monocultures. Different phytoplankton groups displayed diverse responses when exposed to water previously inhabited by the macroflora. Cyanobacteria and diatoms generally had a lower growth potential when Elodea and Myriophyllum had previously grown in the water, while green algae generally were not affected by the macroflora. Efficient wetlands might mitigate eutrophication and brownification, but inefficient wetlands may instead exacerbate these environmental problems, harming downstream lake ecosystems by increasing nutrient and water color levels. Efficient wetlands can delay the progress of eutrophication and brownification in aquatic systems, providing a time window which could be of importance for implementing additional restoration efforts or exploring alternative solutions to counteract or even reverse these ongoing environmental problems. Collectively, my thesis shows that constructed wetlands often, even in winter, have the potential to reduce nitrogen, phosphorus, organic carbon, water color as well as reducing the growth potential of different phytoplankton groups. A high biodiversity of plants could also prove beneficial to increase the efficiency of the wetlands. Hence, my thesis highlights the potential of some, but not all, wetlands as multifunctional tools for mitigation of both eutrophication and brownification.

• author: Borgström, Anna
• supervisor: Lars-Anders Hansson; Johanna Sjöstedt; Anders Persson
• opponent: Professor Hambäck, Peter, Stockholm University
• organization: Aquatic Ecology (research group)
• publishing date: 2024
• type: Thesis
• publication status: published
• subject: Ecology
• keywords: Brownification, Constructed wetlands, Cyanobacteria, Eutrophication, Filamentous algae, Green algae, Macrophytes, Multifunctionality, Nitrogen, Phosphorus, Total organic carbon, Water color
• pages: 200 pages
• defense location: Blue Hall, Dep. of Biology.
• defense date: 2024-04-05 09:00:00
• ISBN: 978-91-8039-958-6
• ISBN: 978-91-8039-959-3
• project: Local scale action against global threats: Securing freshwater ecosystem function and services in a future scenario
• language: English
• LU publication?: yes
• id: 11b56558-9a39-4f1b-ac3c-e2bde138fbf2
• date added to LUP: 2024-03-01 14:21:44
• date last changed: 2024-03-12 12:50:32