Growth Response of Submerged Macrophyte Vallisneria denseserrulata to Water Depth (Light Intensity) Changes Varies with Sediment Nutrient Level

Abstract

The re-establishment of submerged macrophytes is crucial for the ecological restoration of eutrophic lakes. Water depth (light intensity) and sediment nutrient levels are key factors influencing the growth of these macrophytes. Although their individual impacts have been extensively studied, their interactive effects remain unclear. We conducted a two-factor experiment to investigate the interactive effects of different water depths (50 cm and 190 cm) and sediment nutrient levels (fertile and infertile) on the growth and morphological traits of Vallisneria denseserrulata. We found that biomass, relative growth rate, below/above-ground biomass, ramet number, and leaf number significantly increased with decreasing water depth in fertile sediments, while no significant or less pronounced changes occurred for infertile sediments. The absence or weak responses to increased light intensity in infertile sediments are likely due to photoinhibition, which may be alleviated at higher nutrient levels in fertile sediments. Additionally, V. denseserrulata, in adapting to low-light environments (deeper water), increased plant height at the cost of decreased leaf number and below-ground biomass as water depth increased in fertile sediments. Our study demonstrated significant interactive effects between water depth (light intensity) and sediment nutrient levels on the growth and morphological traits of V. denseserrulata, indicating that their response to water depth (light intensity) strongly depends on sediment fertility.

1. Introduction

Submerged macrophytes play a critical role in maintaining the functions of lake ecosystems [1,2,3]. Their growth and distribution are influenced by a range of environmental factors, including nutrients, light intensity, temperature, and substrate characteristics [4,5].

Water depth primarily influences the growth of submerged macrophytes by altering the intensity of available underwater light [6,7]. Light intensity decreases with increasing water depth. Submerged macrophyte distribution along a depth gradient is primarily determined by light availability [8]. Submerged macrophytes exhibit high phenotypic plasticity, allowing them to adapt to reduced light intensity as water depth increases. This adaptation includes increased biomass allocation to above-ground growth, increased plant height, and reduced branch or leaf numbers [9,10]. Conversely, high light intensity can induce photoinhibition in these plants and thus reduce their growth rate [11].

Nutrients in both water and sediment can affect the growth and morphology of submerged macrophytes [12,13]. Some species, such as Vallisneria spp., take up more nutrients from sediment than from water [14,15]. Many submerged macrophytes adapt to changes in nutrient availability. For instance, in an experiment with Myriophyllum spicatum, Xie et al. (2013) found that plants grown at high nutrient levels produced longer shoots and shoot internodes than plants grown at low nutrient levels [16].

In shallow lakes, submerged macrophytes are often the dominant primary producer, but their biomass and coverage decline with eutrophication. Submerged macrophyte loss is mainly attributed to reduced light condition, resulting from phytoplankton shading [17]. The restoration of shallow lake ecosystems aims to increase light intensity above the sediment to meet the minimum light requirement for submerged macrophyte growth [18]. To achieve this, improvements in water clarity or reductions in water levels are often applied in practice [19,20]. The removal of nutrient-enriched surface sediments is a commonly used method to lower internal loading and enhance lake ecosystem recovery [21].

Although the effect of water depth (light intensity) [22,23,24] and sediment nutrients [25,26,27] on submerged macrophyte growth is well studied, the interactive effects of these two factors are not clear. In this study, a two-factor experiment was designed to study the interactive effects of sediment nutrient level and water depth (light intensity) on the growth and morphological traits of V. denseserrulata, a common submerged macrophyte in lakes in China. We hypothesized that water depth would significantly affect the growth and morphological traits of V. denseserrulata, with these effects depending on the sediment nutrient levels. The results have implications for the restoration of eutrophic lakes, as submerged macrophytes play key roles in determining the success of lake restoration [1,19,28].

2. Materials and Methods

2.1. Experimental Design

The experiment was initiated on 7 September 2022 in a restored area of Lake Yi’Ai, a subtropical lake located in Huangzhou District, Huanggang, Hubei Province, China [20], lasting 40 days. During the experiment, the mean concentrations of total nitrogen and total phosphorus were 1.69 (±0.02) and 0.027 (±0.004) mg/L, and the mean temperature and Secchi depth were 24.9 (±4) °C and 1.77 (±0.24) m, respectively. The following four treatments were applied: two water depths (50 and 190 cm) and two sediment nutrient levels (fertile and infertile,

Table 1. Total nitrogen (TN), total phosphorus (TP), and organic matter (OM) contents in two different sediment types.

Sediment Type	TN/g·kg−1	TP/g·kg−1	OM/%
Fertile sediment	1.52	0.77	6.7%
Infertile sediment	0.27	0.12	3.4%

), taken from different areas of the lake. Each treatment was replicated three times. Plastic pots with a bottom diameter of 20 cm and a height of 16 cm were used for planting the macrophytes and were suspended by nylon ropes from a floating bed on the lake surface at depths of 50 and 190 cm. Five V. denseserrulata seedlings with similar sizes and a total fresh weight of 2.0 ± 0.5 g were planted in each pot.

2.2. Plant Growth and Traits

At the end of the experiment, all plants were collected, rinsed with lake water, and brought to the laboratory. The fresh weight, plant height, and relative growth rate of the V. denseserrulata were measured and calculated. Plant biomass was calculated by fresh weight per plot area. The relative growth rate (RGR) of submerged plants is calculated as follows:

$$\mathrm{RGR}\left(\mathrm{mg}·{\mathrm{g}}^{-1}·{\mathrm{d}}^{-1}\right)=1000\ast \ln \left({\displaystyle \frac{{\mathrm{W}}_{\mathrm{f}}}{{\mathrm{W}}_{\mathrm{i}}}}\right)/\mathrm{D}$$

where W_i and W_f are fresh weight at the beginning and end of the experiment, respectively, and D is the number of experimental days [29].

2.3. Statistical Analysis

We used two-way ANOVA to explore the independent and interactive effects of water depth and sediment type on the plant growth and traits (biomass, relative growth rate (RGR), ramet number, below-ground to above-ground biomass ratio, leaf number and plant height). Data that did not satisfy normality and homoscedasticity were log-transformed and then analyzed. The difference was significant when p < 0.05. All analyses were conducted in SPSS (version 26.0, IBM Corp., Armonk, NY, USA).

3. Results

The results of the two-way ANOVA indicated a strong interaction effect between water depth and sediment nutrient level on the biomass, relative growth rate (RGR), ramet number, below-ground to above-ground biomass ratio, leaf number, and plant height of V. denseserrulata (

Table 2. Two-way ANOVA results on effects of sediment and water depth on biomass, relative growth rate, ramet number, below/above-ground biomass ratio, plant height, and leaf number of Vallisneria denseserrulata. p values below significance level (0.05) are in bold.

Factor		Sediment	Water Depth	Sediment × Water Depth
Biomass	F	48.3	0.7	13.2
	p	<0.001	0.4	0.007
Relative growth rate	F	45.5	2.7	13.3
	p	<0.001	0.1	0.007
Below/aboveground biomass ratio	F	2.5	25.2	20.8
	p	0.2	0.001	0.002
Ramet number	F	103.6	155	29.1
	p	<0.001	<0.001	0.001
Plant height	F	6.5	104.9	9.9
	p	0.03	<0.001	0.014
Leaf number	F	87.2	30.7	16.9
	p	<0.001	0.001	0.003

).

Plants grown in shallow water (50 cm) exhibited a significantly greater biomass, relative growth rate, leaf number, and below/above-ground biomass ratio compared to those grown in deep water and in fertile sediment, whereas no significant changes were observed in these metrics between the two water depths in infertile sediment (Figure 1).

In both fertile and infertile sediments, ramet number was significantly higher at 50 cm water depth than at 190 cm, while the magnitude of the difference between the two water depths was significantly larger in fertile sediments than in infertile sediments (Figure 1; Table 2).

Plant height showed a significant increase with water depths in both types of sediment, while the magnitude of increase was larger in the fertile sediments (Figure 1; Table 2).

4. Discussion

Our experiment showed a strong interaction effect between water depth and sediment fertility on the growth traits of the submerged macrophyte Vallisneria denseserrulata, indicating that the response of the plants to water depth (light intensity) is dependent on sediment nutrient availability.

Water depth is the main determinant of light intensity, affecting submerged macrophyte growth. Studies have generally shown a higher growth and reproduction of submerged macrophytes at increased light intensity resulting from decreasing water depth [30,31,32,33]. Our study revealed that, with decreasing depth, reproduction (ramet number), biomass, and relative growth rate increased in fertile sediments, likely due to increased light. The below/above-ground biomass ratio decreased with increasing water depth in fertile sediments, indicating a biomass allocation favoring leaf growth rather than root growth. We also found that leaf length increased with increasing water depth, while leaf numbers decreased. Strand & Weisner (2001) showed that plant height and branch length increased, but branch number decreased in the submerged macrophyte Myriophyllum spicatum with increasing water depth [9]. Other studies on Vallisneria have shown similar results, i.e., increasing leaf length [10] along with decreasing leaf number [34] and root/shoot ratio [35] with increasing water depth. Switching biomass allocation to produce taller plants at the expense of decreased investment in branch number and below-ground biomass is considered a relevant strategy for maximizing photosynthesis and survival under poor light conditions [9,31]. Biomass and relative growth rate did not change with depth in the infertile sediment group, whereas ramet number increased significantly, though to a lesser extent compared to the increase observed in the fertile sediment group. Leaf number and below/above-ground biomass ratio showed no significant differences in infertile sediments across water depths. The weak responses of V. denseserrulata to decreased water depth may be due to photoinhibition in shallow waters. Yang et al. (2022) experimentally showed that the biomass and relative growth rate of V. natans decreased across water depths from 127 to 60 cm [35], and other studies have reported similar findings [32,36,37,38].

Why did V. denseserrulata grown in fertile sediments not show growth inhibition in shallow waters? The roots of Vallisneria spp. are well developed, and sediment nutrients serve as the main nutrient resource for their growth [39]. High nutrient availability in the fertile sediment likely alleviated the inhibition in our experiment. High light intensity may increase ROS (reactive oxygen species) production, inducing photoinhibition and thus negatively affect plant growth [40,41]. However, sufficient nutrient supply can alleviate plant photoinhibition via the biosynthesis of antioxidant enzymes and the detoxification of ROS [42], likely contributing to the positive responses of biomass and reproduction to improved light conditions (decreasing water depth) in the fertile sediment group.

We found that the below/above-ground biomass ratio decreased with water depth in fertile sediment, indicating a biomass allocation favoring leaf growth over root growth, as an adaptation to reduced light conditions. This adaptation was also evidenced by the increase in leaf length with water depth (reduced light intensity), enhancing light harvesting to maximize the photosynthesis rate. Similar results have been reported in V. natans [32,35]. Strand & Weisner (2001) found increased plant height and branch length and decreased branch number in the submerged macrophyte M. spicatum with increasing water depth [9].

For the restoration of shallow eutrophic lakes, re-establishing submerged macrophytes is critical to achieving a clear-water state [28]. Improving light conditions is essential for submerged macrophyte recovery. Practical measures include lowering water level and increasing water clarity by the flocculation of suspended solids [19,43]. However, our study indicates that submerged macrophytes growth may be inhibited in clear shallow water when the sediment nutrient content is low. Thus, to restore submerged macrophytes in shallow areas (e.g., water depth < 50 cm), both sediment nutrient content and light condition should be considered, as they influence the speed and success of recovery. These factors should be thoroughly assessed in restoration plans.

5. Conclusions

There was a significant interactive effect between water depth (light intensity) and sediment nutrient level on the growth and morphological traits of submerged macrophyte Vallisneria denseserrulata. The response of the plants to water depth (light intensity) was significantly affected by the sediment nutrient level, which should be analyzed in planning and assessing the success of submerged macrophyte restoration.