Harvest of Myriophyllum spicatum Facilitates the Growth of Vallisneria denseserrulata but Has No Significant Effects on Water Quality in a Mesocosm Experiment

Abstract

The recovery of submerged macrophytes is crucial for lake restoration. However, Myriophyllum spicatum usually shows an overgrowth and inhibits the growth of Vallisneria denseserrulata via light shading in many restored shallow lakes after the plant transplantation. So far, harvesting M. spicatum is the primary method to alleviate these shading effects in post-restoration lakes. Nevertheless, the effects of harvesting on the growth of V. denseserrulata and water quality are poorly elaborated. In this study, we conducted a mesocosm experiment, including both monoculture and polyculture groups, to investigate the response of V. denseserrulata growth, light climate, and nutrient concentrations in the water with M. spicatum harvesting. Moreover, the growth and morphology of M. spicatum were also examined. We hypothesized that M. spicatum harvesting would enhance the growth of V. denseserrulata and improve both the light climate and water quality. Our results showed that harvesting M. spicatum in the polyculture mesocosms substantially enhanced the relative growth rate (RGR) of V. denseserrulata compared to the non-harvesting controls. Moreover, harvesting M. spicatum reduced the light attenuation coefficient at 30 cm depth; however, the concentrations of chlorophyll-a, total nitrogen, and total phosphorus did not change significantly. As for M. spicatum, harvesting inhibited the growth of main stem and root but did not significantly affect the cumulative weight and RGR of M. spicatum. In contrast, the presence of V. denseserrulata decreased cumulative weight and RGR while promoting the root parameters of M. spicatum. Our findings imply that harvesting overgrowth nuisance submerged macrophyte species (e.g., M. spicatum) can improve the light climate and reduce its root growth, thereby enhancing the growth of target macrophyte species like V. denseserrulata without changes in the water quality which provides valuable insights for post-restoration lake management.

1. Introduction

Submerged macrophytes play a critical role in maintaining the clear-water state of shallow lakes, as they can improve water quality by absorbing nutrients and reducing sediment resuspension, suppress algal growth through competition, and support the growth and reproduction of aquatic organisms [1,2]. However, excessive external nutrient input accelerates lake eutrophication, which reduces water transparency and consequently leads to the decline or even disappearance of submerged macrophytes [3,4]. Various strategies can be implemented to recover submerged macrophytes, including external loading reduction, sediment removal and biomanipulation [5,6,7,8]. Despite concerted efforts to mitigate nutrient input and improve water clarity, the recovery of submerged macrophytes may be delayed in some Danish and German shallow lakes [9,10,11]. Conversely, in some cases, the excessive macrophyte growth in post-restoration lakes can affect recreational activities, such as hindering people from swimming, fishing and boating, and the navigation of ships by blocking the channel [9,12]. Additionally, if not properly managed, dense macrophyte vegetation can disrupt aquatic ecosystems by limiting light penetration and hindering the growth of other aquatic plants [13]. As such, the reconstruction and management of submerged macrophytes remains a focal challenge in lake restoration.

Harvesting has emerged as an effective measure to control excessive submerged macrophyte growth in many areas, such as Taihu Lake in China and Biwa Lake in Japan [14,15]. This method involves the mechanical or manual harvesting of submerged macrophytes from the water at varying intensities and transported to the shore [16]. At the same time, harvesting submerged macrophytes can remove a large amount of nitrogen and phosphorus, and effectively reduce water nutrient levels, which is an important measurement to reduce nutrients and achieve the balance of water nutrients [12]. However, existing harvesting research often lacks detailed assessments of nutrient levels and environmental conditions [17]. Moreover, it is important to note that harvesting often has repercussions on the community structure of aquatic plants and species that are more resistant to harvesting and recover quickly [18,19]. Compensatory growth of submerged macrophytes can occur post-harvesting, allowing damaged plants to have higher adaptability, higher growth rates, and greater biomass accumulation compared to undamaged counterparts [20,21]. These phenomena may potentially diminish harvesting efficacy and elevate management costs. While previous research has predominantly focused on the effects of harvesting on individual plant species [22], there remains a gap in understanding the impacts of harvesting on the growth and interaction dynamics of mixed species communities.

In freshwater ecosystems, the selection of submerged macrophyte species is crucial for the success of shallow lake restoration. Vallisneria denseserrulata and Myriophyllum spicatum are common submerged plant species in freshwater ecosystems, as well as pioneer species in ecological restoration [23,24]. Early-stage ecological restoration efforts frequently involve planting canopy-forming submerged plants, such as M. spicatum, which can absorb nutrients from the water column and increase water transparency quickly [24]. However, these plants typically have underdeveloped root systems, making them less effective at controlling nutrient release and sediment resuspension. Since sediments are the primary source of nutrients in restored lakes, species with well-developed root systems, like V. denseserrulata, are advantageous. These macrophytes can cover sediments and release oxygen by roots, thus stabilizing sediments and mitigating nutrient release, particularly nitrogen and phosphorus [25]. Nevertheless, when rosette-type and canopy-forming plants coexist, canopy-forming plants often dominate by forming a thick canopy on the water surface, which reduces light availability for rosette-type plants and enhances their competitive edge [24,26,27]. Interspecific competition studies suggest M. spicatum’s dominance over V. denseserrulata, as evidenced by the decreased plant number and height of V. denseserrulata with increasing M. spicatum density [28]. This competitive advantage of M. spicatum would negatively affect the growth of rosette-type V. denseserrulata, potentially leading to vegetation community degradation and hindering the restoration efforts. Furthermore, interspecific competition also occurs below ground [29,30]. Harvesting mainly affects above-ground stems and leaves; however, understanding the responses of root systems to harvesting is crucial since roots are key for nutrient absorption [31,32]. However, there is limited research on the impacts of harvesting on submerged plant root systems.

In this study, through a mesocosm experiment involving harvesting M. spicatum at a constant height, we examine the responses of V. denseserrulata and M. spicatum growth to the harvesting and the effects on water quality. We hypothesized that harvesting of M. spicatum (1) would inhibit the main stem growth of M. spicatum but increase the cumulative weight of M. spicatum, and (2) facilitate the growth of V. denseserrulata by improving light conditions, therefore, reducing nutrient concentrations in the water. Our results may offer new insights into how harvesting impacts species interaction and provide important implications for the management of restored lakes after transplanting submerged macrophytes when facing the overgrowth of canopy-forming species, e.g., M. spicatum.

2. Materials and Methods

2.1. Experiment Set-Up

The experiment was conducted at Dongshan station, Suzhou, China (31°02′ N, 120°25′ E) from August to October 2021, lasting for 78 days. Throughout the experiment, the average water temperature was 24.9 ± 4.2 °C. Sixteen tank mesocosms (height, 83 cm; bottom diameter, 67 cm; top diameter, 82 cm; total volume, 300 L) were set. These tanks were subdivided into four treatments: monoculture control (only M. spicatum planted), monoculture harvest (M. spicatum with harvesting), polyculture control (V. denseserrulata and M. spicatum), and polyculture harvest (V. denseserrulata and M. spicatum with harvesting), with four replicates for each treatment (Figure 1). Each tank was filled with 10 cm sediments and 60 cm water collected from the pond at Dongshan station. The sediments were mixed well and were subsequently sieved through a 0.5 cm mesh to eliminate large particles. The concentrations of total nitrogen (TN) and total phosphorus (TP) of the sediments were 1.58 ± 0.05 mg·g⁻¹ and 0.15 ± 0.01 mg·g⁻¹, respectively. Water was similarly filtered through a 64 μm mesh to remove extraneous organisms and large particles, with initial TN and TP concentrations in the water column recorded at 0.67 ± 0.06 mg·L⁻¹ and 0.02 ± 0.00 mg·L⁻¹, respectively. To maintain a consistent water level during heavy rainfall, three holes were drilled in each tank at a depth of 60 cm and an extra tank was prepared as a water reservoir to refill the water reduced by experiment sampling and natural evaporation.

Submerged macrophytes were transplanted into each tank on 2 August 2021. Eight M. spicatum shoots (46.23 ± 0.65 g·m⁻²) were transplanted into each tank of monoculture treatments. Four V. denseserrulata (93.37 ± 2.40 g·m⁻²) and four M. spicatum shoots (26.25 ± 0.64 g·m⁻²) were transplanted into each tank of polyculture treatments. After one month, the submerged macrophytes exhibited robust growth, with M. spicatum reaching harvestable heights. Harvesting in two harvest treatment groups (monoculture harvest and polyculture harvest) commenced on 6 September 2021 and continued at intervals of 9 September, 28 September, 9 October, and 19 October 2021, with harvestings conducted at a water depth of 20 cm.

2.2. Sampling and Measurements

2.2.1. Water Samples

Water was taken from the integrated layers using a 1 L organic glass hydrophore. The concentrations of chlorophyll-a (Chl-a), TN and TP in the water column were determined. Chl-a was determined spectrophotometrically following filtration of water samples through a glass fiber membrane (0.45 μm) and extraction in a 90% (v/v) ethanol/water solution. Water TN and TP concentrations were spectrophotometrically analyzed following digestion with K₂S₂O₈ and H₂SO₄ at 120 °C for 30 min [33]. The water level in each mesocosm was kept constant by using the initial prepared water in a 3000 L bucket.

Light intensity (μmol m⁻² s⁻¹) at the water surface and 30 cm was measured using an underwater photosynthetically active radiation meter (Apogee MQ510, Logan, UT, USA), thereafter light attenuation coefficient (K, m⁻¹) in the water was calculated using the following equation [34]:

$$K={\displaystyle \frac{ln{I}_0-ln{I}_Z}{Z}}$$

where $Z$ represents the water depth, while $I_0$ and $I_Z$ denote the light intensities at the water surface and at depth $Z$, respectively.

2.2.2. Plant Morphophysiological Traits

At the end of the experiment, all submerged macrophytes in each of the mesocosms were collected and cleaned with tape water. Thereafter, the plants in each mesocosm were counted to record the total plant number, and then weighted together to calculate biomass of the plants. For the harvesting groups, the cumulative weight of M. spicatum was calculated as the sum of the weight of the plants remaining in the mesocosm and the harvesting weight of M. spicatum during the experiment [21].

Five V. denseserrulata or M. spicatum were randomly selected from each mesocosm for the trait measurements. The maximum width, length and thickness of the leaf, and the internode length of stolon, and stolon diameter of V. denseserrulata were measured at the end of the experiment. In addition, the stem length, and diameter, internode length and node number of the main stem, and branch number of M. spicatum were recorded. The maximum leaf width, maximum leaf thickness, stolon diameter, and main stem diameter were measured by a vernier caliper, while measurements of maximum leaf length, maximum root length, internode length of stolon and the main stem, and stem length were recorded using a ruler. Subsequently, the plants were divided into above-ground and below-ground parts and dried in an oven at 60 °C for 48 h to determine water content and the below/above-ground biomass ratio (B/A ratio). Furthermore, five V. denseserrulata or M. spicatum were randomly selected from each mesocosm and subjected to analysis for total root length, total root surface area, mean root diameter, total root volume, number of root tips, number of forks, and number of crossings using root scanning instrument and software (WinRHIZO Pro 2019, Regent Instruments Inc., Québec City, QC, Canada) [23].

Furthermore, the relative growth rate (RGR) of submerged macrophytes was calculated using the equation [35]:

$$RGR(\mathrm{g}·{\mathrm{g}}^{-1}·{\mathrm{d}\mathrm{a}\mathrm{y}}^{-1})={\displaystyle \frac{\mathrm{l}\mathrm{n}({\mathrm{W}}_f/{\mathrm{W}}_i)}{\mathrm{d}\mathrm{a}\mathrm{y}\mathrm{s}}}$$

where ${\mathrm{W}}_f$ (g) and ${\mathrm{W}}_i$ (g) represent the final and initial total biomass of V. denseserrulata or M. spicatum in each mesocosm, respectively.

2.3. Statistical Analyses

Linear mixed effect models with a Gaussian distribution were employed for statistical analysis, considering the non-independence of K, Chl-a, TN, and TP in the water column collected from each mesocosm during the experiment (“lmer” function in the “lme4” R (v4.2.3) package). Effects of the presence of V. denseserrulata, harvest of M. spicatum and time on physicochemical variables in the water column were examined using linear mixed effect models, with V. denseserrulata, harvest and time as fixed factors and mesocosms as random factors [36]. Then, using the ANOVA function and Wald χ² tests from the “car” R package, the analysis of variance parameters for the fixed factors of the linear mixed-effects models was calculated. Before analysis, all data were logarithmically transformed to meet the assumptions of the linear mixed models.

ANOVA analysis with Tukey’s test was used to compare the differences in the means of growth and morphological traits of submerged macrophytes among treatments. Prior to analysis, all data were tested for normality of distribution and variance of homogeneity using Shapiro–Wilk’s and Levene’s tests, respectively. When necessary, data were logarithmically transformed to meet assumptions. Visualizations of plant growth and morphological trait parameters among treatments were performed after Z-score standardization using the “pheatmap” R package. To identify disparities among plant traits, we used a hierarchical clustering algorithm with complete linkage based on their Euclidean distances. The ANOVA results were also displayed on the heatmap.

3. Results

3.1. Nutrient Concentrations

Harvesting M. spicatum had no significant effect on TN or TP concentrations (

Table 1. Linear mixed effects model results of total nitrogen (TN), total phosphorus (TP), light attenuation coefficient at the depth of 30 cm (K₃₀) and chlorophyll-a (Chl-a) in the water column.

	TN		TP		Chl-a		K30
	χ2	p	χ2	p	χ2	p	χ2	p
V. denseserrulata	6.3	0.01	2	0.2	3.9	0.05	19.3	<0.001
Harvest	0.7	0.4	0	0.8	0.1	0.8	4.1	0.04
Time	3.2	0.1	1.1	0.3	156.4	<0.001	4.0	0.04
V. denseserrulata × Harvest	0.9	0.3	1.2	0.3	0.3	0.6	4.2	0.04
V. denseserrulata × Time	4.4	0.04	0	0.9	9.6	0.001	14.0	<0.001
Harvesting × Time	0.5	0.5	0.1	0.8	0.1	0.8	4.7	0.03
V. denseserrulata × Harvest × Time	1	0.3	0.3	0.6	8.8	0.003	1.1	0.3

Note: Significant effects were marked in bold.

; Figure 2a,b). The presence of V. denseserrulata significantly reduced the concentrations of TN, and the effects varied over time (Table 1; Figure 2a). However, V. denseserrulata had no substantial effects on TP (Table 1; Figure 2b). Moreover, harvesting and V. denseserrulata presence did not interactively affect the concentrations of TN and TP during the experiment (Table 1).

3.2. Phytoplankton Biomass (Chlorophyll a Contents)

The biomass of phytoplankton (Chl-a concentration) varied significantly over time during the experiment (Table 1). In our study, both the harvesting of M. spicatum and the presence of V. denseserrulata had no significant effects on the phytoplankton biomass (Table 1; Figure 2c). Furthermore, the interaction effects of harvesting and V. denseserrulata on phytoplankton biomass were also not significant (Table 1, Figure 2c).

3.3. Light Attenuation

Both harvesting of M. spicatum and the presence of V. denseserrulata significantly reduced the light attenuation coefficient at 30 cm (K₃₀), and these effects changed with time (Table 1, Figure 2d). Moreover, the interaction effects between harvesting and V. denseserrulata presence on light attenuations were significant (Table 1, Figure 2d).

3.4. Growth and Morphological Traits of M. spicatum

At the end of the experiment, harvesting M. spicatum showed no effects on the cumulative weight and relative growth rate (RGR) of M. spicatum (

Table 2. Results of two-way ANOVA examining effects of harvesting and polyculture with V. denseserrulata on the growth and morphological traits of M. spicatum.

Traits	V. denseserrulata		Harvest		V. denseserrulata × Harvest
	F	p	F	p	F	p
Cumulative weight	72.5	<0.001	0.1	0.7	0.3	0.6
RGR	25.8	<0.001	0.1	0.8	0.4	0.5
Below/Above-ground biomass ratio	1.4	0.3	10.2	0.008	9.6	0.009
Branch number	6.1	0.02	2.0	0.2	0.1	0.8
Internode length of main stem	11.9	<0.001	4.2	0.04	0.2	0.6
Plant number	11.9	0.005	1.0	0.3	0.1	0.8
Root surface area	13.2	<0.001	14.0	<0.001	4.6	0.03
Root volume	6.5	0.01	10.8	0.002	4.2	0.04
Root number per plant	13.0	<0.001	0.1	0.8	0.8	0.4
Maximum root length	32.0	<0.001	1.2	0.3	1.9	0.2
Root crossings	62.3	<0.001	3.5	0.07	2.3	0.1
Root tips	27.6	<0.001	6.5	0.01	6.2	0.02
Root forks	47.1	<0.001	10.7	0.002	6.0	0.02
Total root length	32.6	<0.001	8.6	0.004	2.5	0.1
Diameter of main stem	5.2	0.03	2.9	0.09	0.6	0.4
Individual weight	0.0	0.96	18.8	<0.001	0.0	0.99
Root average diameter	0.4	0.5	15.5	<0.001	1.9	0.2
Stem length	18.6	<0.001	183.0	<0.001	19.4	<0.001
Leaf number per plant	0.2	0.6	28.4	<0.001	3.1	0.08
Node numbers of main stem	0.0	0.9	89.9	<0.001	4.29	0.04

Note: significant effects were marked in bold.

). However, the presence of V. denseserrulata significantly reduced the cumulative weight and RGR of M. spicatum (Table 2, Figure 3a,b).

The responses of the morphological traits of M. spicatum to harvesting can be categorized into three clusters (Figure 3c). The first cluster mainly included internode length, branch number, and plant number of M. spicatum, all of which significantly decreased when V. denseserrulata presented (Table 2, Figure 3c). In contrast, the second cluster, which comprised most of the root parameters (surface area, volume, length, etc.) of M. spicatum increased in the presence of V. denseserrulata (Table 2, Figure 3c). Additionally, harvesting M. spicatum significantly reduced surface area, volume, tips, forks, and total length of root variables of M. spicatum (Table 2, Figure 3c). The final category comprised stem dimensions, leaf number, and individual plant weight of M. spicatum, all of which significantly declined after harvesting (Table 2, Figure 3c).

3.5. Growth and Morphological Traits of V. denseserrulata

At the end of the experiment, harvesting M. spicatum significantly increased the total biomass and RGR of V. denseserrulata (

Table 3. Results of one-way ANOVA examining effects of harvesting M. spicatum on growth and morphological traits of V. denseserrulata.

Traits	F	p
Total biomass	7.4	0.03
RGR	6.6	0.04
Plant number	1.0	0.4
Height	8.3	0.01
Individual weight	4.8	0.04
Below/Above-ground biomass ratio	0.0	0.9
Leaf number per plant	0.36	0.6
Leaf length	7.6	0.01
Leaf width	6.9	0.01
Leaf thickness	0.6	0.4
Internode length of stolon	0.9	0.4
Diameter of the stolon	0.0	0.98
Root number per plant	0.0	0.9
Maximum root length	0.0	0.98
Total root length	0.6	0.4
Root surface area	0.0	0.98
Root average diameter	0.77	0.4
Root volume	0.2	0.7
Root tips	2.7	0.1
Root forks	0.0	0.9
Root crossings	1.0	0.3

Note: Significant effects were marked in bold.

, Figure 4a,b). Moreover, individual weight, height, length and width of the leaf of V. denseserrulata also increased after the harvesting of M. spicatum (Table 3, Figure 4c).

4. Discussion

Our results did not support the first hypothesis, as the cumulative weight of M. spicatum did not show significant differences between the harvest and control treatments. However, the stem length of M. spicatum was substantially reduced by harvesting. Our second hypothesis was partially supported as the harvesting of M. spicatum significantly enhanced the growth of V. denseserrulata, whereas water quality was not significantly affected by the harvest.

In our study, the length of the main stem of M. spicatum was suppressed by harvesting; however, the harvest did not result in notable changes in branch number, cumulative weight and RGR. Previous study emphasized the concept of apical dominance in plant growth, where nutrient allocation favors the apical meristem of the main stem, encouraging vertical growth [37]. The removal of apical dominance is expected to augment branch proliferation and induce compensatory growth in plants, potentially enhancing adaptability, growth rates, and biomass accumulation in damaged plants compared to undamaged ones [20,21]. However, in our experiment, the harvest of M. spicatum revealed no significant changes in branch number, the disparity in the cumulative weight and RGR. One plausible explanation for these findings could be the collection and removal of plant fragments post-harvest in our experiment, which may have hindered the formation of new vegetation and subsequent compensatory growth [16]. Additionally, resource limitations in our experiment could also have played a significant role. Li et al. found that cutting the leaves of V. natans stimulated the compensatory growth, and these effects were more pronounced in high nutrient content sediment treatment (TN: 1.49 mg/g; TP: 0.67 mg/g) [21]. In our study, the relatively low phosphorus content in the sediment (0.15 mg/g) does not seem to adequately support the compensatory growth of M. spicatum after harvesting.

Resource allocation is a critical aspect of life history strategies that submerged macrophytes can automatically adjust the biomass allocation of their roots, stems and leaves due to the changes in the environment, thereby enhancing their adaptive capacity in heterogeneous environments [29,38,39]. Harvesting of above-ground parts of M. spicatum likely induces stress, prompting a conventional expectation of reallocating resources from above-ground to below-ground parts [29]. However, in our experiments, various root parameters (surface area, volume, total length, etc.) of M. spicatum exhibited a significantly declining trend after being harvested, indicating a significant reduction in resource allocation to below-ground structures. This phenomenon may be attributed to shifts in the plant’s capacity to compete for above-ground resources (photosynthetic), impacting the plant’s below-ground competitive ability, or the utilization of stored resources below-ground to supplement the regrowth of the above-ground tissues [29,40]. Consequently, the reduced below-ground competitive potential of M. spicatum after harvesting could lessen its competition with V. denseserrulata for below-ground resources, which might be another pathway for stimulating the growth of V. denseserrulata in our study. Thus, harvest can be an effective way of controlling excessive submerged macrophyte growth, like M. spicatum, by simultaneously reducing competitive pressures both above-ground and below-ground.

The dynamics of submerged macrophyte communities often involve intricate competitive interactions for limited resources such as light and nutrients, which subsequently affect species richness and relative growth advantages [27,41]. The competitive dominance of canopy-forming submerged plants over rosette submerged plants like V. denseserrulata is well-documented [26,42]. Previous studies have shown that these canopy-forming species like H. verticillata and M. spicatum can cover the water surface and significantly restrict the growth of rosette species like V. natans [17,28]. However, harvest of canopy-forming or floating-leaved aquatic vegetation can enhance the growth of meadow-forming submerged macrophytes by eliminating the shading impacts in previous studies [16,43]. In our study, we also found that the harvesting of the canopy-forming M. spicatum significantly enhanced the RGR of the meadow-forming V. denseserrulata. This mainly results from the substantially lower light attenuation coefficient in the harvest group than that in the controls. Specifically, significant increases in total biomass and RGR of V. denseserrulata were recorded in our study. This phenomenon is consistent with the competitive release hypothesis, which suggests that a reduction in interspecific competition can lead to increased resource acquisition and growth in the released species [44]. Additionally, there was a discernible trend when investing more resources into above-ground growth, such as leaf expansion and height increase, to capitalize on the newly available resources. This is consistent with literature where harvesting of H. verticillata induced comparable outcomes of V. natans [17].

Harvesting M. spicatum can reduce the water nutrient concentrations by removing a significant amount of nitrogen and phosphorus from the water, therefore, improving the water quality [16]. Whereas, in our study, the concentrations of Chl-a, TN and TP were not significantly affected by harvesting of M. spicatum. Similar results were also found in a shallow eutrophic lake study that mechanical cutting of submerged macrophytes did not substantially affect the water quality, including variables of the concentrations of dissolved reactive and unreactive phosphorus, dissolved organic carbon and seston [45]. The insignificant effects of harvesting of M. spicatum on water quality in our study may be due to the initial low nutrient concentrations in all treatments and both M. spicatum and V. denseserrulata absorbed nutrients mainly from the sediments [31,32]. Additionally, harvesting-induced disturbance may lead to the resuspension of nutrients adhered to plant leaves or sediments [45,46]. This process can counteract the immediate nutrient removal benefits of harvesting. Furthermore, the exudates from damaged tissues during harvesting may alter the chemical composition and metabolic processes of the water, potentially diminishing the significance of harvesting on water quality [45]. Despite harvesting did not improve water quality as hypothesized, the light climate in the water was improved, indicating a useful strategy in the control of the overgrowth of canny-forming species of submerged macrophytes, such as M. spicatum in our study.

For the restoration and management of shallow lakes facing a high coverage of canopy-forming aquatic vegetation, our results suggest that harvesting of overgrowing species, such as M. spicatum in our study, can effectively enhance the growth of meadow-forming submerged macrophytes by eliminating their shading impacts [17,18,43], and reducing below-ground resource competition. The meadow-forming species, e.g., V. denseserrulata, have well-developed root systems that can effectively stabilize the sediment, thus maintaining a stable clear-water state of the restored lake. Nevertheless, there are limitations to our study. Our results were based on a mesocosm experiment without setting harvest intensity gradients, whereas different harvest intensities may have varied impacts on plant growth, competitive interactions and water quality [17,22,47]. In addition, factors such as nutrient loading and water depth, which can affect plant responses to harvesting, were not considered in our study [47]. Future studies should address these factors and explore interactions between M. spicatum and other macrophyte species under varying environmental conditions to refine harvest management strategies.

5. Conclusions

We found that the harvest of M. spicatum significantly improved light conditions by inhibiting the main stem growth of M. spicatum, and changing its root parameters, therefore, facilitating the growth of V. denseserrulata. However, the brunch number, RGR and cumulative weight of M. spicatum were not pronouncedly affected by harvesting. Contrary to our expectations, the harvest of M. spicatum showed insignificant effects on the concentrations of TN and TP, and the biomass of phytoplankton (Chl-a). In conclusion, the strategic harvesting of overgrowing submerged macrophyte species represents a sustainable and effective method for promoting the growth of desired aquatic vegetation while maintaining water quality.