Water level management of wetlands in response to current and future RCP climate change scenarios

Wetlands are integral components of our natural environment since they provide many critical ecosystem services to humanity, such as water purification, climate change mitigation, flood control, and biodiversity. Although wetlands can convey resilience to some degree of changes, they might be vulnerable to climate change. Altered hydrology and rising temperatures can turn the wetland services into disservices. This thesis investigates the impact of different climate scenarios, both current and future climate scenarios, on wetland ecosystems, natural (peatland), and constructed wetlands. Since 2017, a mesocosm experiment has been conducted within four climate chambers to simulate current (2017-2020 in the experiment and 2016-2019 in real-time) and future climate scenarios based on representative concentration pathways scenarios (RCP 2.6, RCP 4.5, RCP 8.5) for peatlands and constructed wetland. The main aim of the thesis was to investigate the impact of climate change scenarios and water level management on 1) water quality (physiochemical changes in peatland and constructed wetland outflow) and 2) carbon dioxide emissions from peatland. The results of the experiment revealed that the short-term (one year) impact of climate change simulation on the water purification function of both constructed wetland and peatland (ombrotrophic bog) is insignificant when the hydrology of the systems is not a stress factor (no occurrence of flood or drought). However, the response of the water purification function of two different wetlands, constructed wetland and peatland, showed a contrasting tendency along with the increasing temperature trend of climate scenarios. This suggests that while the water quality of constructed wetland may improve with future warmer climate scenarios (no water stress), the water quality of peatland may gradually deteriorate. The findings of a longer-term (three-year) simulation of climate change and water level control on peatland water quality demonstrated that climate change had still no significant effect on peatland water quality, however, the water level control had a substantial impact. Water level management during the drought and post-drought period could improve the water quality of managed mesocosms by 2-50 times compared to the unmanaged system. The investigation of climate change and water level management impact on CO2 sink function of peatland suggested that climate change alone might not have a significant impact. However, the influence of water level management, both alone and in interaction with climate change, can have a significant impact on the CO2 sink function of peatland. The most favorable influence of water level management on CO2 sink function was found in peatland mesocosms under warmer climate scenarios (RCP 4.5 and RCP 8.5) due to the enhanced growth of vascular plants, but this was not the case for the systems under the colder climate scenarios, current and RCP 2.6 climate scenario. In brief, water level management during drought is necessary for warmer climate scenarios such as RCP 4.5 and RCP 8.5 to prevent the system from shifting from CO2 sink to source, while it is unnecessary for the current climate and RCP 2.6.

Wetlands are critical components of our natural environment. They cover only about 8% of the world's land surface but store 29-45% of the terrestrial organic carbon (C). Wetlands provide many critical ecosystem services, including flood control, water purification, biodiversity, cultural values, recreation, tourism, and climate change mitigation. Moreover, wetlands have been acknowledged as nature-based solutions due to their regulatory services, the most important of which are climate and water quality regulation. A well-managed and healthy wetland ecosystem can be resilient to the natural extreme events such as drought and flooding. This resilience, however, is dependent on the intensity and degree of management.
Wetlands, despite their resilience and benefits, are vulnerable to changes, particularly climate change. There are a lot of concerns regarding wetland ecosystems under climate change due to their susceptibility to hydrology. This is due to the fact that wetlands are regarded as transitional land between terrestrial and aquatic systems, with the water table typically being at or near the land. In a natural state, this waterlogged condition causes an anoxic environment that result in inefficient decomposition, exceeding the production, storing a large amount of carbon in the wetland, making them a sink of carbon. Furthermore, one of the most important regulatory services of wetlands, climate change mitigation, owes to high productivity of this ecosystem.
These essential services of wetlands might shift to disservices under climate change due to higher temperature and lowered water levels as a result of increased rates of evapotranspiration, that might cause higher rate of aerobic decomposition. For example, studies have reported that decomposition of wetland can lead to nutrient release into the soil water and eventually can be exported downstream, causing major environmental issues such as eutrophication, acidification, and brownification in the aquatic system. Furthermore, higher temperatures stimulate heterotrophic respiration of organic matter by aerobic bacteria that might result in greater rate of CO2 release than CO2 uptake, causing the wetland to switch from sink to source of CO2. Given all of the concerns about the wetland ecosystem and its functions, the management strategies that can protect the essential functions of wetlands in the face of climate change are crucial.
This thesis investigates the impact of different climate scenarios, both current and future climate scenarios, on wetland ecosystems, natural (peatland), and constructed wetlands. Since 2017, a mesocosm experiment has been conducted within four climate chambers to simulate current (2017-2020 in the experiment and 2016-2019 in real-time) and future climate scenarios based on representative concentration pathways scenarios (RCP 2.6, RCP 4.5, and RCP 8.5) for peatlands and constructed wetland. The main aim of the thesis was to investigate the impact of climate change.
scenarios and water level management on 1) water quality (physiochemical changes in peatland and constructed wetland outflow) and 2) carbon dioxide emissions from peatland.
The thesis includes a critical literature review (Paper I) in which the important experimental wetland studies in the literature were reviewed. Moreover, the main climate change drivers, which affect the wetland functions, water availability, and temperature, were identified and discussed. The uncertainties in the literature, as well as existing gaps in the methodologies for climate change research on wetlands, were highlighted, and a new comprehensive framework was introduced for future climate change studies on wetlands. Moreover, the results of the experiment were presented in three peer-reviewed journal papers demonstrating: 1) the impact of climate change scenarios and wetland type on wetland water quality when all the mesocosms were subjected to water level management during the first year of the experiment (Paper II); 2) the impact of climate change, drought, and water level management on the carbon dioxide (CO2) sink function of peatland mesocosms (during 2018-2020), when the water level was managed in half of the mesocosms and left unmanaged in the other half (Paper III); 3) the impact of climate change and water level management on the water quality of peatland mesocosms over the whole period of the experiment for this Ph.D. thesis (2017-2020) (Paper IV).