Abstract
To investigate the distribution characteristics and driving factors of the biomass of the endangered seagrass Halophila beccarii along the coast of South China, this study focused on H. beccarii across six regions (Huachang Bay, Yangpu, Shajing, Tangjiawan, Yifengxi, and Zhao'an) in four provinces (autonomous regions) of South China. The spatiotemporal dynamics of its biomass and key environmental influencing factors were systematically analyzed.
The results indicated that: (1) The average above-ground biomass (AGB), below-ground biomass (BGB), and total biomass (TB) of H. beccarii along the South China coast were $(11.98 \pm 13.06)$ g·m⁻² DW (mean $\pm$ standard deviation SD, the same below), $(12.06 \pm 12.96)$ g·m⁻² DW, and $(24.05 \pm 23.70)$ g·m⁻² DW, respectively. Among them, the biomass in Tangjiawan was significantly lower than that in other study sites ($P < 0.05$). The biomass exhibited distinct seasonal variations, being lower in winter and spring and higher in summer and autumn. (2) Except for pH and nitrite concentration, other environmental factors (such as water temperature, salinity, inorganic phosphorus, nitrate, and ammonia nitrogen) showed significant differences among different study sites ($P < 0.05$).
(3) Correlation analysis revealed that AGB was significantly positively correlated with water temperature and inorganic phosphorus concentration ($P < 0.05$), but significantly negatively correlated with ammonia nitrogen concentration ($P < 0.05$); BGB was significantly positively correlated with inorganic phosphorus and nitrate concentrations ($P < 0.05$); TB was significantly positively correlated with inorganic phosphorus and nitrate concentrations ($P < 0.05$), but significantly negatively correlated with ammonia nitrogen concentration ($P < 0.05$). (4) Principal component analysis (PCA) results showed that water temperature and nitrite were the primary positive factors promoting total biomass accumulation, while ammonia nitrogen exerted an inhibitory effect. (5) Linear regression further confirmed that the influence of pore water physicochemical factors on total biomass exhibited a significant linear relationship ($R^2 = 0.118, P < 0.001$). The findings of this study possess significant scientific value for deeply understanding the ecological habits, environmental adaptation mechanisms, and causes of endangerment of H. beccarii, while providing a theoretical basis for the conservation and management of this species.
Full Text
Spatiotemporal Variations and Key Influencing Factors of Biomass in the Endangered Seagrass Halophila beccarii
Guangxi Academy of Sciences; Guangxi Marine Academy; Guangxi Mangrove Research Center; Guangxi Key Laboratory of Mangrove Conservation and Utilization; Beibu Gulf Coastal Wetland Ecosystem Field Scientific Observation and Research Station, Ministry of Natural Resources.
Abstract
To investigate the distribution characteristics and driving factors of the biomass of the endangered seagrass Halophila beccarii along the coast of South China, this study focused on populations in Yifengxi, Tangjiawan, and Zhao'an. We systematically analyzed the spatiotemporal dynamics of biomass and identified key environmental drivers. The results showed that the average above-ground biomass (AGB), below-ground biomass (BGB), and total biomass (TB) of Halophila beccarii varied across the South China coast. Notably, the biomass at Tangjiawan was significantly lower than at the other study sites ($p < 0.05$). Furthermore, the biomass exhibited distinct seasonal fluctuations, characterized by lower levels during winter and spring and higher levels during summer and autumn.
With the exception of nitrite concentration, all other environmental factors—including salinity, inorganic phosphorus, nitrate, and ammonium nitrogen—showed significant differences across the various study sites ($p < 0.05$). Correlation analysis revealed that above-ground biomass was significantly positively correlated with water temperature and inorganic phosphorus concentration, while it was significantly negatively correlated with ammonium nitrogen concentration. Below-ground biomass showed significant positive correlations with inorganic phosphorus and nitrate concentrations. Similarly, total biomass was significantly positively correlated with inorganic phosphorus and nitrate concentrations but significantly negatively correlated with ammonium nitrogen. Principal Component Analysis (PCA) indicated that water temperature and nitrite are the primary positive factors promoting total biomass accumulation, whereas ammonium nitrogen exerts an inhibitory effect. Linear regression further confirmed a significant linear relationship between the physicochemical factors of interstitial water and total biomass ($R^2 = 0.118, p < 0.001$). These findings provide important scientific value for understanding the ecological habits, environmental adaptation mechanisms, and causes of endangerment of Halophila beccarii, while offering a theoretical foundation for the conservation and management of this species.
Keywords: Halophila beccarii, biomass, interstitial water physicochemical factors, multivariate statistical analysis, South China coast
1. Introduction
Seagrasses are submerged angiosperms widely distributed across temperate and tropical waters, typically inhabiting intertidal and shallow subtidal zones \cite{Short 2007}. These plants exhibit high primary productivity, serving as critical components of shallow marine ecosystems by providing nutrients and energy through food chains and detrital cycles, while also transporting nutrients and organic carbon to the deep ocean \cite{Duarte Dorte, 2017}. Consequently, they contribute significantly to global carbon sequestration \cite{Mcleod 2011}. Furthermore, seagrasses protect coastlines by attenuating wave energy, trapping sediments, and regulating nutrient cycling; they also serve as effective bioindicators for coastal pollution \cite{Lewis Richard, 2009; Ondiviela 2014; Costanza 2017}.
Despite their immense value, seagrasses have received insufficient attention compared to coral reefs and mangroves \cite{Kumar Deepak, 2021}. Global climate change and anthropogenic stressors are currently accelerating the decline of seagrass meadows worldwide \cite{Waycott 2009}. Among various species, Halophila beccarii Ascherson is classified as "Endangered" by the IUCN due to its habitat vulnerability \cite{Short 2010}. In China, its distribution is localized within the coastal provinces of Guangxi, Guangdong, Fujian, and Hainan.
Biomass serves as the fundamental basis for energy flow and is a key indicator of ecosystem health \cite{2020}. Studying the relationship between the biomass of H. beccarii and key environmental factors not only helps predict ecosystem productivity but also provides a scientific basis for the protection and restoration of this species. This study aims to explore: 1) the magnitude and spatiotemporal differences of H. beccarii biomass along the South China coast; 2) the key environmental factors influencing biomass; and 3) the mechanisms underlying population development and self-recovery.
2. Materials and Methods
2.1 Study Area and Sampling Design
The study area includes: Huachang Bay (HCW) and Yangpu (YP) in Hainan Province; Shajing (SJ) in Guangxi; Tangjia Bay (TJW) and Yifengxi (YFX) in Guangdong Province; and Zhao'an (ZA) in Fujian Province. The study areas are characterized by a tropical marine monsoon climate.
[TABLE:1]
2.2 Sample Collection and Processing
In 2022, sampling sites were established based on the spatial extent of H. beccarii beds. At each location, transects were set perpendicular to the shoreline. Cylindrical samplers were used to collect sediment core samples, which were then rinsed through a 1 mm mesh sieve. Plants were separated into above-ground (AGB) and below-ground (BGB) components and dried at $60^{\circ}C$ to a constant weight to determine dry mass (DW).
2.3 Physicochemical Parameter Measurement
During low tide, sediment pore water was extracted using a syringe from a depth of approximately $10\text{--}15\text{ cm}$. Samples were analyzed for inorganic phosphate, nitrite, nitrate, and ammonium nitrogen. In-situ parameters, including salinity and total dissolved solids (TDS), were measured using a multifunctional water quality meter.
2.4 Statistical Analysis
Data were tested for normality using the Shapiro-Wilk test. One-way ANOVA and Tukey-Kramer post-hoc tests were used to identify significant differences ($p < 0.05$). Pearson correlation and Principal Component Analysis (PCA) were employed to explore relationships between biomass and environmental variables. Linear regression models were constructed to evaluate the influence of pore water factors on total biomass.
3. Results and Discussion
3.1 Spatiotemporal Variation in Biomass
The average AGB, BGB, and total biomass (TB) of H. beccarii along the South China coast were $11.98 \pm 13.06\text{ g DW/m}^2$, $12.06 \pm 12.96\text{ g DW/m}^2$, and $24.05 \pm 23.70\text{ g DW/m}^2$, respectively. Tangjiawan exhibited significantly lower biomass than other sites ($p < 0.05$), likely due to higher anthropogenic interference. Seasonally, biomass was generally lower in winter/spring and higher in summer/autumn, following the species' natural growth cycle.
[FIGURE:1]
3.2 Spatial Differences in Interstitial Water Factors
Except for nitrite, all physicochemical factors showed significant spatial heterogeneity ($p < 0.05$). Salinity followed the order HCW > ZA > YP > TJW > SJ > YFX. Nutrient concentrations also varied significantly, with inorganic phosphorus and nitrate being highest at HCW and lowest at SJ.
[FIGURE:2]
3.3 Multivariate Statistical Analysis
Pearson correlation revealed that TB was highly positively correlated with AGB ($r = 0.91$) and BGB ($r = 0.88$). Inorganic phosphorus showed a highly significant positive correlation with all biomass metrics ($p < 0.01$). Conversely, ammonium nitrogen was significantly negatively correlated with TB.
PCA identified components accounting for 77.19% of the variance. PC1 reflected water mineralization (TDS, salinity), while PC2 and PC3 reflected nitrogen cycling and temperature-driven nutrient release. Linear regression confirmed that TDS and water temperature were positive drivers, while ammonium nitrogen exerted an inhibitory effect ($R^2 = 0.118, p < 0.001$).
Moderate nutrient levels promote growth, but excessive ammonium nitrogen can be toxic, inhibiting photosynthesis. The results suggest that phosphorus limitation may hinder growth in Tangjiawan, while nitrate is a limiting factor in Huachang Bay.
4. Conclusion
This study reveals significant spatial heterogeneity and seasonal fluctuations in H. beccarii biomass along the South China coast. Water temperature, inorganic phosphorus, and nitrate are key promoting factors, while ammonium nitrogen acts as a primary inhibitor. These findings provide a scientific basis for the conservation of this endangered species, suggesting that management should prioritize monitoring nitrogen loads and selecting restoration sites with optimal thermal and nutrient conditions.