Influence of Distribution Spacing on Intraspecific Competition in the Brown Seaweed Sargassum thunbergii Along the Luhua Coast, China

Abstract

Sargassum thunbergii is a dominant seaweed species in the intertidal zone along the coast of China. It provides various ecological services, such as primary productivity, marine carbon sequestration, and water purification. To investigate the population structure characteristics of Sargassum thunbergii, the Hegyi competition model was employed to quantify intraspecific competition within populations in the intertidal zone of Luhua Island, China. The results showed that the competition intensity decreased as a power function (y = 1.93x^−0.89, R² = 0.28) with increasing seaweed height. Intraspecific competition had minimal effects on seaweeds taller than 50 cm. Seaweeds at lower population levels exhibited more stable competition indices. Therefore, the model can reliably predict intraspecific competition intensity in Sargassum thunbergii. The sample circle method was applied to identify an optimal intraspecific competitive range of 50 cm for intertidal populations of Sargassum thunbergii. This study provides scientific guidance for seaweed spacing and rational harvesting during ecological restoration. Moreover, it offers valuable insight for conserving other macroalgae, such as Sargassum fusiforme, and restoring seaweed beds ecologically.

1. Introduction

Research on seaweed population ecology significantly lags behind terrestrial plant ecology. Current studies on seaweed populations primarily focus on fundamental aspects, such as taxonomy and distribution. Internal ecological relationships within seaweed populations, especially intraspecific and interspecific competition mechanisms, remain poorly understood [1,2]. Sargassum thunbergii (mouse tail seaweed) is a keystone species in the intertidal zones of China, possessing substantial ecological significance [3]. It provides critical habitat and food resources for marine organisms and plays a pivotal role in marine carbon cycling and water purification. In marine environments, interactions between water flow and vegetation significantly influence the survival of Sargassum thunbergii. Water flow alters material transport and affects nutrient distribution around seaweed, thereby influencing its efficiency in acquiring nutrients and light [4,5]. Investigating intraspecific competition within Sargassum thunbergii populations is necessary to understand their eco-logical relationships and to establish a scientific basis for marine ecosystem management and conservation.

Due to their shared ecological niche, neighboring plants of the same species compete for limited resources, including light, space, water, and nutrients. Such competition affects growth, survival, and development, causing spatial self-thinning [6,7]. This phenomenon commonly occurs in natural communities of seaweeds, including Sargassum thunbergii [8]. Intraspecific competition significantly influences growth, survival, distribution, density, community structure, and stability. Consequently, it impacts ecosystem services, such as nature-based solutions for coastal flooding and erosion control, and the ecological health of Sargassum thunbergii populations [9]. Compared with terrestrial plants, studies on the population ecology of seaweeds are considerably fewer [10]. Understanding macroalgae, such as Sargassum thunbergii, from an intraspecific competition perspective has long been a notable research gap in macroalgal ecology.

Sargassum thunbergii is a warm-temperate species within the Phaeophyta genus Sargassum, widely distributed in rocky reef matrices in the western Pacific Ocean [11,12,13]. It has crucial ecological functions, including high primary productivity in nearshore marine ecosystems and serving as an excellent habitat for marine organisms like crustaceans and annelids [14]. Sargassum thunbergii adsorbs heavy metals such as iron and arsenic, thereby improving water quality [15]. It is an important dominant species in intertidal zones along the Chinese coast, distributed from Liaoning Province to Guangdong Province [16]. Recently, under the global concept of green and low-carbon responses to climate change, macroalgae, including Sargassum thunbergii, have been recognized as key marine carbon sinks due to their substantial biomass and high carbon sequestration capacity [17]. Furthermore, Sargassum thunbergii plays a critical role in the restoration of nearshore seaweed beds in China and serves as an important bait and food source for economically valuable aquatic products, such as shellfish, sea cucumbers, and abalone [18,19].

The distribution of Sargassum thunbergii in the intertidal zone is striped and patchy [20]. The local population represented by each patch is often used as the study unit for analyzing population structure [21]. Currently, ecological research on Sargassum thunbergii primarily focuses on geographical distribution [22], physiological and biochemical responses to environmental stress, and carbon sequestration capacity assessments. These studies mostly examine individual plants and rarely analyze ecological relationships between local populations of Sargassum thunbergii and other species at the population level.

Luhua Island is located in the Ma’an Archipelago of Shengsi County, Zhejiang Province (Figure 1). Situated in a subtropical region and influenced by warm ocean currents, the island experiences moderate temperatures, balanced salinity, ample sunlight, and high water clarity. Due to runoff from the Yangtze River, Qiantang River, ocean currents, and tides, the surrounding waters are rich in nutrients. Influenced by winds and waves, Sargassum horneri primarily inhabits the northwestern coast of Gouqi Island [23]. The island’s rugged and winding coastline, abundant with reefs and rocks, provides favorable substrates for seaweed attachment with minimal human disturbance. This study focuses on the population dynamics of the dominant species, Sargassum thunbergii, in the intertidal zone of Luhua Island, Zhejiang Province. An intraspecific competition index model was developed to examine relationships between morphological characteristics and competition indices during the growth period of Sargassum thunbergii. The findings provide guidance for seaweed population management and the restoration of seaweed beds.

2. Methods

2.1. Study Site

Luhua Island, located in the northern part of Zhejiang Province, is a significant island within the Ma’an Archipelago. Due to the perennial influence of Yangtze River freshwater, the area supports abundant macroalgae and fishery resources. The average seawater temperature in the sampling area ranges between 14 and 15 °C, salinity is 27.5‰–29.4‰, pH is 8.02–8.21, and dissolved oxygen content ranges from 9.80 to 10.76 mg·L⁻¹. After ebb tides, numerous rock pools form within seaweed reef areas, supporting diverse macroalgae species. At noon, the maximum light intensity in the intertidal zone exceeds 1500 μmol photons·m⁻²·s⁻¹, while at depths of 5–10 cm below the water surface, it exceeds 900 μmol photons·m⁻²·s⁻¹. Water temperature in rock pools can surpass 25 °C [24]. According to ecological surveys conducted over the past ten years [25], seaweed biomass at Luhua Island is 5.27 kg/m². Dominant species include Sargassum thunbergii, Sargassum fusiforme, and Ulva pertusa. Seaweed such as Sargassum thunbergii provides continuous primary productivity for nearshore ecosystems and plays crucial roles in heavy metal absorption and water quality improvement [26].

The surrounding waters of Luhua Island exhibit relatively high seaweed coverage. High biomass species include Sargassum thunbergii, Hizikia fusiformis, and Undaria pinnatifida, with plant heights between 20 and 30 cm and average coverage exceeding 20%. The biomass of Sargassum thunbergii in the intertidal zone of Luhua Island is approximately 3759.21 g/m² (wet weight), representing 65.81% of the total macroalgal biomass in this area [27]. During the seedling stage, the distribution of Sargassum thunbergii is most dense, subsequently decreasing gradually over time, and stabilizing at maturity (Figure 2). The seaweed bed covers an area of approximately 36.86 hectares, extending to depths between 1 and 8 m.

2.2. Study Period

After conducting a field survey on Sargassum thunbergii distribution, a representative site in the intertidal zone of Luhua Island was randomly selected for detailed investigation. This survey occurred during the thriving period of Sargassum thunbergii, from 1 May 2024, to 1 July 2024. Each survey plot measured 2 m × 2 m, divided into four smaller squares (100 cm × 100 cm each). A rectangular coordinate system was established using the horizontal and vertical edges of each plot as references, and the relative coordinates of each seaweed within the plot were recorded. Macroalgae samples were collected to measure their height and wet weight. Using this dataset of population distribution within the survey plots, the competitive distance was calculated based on the relative coordinates of the target and competing seaweeds.

2.3. Determination of Competition Index

Among various numerical models for calculating intraspecific and interspecific competition indices of plant populations, the Hegyi competition model is widely used. This model calculates the resource demand of individual seaweeds and the competition indices between neighboring seaweeds, incorporating coordinates and height data from field surveys [28]. Results are expressed as mean ± standard deviation. Therefore, in this study, the Hegyi model [29] was utilized to determine the intraspecific competition index for Sargassum thunbergii:

$$C_I=\sum _{j=1}^N{\displaystyle \frac{D_j}{D_i}}\times {\displaystyle \frac{1}{L_{ij}}}$$

where $C_I$ is the competition index, $D_j$ is the height of the competitive seaweed, $D_i$ is the height of the target seaweed, $L_{ij}$ is the distance between the target and competitive seaweed, and N is the number of competitive seaweeds. A smaller $C_I$ value indicates weaker competitive intensity between the competitive and target seaweeds. The competition indices between each competitive plant and the target plant were first calculated using this formula, summed, and then averaged to obtain the intraspecific competition index of Sargassum thunbergii.

Coordinates of individual seaweeds collected during the field investigation of Sargassum thunbergii were used to determine distances between seaweed individuals using AutoCAD 2007 (Autodesk, San Rafael, CA, USA). Figures were generated using Origin 2018 (OriginLab, Northampton, MA, USA).

Based on seaweed coverage, topography, and distribution, a circular method with gradually increasing competition radii was employed to calculate intraspecific competition indices for Sargassum thunbergii. The competition radii examined in this study were 10 cm, 20 cm, 30 cm, 40 cm, 50 cm, 60 cm, 70 cm, 80 cm, 90 cm, and 100 cm. The optimal competition radius was determined by identifying the inflection point from the graph depicting relationships between competition radii and intraspecific competition indices. Seaweeds located at the sample edges may have competing seaweeds outside the survey area; thus, edge correction was required to eliminate edge effects on the calculated competition indices. Edge correction was performed using the wrap-around edge method, which involved extending the actual sample boundary to create a larger sample (6 m × 6 m), composed of nine smaller (2 m × 2 m) squares (Figure 3) [30]. During calculations, each seaweed in the actual sample served as the target seaweed, and competing seaweeds were identified from the actual sample and the surrounding eight replicated central plots.

3. Results

3.1. Distribution

A total of 32 seaweeds were identified at the survey site. Their heights ranged from 45.20 cm to 102.50 cm, with an average height of 72.69 ± 16.85 cm, and wet weights ranged from 53.26 g to 247.61 g, with an average wet weight of 144.04 ± 64.34 g per individual Sargassum thunbergii. Values following “±” represent standard deviation.

Sargassum thunbergii was categorized into seven groups based on height (

Table 1. Height distribution of objective and competitive seaweeds.

Height (cm)	Objective Seaweed			Competitive Seaweed
	Percentage	Average Height	Wet Weight	Number	Percentage	Average Height
40 ≤ H < 50	9.38%	46.50 ± 1.18	93.68 ± 26.13	19	4.40%	74.17
50 ≤ H < 60	18.75%	55.65 ± 2.53	109.01 ± 29.80	40	9.26%	74.69
60 ≤ H < 70	21.88%	66.89 ± 2.56	116.72 ± 51.27	81	18.75%	74.61
70 ≤ H < 80	15.63%	75.18 ± 3.68	161.23 ± 73.93	71	16.44%	71.4
80 ≤ H < 90	18.75%	85.17 ± 2.17	157.63 ± 77.45	104	24.07%	72.47
90 ≤ H < 100	6.25%	95.7	/	39	9.03%	75.39
100 ≤ H	9.38%	102.1 ± 0.35	203.34 ± 6.74	78	18.06%	71.21

). Individuals with heights ranging from 50 cm to 90 cm comprised 75.01% of the total population. Among these groups, the 60 ≤ H < 70 cm group showed the highest frequency (21.88%), with an average wet weight of 66.89 ± 2.56 g. Additionally, the 80 ≤ H < 90 cm group contained the highest number of competitive seaweeds (24.07%), followed by the 60 ≤ H < 70 cm group, which had 81 competitive seaweeds and an average height of 74.61 cm. The 40 ≤ H < 50 cm group contained only 19 competitive seaweeds. These results suggest a stable population structure of intertidal Sargassum thunbergii on Luhua Island.

3.2. Optimal Competitive Distance

The intraspecific competition index in Sargassum thunbergii evaluates the ability of individual macroalgae within a population to compete with neighboring plants for resources. The optimal competitive distance is a critical parameter for calculating the competition index [31,32]. The competition index was determined by incrementally expanding the radius of the sample circle (Figure 4). As shown in the figure, when the competition distance was less than 50 cm, the intraspecific competition index of Sargassum thunbergii decreased as the competition distance increased. When the distance exceeded 50 cm, although the number of competing seaweeds continued to increase, the spatial and nutrient competition between competing and target seaweeds diminished due to greater spacing, thus reducing the effective competition index. Therefore, using competing seaweeds within a 50 cm radius from the target seaweed is optimal for representing the intraspecific competition of Sargassum thunbergii in the intertidal zone of Luhua Island.

3.3. Optimal Competitive Height

Seaweeds oscillate irregularly under frequent disturbances from nearshore waves, causing friction, entanglement, and light blockage among neighboring seaweeds. Consequently, seaweed height significantly influences the intraspecific competition index of Sargassum thunbergii [33]. Based on the collected macroalgal samples, seaweeds were grouped by height in 10 cm increments, and the average competition index was calculated for each height group (Figure 5). Analysis showed that the competition indices imposed by neighboring seaweeds decreased with increasing height. The overall average intraspecific competition index for intertidal Sargassum thunbergii on Luhua Island was 0.32.

The 40 ≤ H < 50 cm group experienced the most intense competition (0.08 ± 0.03), accounting for 24.62% of the total intraspecific competition. When seaweed heights exceeded 50 cm, the competition index declined and stabilized. The H ≥ 100 cm group exhibited the lowest average competition index (0.03 ± 0.01). Figure 5 indicates that growing seedlings were most influenced by neighboring seaweed competition. As seaweed size gradually increased beyond 50 cm, seaweeds became more mature, enhancing their ability to compete for space and nutrients, thus stabilizing the population structure.

3.4. Relationship Between the Competitive Index and Height of Sargassum thunbergii

The distribution and growth of Sargassum thunbergii are influenced by a combination of individual performance, spacing, and environmental factors. Height significantly affects the competition index among neighboring seaweeds due to wave disturbances. Nonlinear regression analysis between seaweed height and the average competition index showed a negative correlation described by a power function (y = 1.93x^−0.89, R² = 0.28) (Figure 6). Figure 5 indicates that the average intraspecific competition index decreases with increasing height. A higher competition index was observed when seaweed height was lower, indicating weaker competitive ability in seedlings, placing them at a disadvantage within the population. As seedlings mature, competitive strength improves, reducing the impact from neighboring seaweeds and ultimately stabilizing population structure. Therefore, protection efforts should prioritize Sargassum thunbergii seedlings shorter than 50 cm.

The fitted curve for the straight-line distance between each seaweed and neighboring seaweeds versus the competition index is shown in Figure 7. The competition index decreased with increasing spacing, following a power function (y = 1.04x^−0.99, R² = 0.75).

4. Discussion

Intraspecific competition specifically involves mutual competition among multiple individuals of Sargassum thunbergii for light, nutrients, and living space, influencing their biological morphology, life history strategy, and community structure dynamics [34]. The Hegyi competition index model used in this study accurately evaluates the competition index among individual macroalgae for these resources [35].

Unlike rooted plants, macroalgae such as Sargassum thunbergii comprise holdfasts, fronds, branches, air sacs, and reproductive trays, but lack roots. Instead, macroalgae attach to rocky substrates using holdfasts and primarily absorb nutrients from the surrounding water through fronds. Under wave action, nutrient-rich water causes high-frequency oscillatory movements of the seaweed. Compared with rooted plants, nutrient competition among adjacent macroalgae is relatively weak, while spatial competition is relatively strong [36,37]. As macroalgae populations grow, wave forces acting on macroalgae increase with density, size, and blade width [38,39]. Therefore, wave energy attenuation and coastal protection provided by macroalgal canopies are enhanced. Frequent wave disturbances increase friction between macroalgae and rocky substrates, as well as neighboring macroalgae bodies, thereby raising the likelihood of macroalgae breakage and detachment, especially during storms.

The intraspecific competition index is closely related to the growth stage of seaweed. Based on long-term ecological studies and observations of Sargassum thunbergii [40], the distribution density during the seedling stage is higher, and distances between plants are shorter. As macroalgae gradually mature, weaker individuals are eliminated, and distribution density decreases. Since individuals of the same species occupy identical ecological niches, intensive intraspecific competition occurs between neighboring macroalgae, resulting in spatial self-thinning within intertidal populations [6,7]. Additionally, wave-induced oscillations of macroalgae exacerbate spatial competition, and such competition persists throughout population development [8]. Thus, density regulation driven by intraspecific competition is an important structural characteristic of local Sargassum thunbergii populations.

These findings align with results on intraspecific competition reported for Heptacodium miconioides by Jin et al. (2004), Quercus mongolica by Liu et al. (2020), and Torreya yunnanensis by Yu et al. (2022) [24,31,41,42]. Shorter macroalgae are shaded by taller individuals, which impedes their growth and increases competitive pressure. Taller macroalgae occupy the upper layers, receive more light and other growth resources, exhibit improved growth, and experience reduced competitive pressure. When plant height exceeds 50 cm, the intraspecific competition index stabilizes.

To the authors’ knowledge, this study represents the first attempt to link ecological characteristics of macroalgal populations with intraspecific competition. The study was limited to one location, and the sample size was relatively small. Furthermore, this research focused exclusively on population characteristics of Sargassum thunbergii during its prosperous period, without categorizing intraspecific competition dynamics at other life history stages, such as the seedling stage and rapid growth stage. Many macroalgae, including Ulva pertusa and Ishige okamurae, co-existed with Sargassum thunbergii, competing for light, nutrients, and living space, establishing specific interspecies competitive relationships.

Intraspecific competition among Sargassum thunbergii individuals is influenced by external factors, particularly wave action. Along the northeastern US coast, tidal and storm surges can elevate coastal wave heights by 0.7 m, significantly impacting coastal ecosystems [43]. From a theoretical modeling perspective, wave energy is dissipated by seaweed presence, generating wave-induced circulation around kelp beds, thereby altering local water flow, nutrient transport, and nutrient uptake by Sargassum thunbergii [44,45]. Under such conditions, intraspecific competition for nutrients and light among Sargassum thunbergii individuals may be complexly altered. Additionally, Sargassum thunbergii interacts with other species in intricate ecological relationships. In intertidal ecosystems, competition between Sargassum thunbergii and species such as Ulva pertusa influences species composition and community structure [46]. Comparing vegetation interactions in different ecological contexts reveals that in regions dominated by large kelp species, growth space for Sargassum thunbergii is constrained. Conversely, in shellfish farming zones, Sargassum thunbergii provides a habitat for shellfish, forming mutualistic relationships. These varying ecological scenarios offer broader insights into the dynamics of intraspecific and interspecific competition involving Sargassum thunbergii, contributing new perspectives on its ecological dynamics and supporting marine ecosystem conservation and restoration [47,48].

Future studies investigating various growth stages of Sargassum thunbergii are necessary to better understand the dynamics of intraspecific and interspecific competition within intertidal populations. Such research would offer technical support for conserving Sargassum thunbergii and restoring other macroalgal ecosystems.

5. Conclusions

During the investigation of intraspecific competition in the intertidal Sargassum thunbergii population at Luhua Island, the average height of macroalgae during the bloom period was 72.69 ± 16.85 cm, the average wet weight was 144.04 ± 64.34 g, and the total intraspecific competition index was 0.32, indicating a stable population structure.

The intraspecific competition of macroalgae depends on their spatial distribution within the population. At the seedling stage, Sargassum thunbergii exhibits limited access to light, nutrients, and space, resulting in higher intraspecific competition and a disadvantageous position in the population. As the macroalgae grow, the competition index decreases following a power function (y = 1.93x^−0.89, R² = 0.28), stabilizing once plant height exceeds 50 cm.

Additionally, analysis of the relationship between macroalgal spacing and the competition index revealed that as spacing increases, the competition index decreases (y = 1.04x^−0.99, R² = 0.75). An optimal competitive distance of 50 cm was determined for Sargassum thunbergii. Therefore, for effective ecological protection, special attention should be given to macroalgae with plant heights less than 50 cm. Moreover, selective harvesting and nurturing of seedlings could be used to adjust the distribution density of Sargassum thunbergii, maintaining community stability.

The findings from Luhua Island provide a valuable reference for seaweed research in similar coastal environments across China, including Shandong and Fujian. The Hegyi competition model used in this study effectively quantifies intraspecific competition and can be applied to other large macroalgae populations. Such applications can assist in understanding population structures and competition mechanisms, contributing to scientific management strategies, optimal spacing arrangements, and sustainable harvesting practices.

Globally, many coastal ecosystems face similar challenges regarding seaweed population dynamics. This study’s insights into the relationship between macroalgal height, spacing, and competition can guide seaweed research under various latitudes and marine conditions. Understanding how environmental factors and morphological characteristics influence competition will help researchers predict seaweed responses to climate change, including rising seawater temperatures and altered nutrient availability, thereby promoting global seaweed conservation and sustainable utilization.

Future research should focus on the annual and seasonal distribution, coverage, and density of Sargassum thunbergii. Long-term monitoring will document population changes and periodic patterns in the intertidal zone, alongside the impacts of environmental and climatic variability. Seasonal analyses will explore how factors such as light and temperature affect growth mechanisms. A combination of high-resolution satellite remote sensing and field quadrat surveys will precisely measure dynamic changes in coverage and density. Predictive models will be developed to forecast future trends, providing robust theoretical and empirical support for marine ecological protection, management, and the sustainable utilization of Sargassum thunbergii resources.