Open Access
Issue
Knowl. Manag. Aquat. Ecosyst.
Number 422, 2021
Article Number 23
Number of page(s) 6
DOI https://doi.org/10.1051/kmae/2021020
Published online 28 May 2021

© P. Manolaki et al., Published by EDP Sciences 2021

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License CC-BY-ND (https://creativecommons.org/licenses/by-nd/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. If you remix, transform, or build upon the material, you may not distribute the modified material.

1 Introduction

Constructed wetlands are likely to become more widely used as a tool to reduce the transport of nutrients from agriculture to aquatic environments by increasing water residence time which enhance the nutrient retention and removal from the water. Aquatic plants have been effectively used in constructed wetlands (CWs), as they enhance retention and removal of nitrogen (N) and phosphorus (P) via multiple direct and indirect pathways including direct macrophyte assimilation, positively linked to plant biomass and denitrification in the sediment (e.g. Bachand and Horne, 2000; Levi et al., 2015). Moreover, macrophytes serve as habitat for bacteria and algae, which also assimilate nutrients and perform denitrification (Srivastava et al., 2017).

The most common vegetation in European CWs with free surface flow consists of monocultures of Phragmites australis or Typha latifolia (Vymazal, 2013). However, recent studies suggest that higher biodiversity both in terms of species number and of species growth forms (such as more emergent species, floating leaved and submergent species) can significantly increase nutrient removal from a system due to important differences in their nutrient uptake strategies (e.g. Bouchard et al., 2007; Choudhury et al., 2018; Manolaki et al., 2020) via niche complementarity (Choudhury et al., 2018; Olesen et al., 2018) and the extension of the nutrient period across seasons (Manolaki et al., 2020).

In Denmark, a plan is underway to create a thousand new constructed wetlands until 2021 in order to reach the N reduction target of the EU Water Framework Directive WFD; (European Commission, 2000). Species propagation and biomass accumulation rate are among the most important parameters that affect the efficacy of CWs in nutrient removal (Chambers and McComb, 1994). Therefore, information about the propagation of tolerant species in relation to species establishment, survival rate and size suitability for planting, will support the high number of plants required for the new CWs. Still, their survival and establishment rate can vary among plant species during the growing season, with some species being most productive in spring, summer and/or autumn (Manolaki et al., 2020).

Seasonal variations in establishment and reproduction influenced primarily on temperature, day length and light quality (Warwick and Brock, 2003). Therefore, plant adaptations to these parameters affect their productivity (Lambers et al., 2008) with some species being more productive than others at some point during the growing season. In order to optimize the efficacy of CWs in nutrient removal by increasing the plant functional diversity, it is important to ensure high establishment success for a number of alternative species. Therefore, it is necessary to know the most suitable time for planting in order to ensure high propagation success.

In this study, we test the establishment and propagation rate of six emergent macrophytes suitable for use in constructed wetlands in five outdoor experiments from March to October in order to identify the most efficient time period for planting based on the plants' growth rate. The results will support a more diverse, efficient and successful planting of constructed wetlands when using the selected species.

2 Materials and methods

Six emergent macrophyte species collected from streams and small lakes located in Mid Jutland (Denmark) were used in this study namely: Nasturtium officinale R. Br., Glyceria fluitans (L.) R. Br., Berula erecta (Huds.) Coville, Veronica beccabunga L., Butomus umbellatus L. and Ranunculus hederaceus L. The species were chosen based on their presence in early spring and their potential for water purification through nutrient uptake. All species have a vegetative growth form. For all of them, shoots with at least two nodes were included in the study. Shoot length depended on the species morphology, as shoots from e.g. B. umbellatus were naturally longer than shoots from B. erecta (Tab. 1).

The outdoor growth experiment for all species was repeated five times during the growing season. Monthly experiments were performed in March (7 March–5 April; 29 days), April (19 April–16 May; 27 days), June (31 May–28 June; 28 days), August (8 August–4 September; 27 days) and October (19 September–24 October; 35 days). Healthy and green plant material was collected from the field one day prior to each experiment. To make sure that the shoots were approximately at the same age and growth stage, the collection of flowering shoots was avoided. The shoots were rinsed and then planted in 16 partially buried tubs (90 L plastic tubs; 67 cm wide and 53 cm high) which consisted of 24 L washed beach sand sediment (5 cm sediment depth) and tap water to a water level of approximately 5 cm. Each day at noon, 0.3 L of water were added to each tub through a drip tube to compensate for evaporation. For each species there were 10 replicates of shoots distributed in 10 tubs. Individuals from each species were planted in different tubs to avoid pseudo replication and thus, the number of shoots in each tub varied between one and six. There was enough distance (>25 cm) between shoots to avoid competition (Olesen et al., 2018). In total 17 tubs were used in the experiment. Shoots were gently planted in a depth of 5 cm approximately. After planting, three slow releasing fertilizer pellets (5–6 months) were added to the sediment (OsmocotePlus Tablet; 15 N − 4 P − 10 K − 1.2 Mg + Mikro) to ensure enough nutrients for plant growth without excessive algae growth.

Once a week, dissolved oxygen concentration and % saturation was measured in the water in each tub (OxyGuard probe). Data for solar radiation and air temperature was collected from a data logger (CR1000) on a climate station close to the study site (Dynamet Weather Station, Dynamax Inc) (Tab. 2). During the experiment conducted in June, water samples of 15 ml were taken every week from each tub and were analyzed for nutrient concentrations of phosphate-P (PO43−-P), ammonium-N (NH4+-N) and nitrate-N (NO3-N) on a flow injection analyzer (QuikChem FIA+ 8000 Series, Lachat instruments). The water supply was similar to all experiments and thus, the water quality was also assumed to be similar (Tab. 2). Once a week, and after intensive rainfall, the water level was manually lowered or increased to approximately 5 cm depending on the present water level.

At the end of the growth experiments, one shoot per replicate (6 species × 10 replicates) was taken up and rinsed before being oven-dried (60 °C). The dry weight (DW) for each individual was measured and the relative growth rate was calculated as RGR (day−1) = (ln DWend − ln DWstart)/time (day). The starting DW for each shoot was estimated in relation to its initial wet weight (WW) and a DW/WW ratio, which was calculated based on 5–10 shoot-replicates similar to the shoots used in the experiment. The doubling time was calculated as DT = ln(2)/RGR (Mitchell, 1974). Relative growth rate (RGR) was used as a measure of propagation success.

We used one-way ANOVA to compare RGR for each species in different months and post-hoc Tuckey-HSD test to search for significant differences among growth periods. Data was log10 transformed when necessary for better achievement of variance homogeneity.

Table 1

Range of shoot length (cm) and number of replicas for shoots used in the experiments for the six studied species.

Table 2

Monthly average values per experiment for weather and climate data. Data were collected from a climate station located near the study site (solar radiation and air temperature) and from an oxygen probe. Solar radiation and air temperature were logged every half hour, and oxygen was measured once a week. Day values are calculated as an average from 07:00 to 18:30 h, and night values are calculated from 19:00 to 06:30 h. Nutrient concentration was only measured in summer but is from the same source and with similar amount of nutrient pellets in the other seasons.

3 Results

The survival rate of shoots was generally high (>70%) for all species during each monthly experiment. However, all 10 shoots from B. umbellatus died in October. Average negative growth rates (=decay rates) appeared in March for N. officinale and in April for B. umbellatus (Fig. 1).

There were significant seasonal differences in RGR for all species except for G. fluitans (Fig. 1). All species had the highest RGR in June and followed the same bell-shaped distribution of rise and fall in RGR.

N. officinale reached a maximum RGR of 0.090 day−1 (doubling time, DT of 7.7 days) and showed high growth efficiency from April to October. V. beccabunga had high growth efficiency from April to August with a maximum RGR of 0.076 day−1 (DT of 9.2 days). Both B. erecta and R. hederaceus showed highest growth efficiency from April to June, with a maximum RGR of 0.068 day−1 (DT of 10.2 days) and 0.106 day−1(DT of 6.5 days), respectively. R. hederaceus showed a significant decrease in RGR in August and October and B. erecta showed high variance in RGR in October. B. umbellatus had the lowest maximum RGR (0.030 day−1, DT of 22.8 days) and showed only good growth efficiency in June (Fig. 1).

thumbnail Fig. 1

Box plots with standard deviation (±SD) of the mean monthly values of the relative growth rates (RGR) for the six species. Different letters indicate significant differences (p < 0.05). ANOVA results: N. officinale: F(4,45) = 55.83, p < 0.0001, V. beccabunga: F(4,45) = 34.93, p < 0.0001, B. erecta: F(4,45) = 4.49, p = 0.0039, R. hederaceus: F(4,42) = 34.160, p < 0.0001, G. fluitans: F(4,44) = 0.48, p = 0.75, B. umbellatus F(3,33) = 59.92, p < 0.0001.

4 Discussion

The results showed differences in relative growth rate (RGR) among species during the growing season with almost all species having the highest growth rate in June and also varying in the range of 0.002–0.106 day−1. The relative growth rate of the specific emergent species used in this study are somewhat in the same range as 0.003–0.13 day−1 found by Manolaki et al. (2020) and within the range of relative growth rates (0.007–0.109 day−1) found by Nielsen and Sand-Jensen (1991) for 14 submerged species. However, the growth capacity of submerged species is much lower than the one recorded in relation to the emergent species (Manolaki et al., 2020).

We intended to identify at which moment during the growing season the selected plant species achieved the highest growth efficiency and therefore achieved the highest biomass accumulation in the shortest amount of time. This information is important to ensure high propagation rate and establishment success when planting into a constructed wetland, especially when fast growing species like Phragmites australis are also present. Our results showed that G. fluitans was the only species suitable for mass propagation in March. Even though, G. fluitans showed the lowest RGR during summer period (June), as it was also illustrated by other studies (Manolaki et al., 2020), it is generally suitable for propagation during the whole growth period (from early spring to autumn) due to low seasonal variation in its growth efficiency.

Species with the highest growth rates were N. officinale, B. erecta, V. beccabunga and R. hederaceus. All of them are suitable for propagation from April to June with the highest growth rates observed in the middle of the growing season. Amphibious species like N. officinale, B. erecta and V. beccabunga did not stored underground organs like other emergent species e.g. G. fluitans, which can translocate the stored dry mass from the beginning of the growing season and show an early high RGR (Howard-Williams et al., 1982). Species that lack underground reserves showed bell-shaped RGR curve, which might causean extension of the growth period. Especially for N. officinale, the RGR curve showed high relative growth rates both in August and October, indicating that this species is appropriate for propagation throughout the whole growing season. This might also suggest that the species adaptations and preferences for light, temperature and day length are similar, as they all show highest propagation potential in June, when the temperature and solar radiation is high. However, N. officinale and G. fluitans showed potential for propagation in autumn (October), suggesting that these species can be planted later in the growing season, since they are better adaptable to lower temperatures and lower light conditions and as such, they are also very useful for constructed wetlands.

The results showed that all species had the highest RGR in June. However several species showed good growth efficiency from April through to August. Hence, it is possible to have a full production of emergent macrophyte species throughout the whole growing season. Based on these results, we suggest the propagation of certain plant species, such as N. officinale and G. fluitans, for enhancing biodiversity and ecosystem functioning in constructed wetlands.

Acknowledgements

We thank the Danish Ministry of Environment and Food for the financial support through the Green Development and Demonstration Project (GUDP), PLANTNAT. We thank Prof. Ioannis Vogiatzakis and Dr. Christina Kkona for English language editing.

References

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Cite this article as: Manolaki P, Olesen A, Graves Hvidt B, Lauridsen TL, Riis T. 2021. Investigating emergent macrophytes establishment rate and propagation towards constructed wetlands efficacy optimization. Knowl. Manag. Aquat. Ecosyst., 422, 23.

All Tables

Table 1

Range of shoot length (cm) and number of replicas for shoots used in the experiments for the six studied species.

Table 2

Monthly average values per experiment for weather and climate data. Data were collected from a climate station located near the study site (solar radiation and air temperature) and from an oxygen probe. Solar radiation and air temperature were logged every half hour, and oxygen was measured once a week. Day values are calculated as an average from 07:00 to 18:30 h, and night values are calculated from 19:00 to 06:30 h. Nutrient concentration was only measured in summer but is from the same source and with similar amount of nutrient pellets in the other seasons.

All Figures

thumbnail Fig. 1

Box plots with standard deviation (±SD) of the mean monthly values of the relative growth rates (RGR) for the six species. Different letters indicate significant differences (p < 0.05). ANOVA results: N. officinale: F(4,45) = 55.83, p < 0.0001, V. beccabunga: F(4,45) = 34.93, p < 0.0001, B. erecta: F(4,45) = 4.49, p = 0.0039, R. hederaceus: F(4,42) = 34.160, p < 0.0001, G. fluitans: F(4,44) = 0.48, p = 0.75, B. umbellatus F(3,33) = 59.92, p < 0.0001.

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