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Article

Field-Scale Floating Treatment Wetlands: Quantifying Ecosystem Service Provision from Monoculture vs. Polyculture Macrophyte Communities

by
Jonathan Fletcher
,
Nigel Willby
,
David M. Oliver
and
Richard S. Quilliam
*
Biological & Environmental Sciences, University of Stirling, Stirling FK9 4LA, UK
*
Author to whom correspondence should be addressed.
Land 2023, 12(7), 1382; https://doi.org/10.3390/land12071382
Submission received: 12 June 2023 / Revised: 30 June 2023 / Accepted: 1 July 2023 / Published: 11 July 2023
(This article belongs to the Section Land Use, Impact Assessment and Sustainability)
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Abstract

:
Global water security is critical for human health, well-being, and economic stability. However, freshwater environments are under increasing anthropogenic pressure and now, more than ever, there is an urgent need for integrated approaches that couple issues of water security and the remediation of degraded aquatic environments. One such strategy is the use of floating treatment wetlands (FTW), which are artificial floating mats that sustain and support the growth of macrophytes capable of removing nutrients from over-enriched waterbodies. In this study, we quantify a range of indicators associated with FTWs, planted with different vegetation community types (i.e., monocultures and polycultures) over the course of a three-year field-scale study. The composition of the two different types of FTWs changed significantly with a convergence in diversity and community composition between the two types of FTWs. Phytoremediation potential of the two FTW communities, in terms of nutrient standing stocks, were also similar but did compare favourably to comparable wild-growing plant communities. There were few substantial differences in invertebrate habitat provision under the FTWs, although the high incidence of predators demonstrated that FTWs can support diverse macroinvertebrate communities. This field-scale study provides important practical insights for environmental managers and demonstrates the potential for enhanced ecosystem service provision from employing nature-based solutions, such as FTWs, in freshwater restoration projects.

1. Introduction

Freshwater environments are under increasing pressure due to a range of stressors such as point source and diffuse pollution, land-use change and associated habitat loss, and climate change [1,2]. Improving freshwater environments and water quality to support these ecosystems, and the people that rely on them, is a key priority and global policy aim, e.g., through SDG 6 [3]. Enhancing water and habitat quality are strongly linked to the ability to provide multiple ecosystem services such as carbon sequestration, water flow regulation, and habitat provision [4,5]. Utilizing so-called ‘nature-based solutions’ (NBS) to enhance freshwater environments and support ecosystem functioning is one strategy to improve habitats and promote ecosystem service provision [6,7].
Floating treatment wetlands (FTW) are a form of phytotechnology and a type of NBS that can assist in freshwater restoration [8]. These buoyant structures allow emergent macrophytes to grow hydroponically in freshwater bodies and facilitate the removal of waterborne pollutants to improve water quality [9,10]. Consequently, FTWs are being increasingly used worldwide as a ‘best practice’ management tool for freshwater restoration in both urban and rural settings, spanning a range of temperate and tropical climatic zones [11]. However, existing studies on FTWs have mainly focused on quantifying removal dynamics of specific pollutants [9]; therefore, there is a clear knowledge gap in understanding the added value of FTWs and how plant ecological processes can influence the provision of ecosystem services [12].
The belowground structures of those macrophytes utilized in FTWs, such as roots and rhizomes, would naturally be mainly anchored in the sediment. However, in FTWs these structures develop hydroponically and are therefore likely to support macroinvertebrate communities and thus higher trophic levels such as fish [13]. Discrete FTW plant communities will provide varying root morphologies for habitat provision for macroinvertebrate communities, and more complex macrophyte root morphology is thought to increase macroinvertebrate diversity and abundance due to niche stratification and an increase in microhabitats [14,15]. Species-rich plant communities are more likely to have more diverse root morphologies compared to monocultures [16], although there are currently no studies that have explored the macroinvertebrate community composition associated with FTWs.
FTWs are governed by the key ecological engineering principle of ‘self-design’, where plant successional processes introduce an element of unpredictability to the eventual outcome [17]. Ecosystem service provision from FTWs is strongly related to specific community compositions [18], which means there can be unintended outcomes (positive and/or negative), together with associated impacts on restoration objectives. Therefore, given the increasing popularity of FTWs as a restoration tool, there is a need to understand how the ‘self-design’ aspect is manifested in these systems. Although FTWs are often viewed as a potentially low-cost intervention and an ideal NBS for decentralized water treatment [8], the challenges of deploying and maintaining FTWs are often unreported. Although there are potential benefits from capitalizing on resource recovery (e.g., nutrients, heavy metals) from field-deployment and subsequent harvesting [19], without a specialist (e.g., a paid consultant) it may be difficult to economically justify incorporating FTWs into environmental management approaches.
Assessing the added value potential of using FTWs as a tool for habitat enhancement or freshwater restoration (beyond pollutant removal) is important for understanding their effect on the provision of ecosystem services. Therefore, the overarching aim of this study was to quantify a range of indicators associated with FTWs (e.g., macroinvertebrate diversity, plant biomass, and tissue nutrient concentrations) and multi-annual changes in vegetation in FTWs planted with different vegetation community types. This was achieved through a three-year sampling campaign on a field-scale trial of FTWs planted with monoculture or polyculture plant communities. In addition, we also report on the logistical challenges of deploying and maintaining FTWs in the field with the objective of providing information to aid the future design and management of these systems.

2. Materials and Methods

2.1. Design of Floating Treatment Wetlands (FTWs) and Experimental Strategy

The location for the field trial was Airthrey Loch (56°8′51.6252″ N, 3°55′0.3278″ W), which is an artificial water body with an area of 7 hectares, mean depth 1.8 m, and a residence time of 0.4 years (Figure 1). Airthrey Loch is primarily fed by a single stream carrying high levels of nitrogen and phosphorus from diffuse catchment sources, although there are multiple other point source inputs of grey water from the surrounding campus. Nutrient-rich inflows have led to the eutrophication of Airthrey Loch, triggering large cyanobacterial blooms. These conditions also help facilitate the persistence of invasive non-native macrophytes such as Azolla filiculoides (water fern), Elodea canadensis (Canadian pondweed), and E. nuttallii (Nuttall’s pondweed) in some areas of the loch, while the native vegetation is characteristic of highly eutrophic water bodies. The loch itself would be classified under the Water Framework Directive criteria as a small, lowland, high-alkalinity, very shallow lake with moderate, bordering on poor ecological status. Therefore, Airthrey Loch provides a model opportunity for deploying FTWs as a freshwater restoration tool and case study for ecosystem service generation in impacted waters. An accessible 40 m stretch of the northern shore of Airthrey Loch was utilized for this study. This field trial was carried out from May 2019 to September 2021, allowing for the establishment of the FTWs within this impacted system and providing enough time for root system development. A video showing experiment set-up in Year 2 can be found here: https://www.youtube.com/watch?v=rCUP5r-Dj5U&t=6s (accessed on 11 June 2023).
The FTWs were composed of HDPE 20 cm diameter hollow tubes, extruded at each end to allow nut and bolt fixtures to produce a modular design (2 m × 2 m). Each FTW when fully constructed was 2 m × 4 m and comprised two modules with each having a 2 m × 2 m zinc coated iron grid underneath (held together by strong cable ties) to support macrophytes and substrate. Fences were attached to the FTWs to discourage nesting or herbivory by waterfowl. Three replicate polyculture FTWs and three replicate monoculture FTWs were arranged alternately along the shoreline to account for variation in the water depth and sediment conditions along the shoreline (Figure 1). Underwater separation fences were constructed using HDPE semi-rigid plastic mesh with a 2 mm × 2 mm diamond hole size, secured to the floor of the loch using fence posts, to facilitate the development of distinct microhabitats between the FTWs and help secure the FTWs in place.
To test the hypothesis that different plant communities lead to varying outcomes in ecosystem services, two different plant communities were used: (1) a Phragmites australis monoculture, and (2) a polyculture community including Alisma plantago-aquatica, Juncus effusus, Lythrum salicaria and Myosotis scorpioides. All planted species also occurred naturally within the surrounding emergent vegetation of the lake and are common throughout the region. Coir-matting was placed within each frame, supported by the attached grids, enabling hydroponic growth and development of root systems within the water. Macrophytes growing in the FTWs were surveyed two months after deployment to assess which species had established. Despite changes in community composition during the duration of the trial, for ease of reference both communities are referred to henceforth as either ‘monoculture’ or ‘polyculture’.
Plant surveys were conducted three times over the course of the field experiment to track changes in the establishment of the FTWs (Table 1). Each wetland was treated as a distinct 2 m × 4 m quadrat, with the Domin scale approach used to estimate coverage of each vegetation type. After one year of root establishment (August 2020), aquatic macroinvertebrates were sampled from each FTW replicate by sweeping a 1 mm mesh net over 1.5 m underneath the FTW, with the net deliberately scraped against the plant roots to dislodge associated invertebrates. This was undertaken four times for each replicate FTW. The content of the net was transferred to a plastic zip-lock bag and immediately preserved by adding 70% industrial methylated spirits (IMS). Macroinvertebrate species were subsequently sorted and identified to the lowest feasible taxonomic level.
Above-ground biomass from each FTW was quantified by using a 0.25 m2 quadrat placed on top of the FTW. All plant biomass within the quadrat was harvested to water level by clipping. For each FTW, three random quadrats were used to gain a representative biomass for each FTW and therefore each community type. This was undertaken on two occasions to relate biomass to community composition over time (Table 1).
To understand the resource recovery gains from harvesting macrophytes growing on FTWs in polluted waters, the concentration of a range of recoverable pollutants (calcium (Ca), potassium (K), magnesium (Mg), nitrogen (N), phosphorus (P), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn), sodium (Na), and chromium (Cr)) were determined in the above-ground biomass samples collected in Year 2. Plant tissue was oven-dried at 75 °C until a constant dry weight was achieved, and then ground using a RETSCH RS200 vibratory disk mill (RETSCH, Haan, Germany) to obtain material for tissue concentration analysis. Milled subsamples were either analysed for total C and N using a C: N analyser (FlashSmart NC ORG, ThermoFisher Scientific, Loughborough, UK), or microwave-digested with 70% nitric acid and analysed for P and metalloid element concentration using inductively coupled plasma spectrophotometry (ICP-Optical Emission Spectrometer, Thermo Scientific iCAP 6000 Series ICP; Thermo Scientific, Loughborough, UK).

2.2. Data Analysis

All data analyses were carried out in R studio version 4.0.3 [20]. For group comparisons, non-parametric Wilcoxon tests were performed as data did not conform to the assumptions required for parametric tests due to small sample sizes. Where there were pseudo replicates from the FTWs (e.g., for biomass, tissue concentrations, and invertebrate species composition) a replicate mean was taken before determining a treatment mean. The total standing stock of each pollutant per 0.25 m2 quadrat was calculated by multiplying the community pollutant tissue concentration (mg/g) by the total community biomass (g dry weight). Standing stocks were multiplied by four to present results on a g/m2 basis for comparability with existing literature.
The Shannon–Weiner Index was used for comparing communities as it accounts for diversity and species’ evenness. This was applied to both plant survey data and macroinvertebrate datasets. To complement these approaches, macrophytes and macroinvertebrates were also assigned to functional groups. For macroinvertebrates, this involved categorizing each species by its functional feeding group, which included collectors, filterers, predators, scrapers, shredders, and others (e.g., parasites). Using the approaches detailed by [21], ratios between functional feeding groups were calculated to help understand community composition and environmental background (Table 2). For example, differences between functional feeding groups can relate to availability of fine particulate organic matter (FPOM) and coarse particulate organic matter (CPOM) or habitat stability. The ‘Habitat stability index’, ‘Shredder index’, ‘Filtering Collector index’, and ‘Top-down predator index’ were the ratios chosen to understand the habitat provision for macroinvertebrate communities by FTWs.
To assess the plant community composition in a summarized form, each surveyed FTW community was categorized by the dominant plant growth strategy following Grime’s C (competitor), S (stress tolerator), and R (ruderal) (CSR) plant growth strategy framework [22]. The component plants from each community were assigned to their primary growth strategies (i.e., C, S, and R) on a continuous scale. For each quadrat, the proportion of each primary growth strategy within the community, weighted by the Domin cover scores of the component species, was calculated following the approach of Willby et al. [23].

3. Results

3.1. Macrophyte Community Composition Changes

One year after deployment, the monoculture FTWs were almost completely dominated by Phragmites australis, while the polyculture contained an even mixture of Juncus effusus, Lythrum salicaria, Alisma plantago-aquatica, and Iris pseudacorus with numerous minor species including Phalaris arundinacea, Caltha palustris, and Lycopus europaeus (Figure 2). Consequently, the polyculture FTWs had significantly higher species richness and levels of diversity in year 1 (p < 0.05), and in the following year, there were shifts in both community types with simultaneous increases in species richness and diversity by 1–2 times (Figure 3; p < 0.05). For example, the proportion of Phragmites australis decreased from near dominance in the monoculture FTWs to around 30–40%, with Lycopus europaeus, Epilobium hirsutum, and Myosotis scorpioides increasingly represented. The composition of the main species in the polyculture FTWs remained stable, although there was an increase in the proportion of Myosotis scorpioides, Mentha aquatica, Lycopus europaeus, Epilobium hirsutum, and Iris pseudacorus, primarily at the expense of the originally planted species, Alisma plantago-aquatica and Juncus effusus (Figure 2). Overall, for the first two years both species richness and community diversity remained significantly higher in the polyculture FTWs (Figure 3).
The proportion of Phragmites australis in the monoculture FTWs was reduced in year 3 with the most dominant species being Lycopus europaeus, Mentha aquatica, and Lythrum salicaria (Figure 2), whereas the most dominant species in the polyculture FTWs in year 3 were Lycopus europaeus, Epilobium hirsutum, Mentha aquatica, and Lythrum salicaria. However, by year 3, there were no differences in species richness or species diversity between the two communities (Figure 3). Both communities showed signs of convergence, sharing several common major species, including Lycopus europaeus, Mentha aquatica, and Lythrum salicaria. The progression of each plant growth strategy was represented in both communities by an increased proportion of ruderal (R) species at the expense of stress tolerators (S) (Figure 4), although there was little difference in the overall representation in growth strategy between the two community types.

3.2. Macroinvertebrate Community Composition

Across both the monoculture and polyculture FTWs, the macroinvertebrate assemblages inhabiting the root zones were mostly populated by molluscs (snails), malacostraca (crustaceans), diptera (fly larvae), and to a lesser extent trichoptera (caddisfly larvae), coleoptera (beetles), and hemiptera (true bugs). However, there were no significant differences between the two types of FTW for either macroinvertebrate richness or abundance, although abundance was slightly higher from the polyculture FTWs (Figure 5). The most represented macroinvertebrate groups in both FTW community types were Planorbidae (ramshorn snails), Pisidium tenuilineatum (pea mussel), Radix balthica (wandering snail), Asellus aquaticus (water louse), Crangonyx pseudogracilis (amphipod shrimp), Chaoborus (glassworm, insect larvae), Chironomidae (nonbiting midge larvae) and Leptoceridae (long-horned caddisflies larvae). However, there were no significant differences between the FTWs in terms of their macroinvertebrate assemblages.
The functional feeding groups from each type of FTW were also similar (Figure 6). In relative order of greatest representation, these were predators (40–45%), shredders (25–35%), collectors (10–22%), scrapers (4–5%), filterers (ca. 4%), and others (e.g., parasites) (<1%) (Figure 6). Functional feeding group representation between the two FTW types was not significantly different for each category. Although not significant, there were ca. 10% more collectors in the monoculture than the polyculture FTW, and slightly more shedders represented in the polyculture FTW (Figure 6).
Calculating ratios of different macroinvertebrate communities can help in understanding habitat type and availability and type of organic matter (Table 2); however, there were no significant differences in functional feeding group ratios between the monoculture and polyculture FTWs (Table 3). For all FTWs, the mean habitat stability index was <0.5, indicating a greater presence of entrained FPOM (Table 3; see Table 2 for ratio thresholds). A large mean shedder index indicated that CPOM availability for shredders is greater than FPOM availability for collectors. Similarly, the mean ‘Filtering collect index’ was <0.5, indicating that suspended FPOM load was lower than storage (entrained) FPOM. The mean ‘Top-down predator index’ indicated that a high predator presence (0.2–0.4) was being sustained.

3.3. Plant Biomass and Nutrient Tissue Concentration

In year 2, biomass was not significantly different between the monoculture and polyculture FTWs (Figure 7). However, in year 3 the biomass from the polyculture FTWs was significantly lower. Polyculture FTWs had higher tissue concentrations of Ca, Cr, Mg, and Mn in year 2, although there were no differences in N and P concentrations between the two community types (Figure 8).

4. Discussion

4.1. Plant Community Succession

Plant community succession is an inevitable feature of natural systems where there is an existing pool of ready colonists; here, we report on the first study on multi-annual changes in plant community composition in FTWs following their deployment in the field. The progression in community composition of both plant community types away from their original assemblages demonstrates the importance of considering ‘self-design’ in utilizing FTWs. However, the passive management strategy used in this study and the resultant compositional changes highlight the risk of deploying FTWs with set expectations in the performance of these systems for pollutant removal and/or ecosystem service provision. This is because performance is often tied to the service provision of certain plant species of specific community assemblages commonly derived from previous FTW studies [24,25]. Arresting succession through active management, such as removing selective species, would protect the original assemblage but increase expense in terms of time and labor [26]; however, this could be offset by additional benefit gained by resource recovery potential [19,24]. The change in plant succession and impact on performance in phytoremediation systems, particularly FTWs, is a knowledge gap, and given these results, suggests that within the time frame of two full years there is considerable potential for deviation from the original planted communities. This is important to consider given the increasing use of FTWs as restoration tools for pollutant removal.
The increase in similarity between the communities due to Lycopus europaeus, Mentha aquatica, and Lythrum salicaria becoming prominent in both FTW community types suggests that a convergence in community composition was taking place [27]. Given that both communities were situated in the same environment with the same pressures, and exposed to the same propagule sources, convergence in community composition was a likely outcome. The loss of stress-tolerant species and increase in ruderal types of species, including Lycopus europaeus and Mentha aquatica, suggests that FTWs can be disturbance-prone habitats. FTWs deployed in the environment are constantly moving due to wind and waves. This, combined with hydroponic growth, might reduce the competitive advantage of normally dominant plant species, meaning that ruderals are better adapted to growth in FTWs. Any move towards harvesting vegetation in FTWs for pollutant export would likely further favour ruderal species.
The prevalence of some species in both types of FTWs may have resulted from either cross-colonization (e.g., Myosotis scorpioides from the polyculture FTW), or immigration from established marginal vegetation; for example, Lycopus europaeus and Epilobium hirsutum were common on the banks of the site and likely colonized the FTWs from here either by aerial dispersal or hydrochory. Therefore, FTWs, like any other ecosystem, can be sinks for colonizing plants and may eventually subsume the surrounding appropriately adapted plant species. The opposite scenario may also occur when FTWs are installed in areas of bare vegetation, e.g., newly constructed stormwater ponds, where they may act as sources of propagules. Using FTWs as a source and facilitator of plant vegetation is an interesting concept and this type of approach with terrestrial vegetation has been found to be useful in unvegetated environments [28]. The concept of FTWs as both sources and sinks in ecosystem restoration is unexplored, but the results highlight additional applied benefits and risks associated with FTWs. For example, if the surrounding environment already contains a diversity of macrophytes, then it may be possible to plant the FTWs with a single species (or even just add some substrate) to commence the colonization process, which would reduce both effort and costs. However, initially planting with a diverse community may facilitate increased diversity during maturation of the community [29]. Conversely, if FTWs are envisaged as a source of colonists in otherwise sparely vegetated sites, it is critical that they are planted appropriately with species that are locally native and non-invasive. In this study, the decrease in diversity and species number suggested that there are limits to the number of species that can be supported in such engineered systems. However, there are potential benefits of ‘self-designed’ systems, e.g., the increased extent of existing habitats or the provision of desirable ecosystem services associated with a mature community. Therefore, it is important for environmental managers to be aware that plant succession, particularly towards ruderal-dominated vegetation, may influence the outcomes of projects employing FTWs.

4.2. Macroinvertebrate Communities and Habitat Provision by FTWs

In year 2 of this study, despite there being differences in the macrophyte composition between the two types of FTWs, the macroinvertebrate communities remained similar in all key variables including diversity, abundance, key species, and functional feeding groups. While there is debate in the literature about the importance of macrophyte species richness for indicators including number of invertebrate taxa, abundance, and diversity, most studies demonstrate that increases in macrophyte diversity and/or structural diversity have significant impacts on at least one of these metrics [13,14,15,30]. It is possible that plant rooting structure for each community type was not sufficiently different to drive differences in invertebrate communities, despite polycultures having a visually denser root network (often due to the higher density of Mentha aquatica) compared to the Phragmites-dominated monocultures.
Ratios of invertebrate functional feeding groups help to construct a picture of the bio-physical environment available to macroinvertebrate communities. The low ‘habitat stability index’, with a predominance of shredders and gatherers consuming entrained FPOM, is indicative of a habitat that is unstable and unsuitable for organisms that require a firm and stable substrate (e.g., filterers and scrapers). The large mean ‘shredder index’ supports this and indicates that CPOM is more widely available (e.g., from decaying plant matter) compared to FPOM that would be expected to settle in undisturbed areas of lakebed beneath the FTWs. A positive design feature of FTWs for their operation is that they move with the water level and current, but the instability this imposes might also constrain invertebrates and obscure any effect of macrophyte composition on invertebrate abundance and richness. For example, Yofukuji et al. [13] found notably higher diversity, richness, and abundance of invertebrates in non-FTW plant communities with similar diversity to those in this study. However, macroinvertebrate predators, as a functional feeding group, were well represented suggesting that there was a sufficient availability of prey, probably supported by an ample supply of CPOM from the dense macrophyte roots and from the turnover of above-ground parts. Furthermore, a higher proportion of macroinvertebrate predators suggests that predation by higher-order consumers such as fish was not a significant pressure in this system, perhaps due to the refuge afforded by the suspended roots.
While it is likely that the habitat provided by the roots of FTWs is different and possibly less stable than other freshwater habitats, the evidence above does demonstrate that FTWs can provide good habitat for certain macroinvertebrate communities (although the study size used here was small). As FTWs can be strategically positioned, it would be possible to create and expand habitats across freshwaters, especially in recently constructed water bodies. Given the promotion, expansion, and regeneration of blue-green spaces, there are clear opportunities for using FTWs to assist the establishment of macroinvertebrate communities and enhance the associated ecosystem, e.g., by creating habitat corridors and supporting higher-order predators [16,31,32].

4.3. Resource Recovery

In year 2 of the experiment, despite some key differences remaining in the macrophyte composition between the two types of FTWs, such as diversity and a greater proportion of Phragmites australis, few differences in the tissue concentrations of pollutants were found. The convergence in composition of the two community types—with several common macrophyte species between them—meant a greater likelihood of similar tissue concentrations of most of the pollutants in the two communities. The differences in the proportion of some plant species between the communities may have led to some of the differences between the monoculture and polyculture FTWs for Ca and Mn concentrations. The higher concentration of Ca in the above-ground plant biomass of the polyculture FTWs may be because although Ca is required to build structural tissue, it is lower in monocots than dicots due to their low concentration of cell wall pectate [33]. The polyculture FTW contained more dicotyledonous species compared to the monoculture FTW because Phragmites australis (a monocot) remained a large component of the latter community (almost 40%). There is also evidence to suggest that dicotyledonous species possess more effective Mn transporter proteins for transporting this pollutant to above-ground tissue [34], potentially explaining the pattern of increased Mn tissue concentration in the plants growing in the polyculture FTWs.
In terms of nutrient standing stocks of key recoverable pollutants such as N and P, there was no difference between the two FTW community types. Given that biomass is likely to be the main driver of pollutant standing stocks for macronutrient-type pollutants [12], it is unsurprising that there were no differences in these pollutants as the biomass between the two community types was similar. Higher standing stocks of Cu in the monoculture FTW containing Phragmites australis as a large community component suggests that this species may have an enhanced ability for Cu sequestration and translocation to above-ground tissues [35].
There are only a small number of published field-scale studies of the performance of FTWs (e.g., [32,36]). The magnitude of N and P standing stocks were similar to Karstens et al. [36], whilst the N standing stocks of the FTWs in this study were lower than Olguín et al. [32], although P was substantially higher. Differences in climate (tropical versus temperate), community composition, local conditions, and water chemistry between available studies makes an overall appraisal of standing stocks in field scale FTWs challenging. A more useful evaluation is to compare FTWs with existing stands of plant communities with similar background environmental variables. By comparing the results from this study to a survey of existing wild plant stands in close proximity to our study site, we found broad similarity in the standing stocks of most pollutants in communities with a similar biomass range of 500–1000 g/m2 [12]. However, some stands of wild macrophytes also had much higher standing stocks, suggesting that macrophytes rooted in sediment can achieve a higher above-ground biomass, especially in the case of larger, competitive species with extensive lateral root systems. Nevertheless, harvesting FTWs can allow nutrients to be exported from freshwaters but the cost–benefit of an active management regime may depend on multiple site-specific factors such as harvesting costs, proximity to biomass reuse location (transportation), biomass quantity related to FTW coverage, and specific plant communities [19].

5. Conclusions

Floating treatment wetlands (FTWs) are novel ecological engineering systems that can assist the remediation of polluted waters, and when used at scale can provide ecosystem services in the form of habitat provision and resource recovery. Through a field-scale experiment, it was found that plant community composition in passively managed FTWs can change substantially in the short term and may impact ecosystem service provision. The two initially different plant community types in the polyculture and monoculture FTWs converged in their community composition and macrophyte diversity. There were few substantial differences in habitat provision for macroinvertebrates between both monoculture and polyculture FTWs, suggesting that any differences in root morphology did not play a significant role in shaping macroinvertebrate community diversity. However, macroinvertebrate indices suggested that habitat provided by FTWs was unstable without adequate provision of FPOM, which may have been a greater limiting factor than root morphologies. The high incidence of predators demonstrates that FTWs can support functionally diverse macroinvertebrate communities and may act as a refuge for predatory invertebrates from predation by fish, which suggests that these systems can be used to locally increase habitat provision within freshwaters. Resource recovery between the two FTW communities in terms of nutrient standing stocks was also similar, but did compare favourably to similar wild-growing plant communities. Therefore, depending on specific cost–benefit scenarios, FTWs in practice can be used to export nutrients from eutrophic waters.
This field-scale study has demonstrated that ‘self-design’ is an important practical factor to consider when employing nature-based solutions in freshwater restoration projects. In the absence of field-scale FTWs studies on the possible added value in ecosystem service provision, this work provides new insights that can be used to inform freshwater management decisions.

Author Contributions

Conceptualization, J.F., N.W., D.M.O. and R.S.Q.; methodology, J.F. and N.W.; investigation, J.F.; writing—original draft preparation, J.F. and R.S.Q.; writing—review and editing, N.W., D.M.O. and R.S.Q.; supervision, R.S.Q.; project administration, J.F. and R.S.Q.; funding acquisition, R.S.Q. All authors have read and agreed to the published version of the manuscript.

Funding

Funding for this work was provided by the Scottish Government Hydro Nation Scholars Programme.

Data Availability Statement

Please contact the authors to request a copy of the raw data.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. Field trial location at Airthrey Loch, Stirling, UK (top, map highlighting study site located within with black rectangle; bottom, photo of field trial set-up).
Figure 1. Field trial location at Airthrey Loch, Stirling, UK (top, map highlighting study site located within with black rectangle; bottom, photo of field trial set-up).
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Figure 2. Examples of each plant community in different phases of development. (A) Polyculture in Year 1 (spring/summer); (B) Year 2 (spring/summer); (C) Year 3 (summer); (D), monoculture in Year 1 (spring/summer); (E), Year 2 (spring/summer); (F) and Year 3 (summer).
Figure 2. Examples of each plant community in different phases of development. (A) Polyculture in Year 1 (spring/summer); (B) Year 2 (spring/summer); (C) Year 3 (summer); (D), monoculture in Year 1 (spring/summer); (E), Year 2 (spring/summer); (F) and Year 3 (summer).
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Figure 3. Mean plant community species richness and species diversity (Shannon–Wiener index) for the monoculture versus polyculture communities in years 1 to 3. Error bars ± 1 SD.
Figure 3. Mean plant community species richness and species diversity (Shannon–Wiener index) for the monoculture versus polyculture communities in years 1 to 3. Error bars ± 1 SD.
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Figure 4. Mean percentage plant strategy representation (competitors, ruderals, stress tolerators) for monoculture and polyculture FTWs from years 1 to 3. Error bars ± 1 SD.
Figure 4. Mean percentage plant strategy representation (competitors, ruderals, stress tolerators) for monoculture and polyculture FTWs from years 1 to 3. Error bars ± 1 SD.
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Figure 5. Macroinvertebrate community (A) abundance, (B) richness, and (C) diversity, associated with the roots of monoculture versus polyculture FTWs for year 2 (note different scales on y axes). Boxes represent interquartile range, horizontal lines the median, and vertical lines the 10th and 90th percentiles.
Figure 5. Macroinvertebrate community (A) abundance, (B) richness, and (C) diversity, associated with the roots of monoculture versus polyculture FTWs for year 2 (note different scales on y axes). Boxes represent interquartile range, horizontal lines the median, and vertical lines the 10th and 90th percentiles.
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Figure 6. Proportions of macroinvertebrate functional feeding groups collected from monoculture and polyculture FTWs (note different scales on y axes).
Figure 6. Proportions of macroinvertebrate functional feeding groups collected from monoculture and polyculture FTWs (note different scales on y axes).
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Figure 7. Plant community dry weight biomass for monoculture versus polyculture FTW communities for year 2 and year 3. Boxes represent interquartile range, horizontal lines the median, and vertical lines the 10th and 90th percentiles.
Figure 7. Plant community dry weight biomass for monoculture versus polyculture FTW communities for year 2 and year 3. Boxes represent interquartile range, horizontal lines the median, and vertical lines the 10th and 90th percentiles.
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Figure 8. Plant community above-ground tissue concentration per element for monoculture versus polyculture FTW for year 2 (note different scales on y axes). Boxes represent interquartile range, horizontal lines the median, and vertical lines the 10th and 90th percentiles.
Figure 8. Plant community above-ground tissue concentration per element for monoculture versus polyculture FTW for year 2 (note different scales on y axes). Boxes represent interquartile range, horizontal lines the median, and vertical lines the 10th and 90th percentiles.
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Table
Table 1. Sampling frequency during field trial.
Table 1. Sampling frequency during field trial.
Ecosystem ServiceMeasurable Variable Sample Timing
Year 1 (2019)Year 2 (2020) Year 3 (2021)
Habitat provisionMacroinvertebrates (abundance and diversity)-August (summer)-
Plant community stability Abundance
(Domin scale)		June (summer)August
(summer)		September
(late summer)
Biomass Dry weight-September
(late summer)		September
(late summer)
Resource recovery Tissue concentration and standing stocks-September
(late summer)
Table
Table 2. Functional feeding group calculations, thresholds, and possible interpretations (assuming that all shredders are herbivore shredders). Based on Cummins [21].
Table 2. Functional feeding group calculations, thresholds, and possible interpretations (assuming that all shredders are herbivore shredders). Based on Cummins [21].
Ratio Name Ratio of Feeding GroupsThresholds and ExplanationsInterpretations
Habitat stability indexfilterers + scrapers
shredders + gatherers		Ratio bigger than 0.5 indicates that suspended fine particulate organic matter is greater than entrained fine particulate matterFiltering collectors require stable locations and scrapers require surfaces that remain in a stable position facing upwards
Shredder index  shredders  
collectors + filterers		A ratio of >0.5 in autumn/winter, and of 0.25 in spring/summer, indicates that CPOM availability for shredders is greater than FPOM availability for collectorsCPOM food support for shredders > than FPOM for collectors
Filtering collector index   filterers  
collectors		A ratio of <0.50 indicates that suspended FPOM load is less than stored (entrained) FPOMFPOM food for collectors at higher density and/or better quality than storage FPOM
Top-down predator index    predators    
all functional feeding groups		Predator:prey ratio 0.10–0.20 to total macroinvertebrate populationThis level of a predator population density (or biomass) is supported by sufficient prey to support them
Table
Table 3. Mean values of indices for each calculated macroinvertebrate index for monoculture, polyculture FTWs, and both combined.
Table 3. Mean values of indices for each calculated macroinvertebrate index for monoculture, polyculture FTWs, and both combined.
Habitat
Stability Index		Shedder
Index		Filtering
Collector Index		Top-Down
Predator Index
Monoculture0.198.360.230.42
Polyculture0.219.950.240.44
All Communities0.209.160.240.43
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Fletcher, J.; Willby, N.; Oliver, D.M.; Quilliam, R.S. Field-Scale Floating Treatment Wetlands: Quantifying Ecosystem Service Provision from Monoculture vs. Polyculture Macrophyte Communities. Land 2023, 12, 1382. https://doi.org/10.3390/land12071382

AMA Style

Fletcher J, Willby N, Oliver DM, Quilliam RS. Field-Scale Floating Treatment Wetlands: Quantifying Ecosystem Service Provision from Monoculture vs. Polyculture Macrophyte Communities. Land. 2023; 12(7):1382. https://doi.org/10.3390/land12071382

Chicago/Turabian Style

Fletcher, Jonathan, Nigel Willby, David M. Oliver, and Richard S. Quilliam. 2023. "Field-Scale Floating Treatment Wetlands: Quantifying Ecosystem Service Provision from Monoculture vs. Polyculture Macrophyte Communities" Land 12, no. 7: 1382. https://doi.org/10.3390/land12071382

APA Style

Fletcher, J., Willby, N., Oliver, D. M., & Quilliam, R. S. (2023). Field-Scale Floating Treatment Wetlands: Quantifying Ecosystem Service Provision from Monoculture vs. Polyculture Macrophyte Communities. Land, 12(7), 1382. https://doi.org/10.3390/land12071382

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