Full title: Optimizing future wetlands for the provision of multiple ecosystem services
In a future, unstable climate with frequent extreme weather events, the wetland ecosystem service water retention will increase in societal relevance, especially in regions where rainfall is already intermittent and droughts increase in frequency. It is thus imperative to design wetlands for high water retention.
We need to understand how water retention can be maximised without affecting other vital ecosystems, such as water purification. In the project we therefore aim to clarify how hydrological regime and optimized water retention in wetlands may affect nutrient removal, and to give recommendations on wetland design and restoration for multi-functionality in a future climate.
In order to ensure versatile wetland function also under future climate scenarios, the project aims to:
Investigate how wetlands optimized for high water retention function in terms of nutrient removal.
Investigate the underlying mechanisms driving both ecosystem services under varying climate scenarios.
Give recommendation on wetland designs that optimize both these ecosystem services, water retention and water purification.
By combining analyses of empirical data from large wetlands across geographical regions, experimental wetlands and literature we will be able to develop a best practice guide for wetland design to help prioritize where and how – according to their geographical location – wetlands should be implemented for optimized wetland multifunctionality.
Our proposed research will aid stakeholders to enhance water retention at the same time as water purification. The generated knowledge from the project is relevant to preserving current and future wetland ecosystem services, and thus increases Sweden’s future sustainability and resilience to climate change.
The experiment is set-up to investigate effects on nutrient removal of optimizing wetlands for water retention. We define wetlands optimized for water retention as wetlands with an outlet design that enables a controlled decrease in water level after wetland saturation. We will implement this design in half the wetlands with a hole at the base of the outlet pipe (see figure below). The holes will be small enough to allow basins to fill during high flow event, but large enough to gradually empty the basin afterwards (thus resetting the wetland’s water retention capacity). Wetlands not optimized for water retention will not have a controlled decrease in water level after a high flow event, although, some decrease is expected due to evapotranspiration. The treatments will be distributed in the experimental wetland facility in a randomized block design (see figure below).
All wetlands will be exposed to hydrological regimes ranging from high flow (flood) to no flow (drought). Under these hydrological regimes, we will test the effects of water retention optimization in wetlands of different designs; depth and size. We will simulate wetland size differences by varying hydraulic load. Wetlands with simulated large size will be shallow and have a lower hydraulic loading rate than other wetlands, as a larger wetland would (even under high flow events).
a) Overview of the experimental wetland facility during the multiple ecosystem services study. Numbers in wetlands represent treatment. b) Modifications of wetland outlets for the different treatments. Empty circles represent holes in the outlet pipe. The figure shows water levels during high flow.
One study period will be three weeks long. The first week will be a high flow period lasting seven days. The inlet flow rates will be high enough to keep all wetlands filled for the duration of the high flow period. At the start of the second week, all inlet flows will be turned off, and the water level in wetlands optimized for water retention will gradually decrease during approximately four days. For the remainder of week two and three, wetlands optimized for water retention will be empty while non-optimized wetlands will remain filled (some variations are expected due to precipitation and evapotranspiration). At the beginning of the fourth week, a new study period begins, and the inflows will be turned on again. This will be repeated for the duration of the whole study.
We will collect water samples for analyses of nitrogen concentrations three times per week during the first two weeks of each study period. After two weeks, the wetlands optimized for water retention should be more or less dried, and we will therefore not collect any samples during the third week of each study period. We have started a collaboration with Joachim Audet at Aarhus University: We will in addition to water samples also collect gas samples from the wetlands, which he will analyse for dissolved greenhouse gas concentrations. When samples are collected, we will also measure flow rates and temperature etc.
John Strand, Hushållningssällskapet Halland
The project is financed by Swedish Environmental Protection Agency
The work in the wetlands has started. Here you see Stefan Weisner and Josefin Nilsson restoring the overgrown wetlands to a standardized volume. Pictures taken on the 27th of November 2020 (c) Antonia Liess.
During March, April and May, we prepared the experimental wetland facility for this study.
In March, we shortened the outlet pipes in the shallow wetlands and drilled holes at the base of the outlet pipes in the wetlands optimized for water retention. After a few weeks of testing the design, we decided to fit nets around the outlet pipes to prevent the drilled holes from clogging. We also added measurement poles in order to easily monitor the decrease in water level.
1) Waterflow through the drilled hole at the base of the outlet pipe in a wetland optimized for water retention, viewed from above. The wetland is filled. Picture taken 2021-03-24. 2) Net and measurement pole fitted around the outlet pipe in a wetland optimized for water retention. Picture taken 2021-05-17.
The vegetation we cut and removed last summer and during the fall has started to grow back in the wetlands.
We began our first study period and took the first set of samples 2021-05-17.
The first study period ended 2021-06-07. After two weeks of drought (no inflow), the wetlands optimized for water retention were emptied while the non-optimized wetlands remained almost full (see picture below). Now we will begin the second study period. A lot of vegetation has regrown in the wetlands at this point.
1) One of the wetlands not optimized for water retention. This wetland is still almost filled after two weeks of drought (no inflow). 2) One of the wetlands optimized for water retention. This wetland is emptied, apart for the deep part around the outlet pipe, after two weeks of drought (no inflow). Pictures taken 2021-06-07.
On every sampling occasion we measure flow rates at each wetland inlet (unless inflows are turned off) and adjust them if needed, and measure flow rates at wetland outlets.
We collect inlet water and gas samples from outlet pipes of tanks B, C and D. Outlet water samples are taken at the ends of wetland outlet pipes, and outlet gas samples are taken close to the outlet pipe inside each wetland.
Water samples are analysed for nitrogen concentrations using a flow injection analyzer. Léa Velut is helping with these analyses as part of her internship project. We always collect two sets of gas samples: one set is sent to Joachim Audet for analysis, and one set is analysed by Maëlys Bockhoff as part of her internship project. Gas samples are analysed for concentrations of greenhouse gases using a gas chromatograph.
Throughout July, the experiment has progressed as planned. Samples have been collected and analyzed, and we have begun the data analyses. Although the wetlands in the facility differ in vegetation cover and composition, initial data analyses show relatively low variations in nitrogen removal within each treatment.
Our French interns, Léa Velut and Maëlys Bockhoff from ENGEES (École Nationale du Génie de l'Eau et de l'Environnement de Strasbourg), have now finished their internship with us. We want to thank them once again for their exceptional work at the facility and in the lab. Both Léa and Maëlys wrote internship reports on their work with us. Léa’s report, How can hydrologic regime and optimization of water retention in wetlands affect nutrient removal?, can be downloaded here.
Before Léa and Maëlys left, we took the last set of gas samples. The rest of the experiment will continue during this and next month.