Distribution and Ecological Risks of Organic Carbon, Nitrogen, and Phosphorus in Dongzhai Harbor Mangrove Sediments, China

Abstract

This study characterized the spatial distribution and assessed the ecological risks of carbon, nitrogen, and phosphorus in sediments of the Dongzhai Harbor mangrove wetland, Hainan, China. Analysis of key environmental indicators (grain size, pH, TOC, TN, TP) across twenty-seven sediment cores (0–100 cm depth) revealed distinct decreasing land–sea gradients and vertical stratification of nutrient concentrations. Mangrove plant debris was identified as the primary source of sedimentary organic matter. Elemental ratio analysis indicated terrestrial inputs as the dominant phosphorus source. Significant positive correlations between TOC, TN, and TP in surface sediments suggested coupled nutrient dynamics. Vertical distribution of C/N to C/P ratios increased with depth, which may be related to increased nitrogen and phosphorus inputs due to regional human activities. Pollution assessment showed significantly higher ecological risks in surface sediments (0–50 cm), particularly near inland areas and dense mangroves, indicating co-regulation by vegetation processes and human impacts. These findings highlight significant spatial heterogeneity in ecological risks, underscoring the need for enhanced monitoring and targeted management strategies in critical land–sea transition zones.

1. Introduction

Mangrove wetlands, predominantly in tropical and subtropical intertidal zones, perform critical functions including wave attenuation, shoreline protection, pollution mitigation, biodiversity conservation, and maintaining coastal ecological equilibrium [1]. Compared to inland wetlands, mangrove systems exhibit greater openness, facilitating tidal exchange of organic carbon, nitrogen, and phosphorus with adjacent waters [2,3]. Functioning as pivotal land–sea ecotones, their geomorphology and root structures establish them as efficient sinks for sequestering materials [4]. However, escalating anthropogenic pressures intensify nutrient inputs into these ecosystems. Consequently, nutrient bioaccumulation occurs within mangrove food webs, with subsequent tidal export contributing to eutrophication in proximate nearshore waters [5,6]. Crucially, the enrichment and release of biogenic elements (notably nitrogen and phosphorus) within mangroves are key drivers of this coastal eutrophication [6]. Carbon, nitrogen, and phosphorus are essential nutrients for marine primary productivity [7]. Critically, nutrient stocks accumulated within sediments can be reactivated via resuspension, diffusion, and advection (natural or anthropogenic), transforming sediments from sinks into sources for the overlying water column [8]. Controlling sedimentary nutrient pools is therefore paramount for mitigating coastal eutrophication and sustaining aquatic ecosystem health.

The Dongzhai Harbor mangrove wetland, situated in the estuarine zone of Hainan Island’s coastline, China, functions as a representative land–sea interface ecosystem fulfilling critical roles in regional environmental regulation and biodiversity maintenance. However, recent rapid industrialization and maritime transport expansion have subjected this ecosystem to escalating pressures. Existing research has primarily focused on assessing heavy metal contamination and associated ecological risks in the area’s sediments [9,10,11,12], particularly their spatial distribution, sources, and risk levels. For instance, Liu et al. [13] identified aquaculture effluent and domestic sewage as dominant sources of Cu, Cd, and Zn; Guo et al. [14] demonstrated aquaculture-induced moderate arsenic contamination; Zhang et al. [15] established significant agricultural influence on Cd and Hg pollution.

In contrast, research on sedimentary organic carbon, nitrogen, and phosphorus remains limited. Existing studies have predominantly addressed nitrogen and phosphorus biogeochemical cycling [16,17], with insufficient attention to their spatial distribution and ecological impacts. The Dongzhai Harbor wetland periphery is characterized by agricultural and aquaculture villages, where intensive application of fertilizers and effluents—rich in nutrients and heavy metals—occurs. These contaminants exhibit high migration and deposition potential under hydrodynamic and topographic influences [18]. Given that mangrove sediments serve as significant nutrient reservoirs but also potential secondary pollution sources under disturbances, systematic investigation of their distribution, enrichment, and ecological consequences is essential for elucidating biogeochemical cycles, evaluating ecosystem health, and guiding regional sustainability. This study examines Dongzhai Harbor mangrove sediments, analyzing physicochemical properties, nutrient concentrations, distribution, and sources. The findings provide insights into sediment status, evaluate nitrogen and phosphorus release potential, and assess associated ecological risks. This work establishes a basis for pollution mitigation and sustainable management of Hainan’s mangrove wetlands and adjacent ecosystems.

2. Materials and Methods

2.1. Study Area

The Dongzhai Harbor Nature Reserve occupies the northeastern sector of Qiongshan District, Haikou City, Hainan Island, situated at the confluence of Wenchang and Haikou urban boundaries within Dongzhai Harbor Bay (Figure 1) [10]. This region experiences a tropical monsoon climate, with mean annual air and sea surface temperatures of 23.8 °C and 24.5 °C, respectively, and receives 1676 mm of annual precipitation. Tidal dynamics exhibit an irregular semidiurnal pattern, featuring a maximum range of 1.8 m and a mean range of 1.1 m [19]. Beigang Island at the reserve’s northern extremity connects to the Qiongzhou Strait via tidal channels. The harbor displays weak hydrodynamic energy, receiving discharges from four rivers originating east, south, and west. Substantial sediment loads delivered by storm-derived runoff provide optimal substrates for mangrove colonization [14]. The coastal terrain is predominantly low-lying, underlain by granitic bedrock [20], with surface mantling by unconsolidated Quaternary sediments constituting the primary harbor deposits [21].

Encompassing a total area of 3337.6 hectares, the reserve supports mangrove forests across 1771.08 ha [22]. It harbors 36 mangrove species spanning 19 families, representing 100% of China’s documented mangrove flora [23], highlighting its outstanding ecological significance. Adjacent administrative regions primarily comprise Yanfeng and Sanjiang townships, where local livelihoods are predominantly reliant on aquaculture and fisheries, with limited rice cultivation. Critically, aquaculture effluents are typically discharged untreated into mangrove wetlands, thereby potentially compromising regional ecological integrity [24].

2.2. Sample Collection

During 2020, 27 sediment sampling sites were established within Hainan’s Dongzhai Harbor mangrove wetland (Figure 1). Sampling employed a stainless-steel static gravity corer fitted with polyvinyl chloride (PVC) liner tubes (110 cm length × 8 cm diameter). Cores were extracted to 100 cm depth and sectioned contiguously into 10 cm intervals onsite. A total of 270 sediment samples were collected, sealed in polyethylene bags, and transported to the laboratory under controlled conditions. Samples underwent natural air-drying followed by removal of macroscopic biogenic debris. Subsequent to sieving through a 100-mesh sieve (<150 μm), homogenized sediments were stored in airtight containers within desiccators pending analysis.

The particle size distribution was measured using a laser diffraction particle size analyzer (Mastersizer 2000, Malvern Panalytical, Malvern, UK), with an analytical error of <1%. According to the World Reference Base for Soil Resources (WRB) edited by the International Union of Soil Sciences (IUSS), soil particles were classified as sand (2–0.05 mm), silt (0.05–0.002 mm), and clay (<0.002 mm) [25]. Soil pH was determined using a Mettler Toledo pH meter (Mettler Toledo Delta 320, Mettler-Toledo International Inc., Zurich, Switzerland). TOC content was measured using an elemental analyzer (VarioMacro-CHNS, Langenselbold, Germany) with an accuracy of 0.5%. TP was measured by X-ray fluorescence spectroscopy (ARL9800XP, Switzerland), with a relative error of less than 10% [26]. TN was determined using the semi-micro Kjeldahl digestion method, which involved digesting the sample with concentrated H₂SO₄ and a catalyst mixture (K₂SO₄:CuSO₄:Se = 100:10:1) at 380 °C for 60 min, followed by distillation and titration with HCl [27]. Quality control was performed using the Chinese national soil standard material GBW07403 (China), with detection results (mg/kg): TN/584, TP/350, and TOC/530. The recovery rates of the standard samples ranged from 91% to 109%, meeting the control requirements.

2.3. Data Processing

Spatial distribution patterns of nutrient concentrations were mapped using geostatistical analysis in ArcGIS 10.7 (ESRI, Redlands, CA, USA). Source apportionment of sedimentary TOC, TN, and TP was performed through descriptive and inferential statistics with SPSS 27 (IBM, Armonk, NY, USA). Data processing, statistical analyses, and visualization were executed in OriginPro 2021 (OriginLab, Northampton, MA, USA) and Microsoft Excel 2021. Multivariate ordination via redundancy analysis (RDA) was conducted using Canoco 5.0 software (Microcomputer Power, Houston, TX, USA) to evaluate soil-environmental factor relationships.

2.4. Risk Assessment Methods

The risk assessment methodology involved in this study is listed in

Table 1. Risk assessment methodology.

Name	Expression	Coefficient of Interpretation	Contamination Degree	Reference
organic nitrogen pollution index (ONI)	organic nitrogen (%) = total nitrogen (%) × 0.95	Indicators of Organic Nitrogen Risk in Surface Sediments of Waterbody Environments.	ONI < 0.033 Uncontaminated 0.033 ≤ ONI ≤ 0.066 Slightly Contaminated 0.066 ≤ ONI ≤ 0.133 Moderately Clean ONI ≥ 0.133 Organically Polluted	[28]
Organic Pollution Index (OI)	Organic Pollution Index = organic carbon (%) × organic nitrogen (%)	An important indicator of the combined risk of organic carbon and organic nitrogen in sediments.	OI < 0.05 Uncontaminated 0.05 ≤ OI ≤ 0.20 Slightly Contaminated 0.20 ≤ OI ≤ 0.50 Moderately Clean OI ≥ 0.50 Organically Polluted	[29]
Single Factor Pollution Index (Si)	${\mathrm{S}}_{\mathrm{i}} = {\mathrm{C}}_{\mathrm{i}}/{\mathrm{C}}_{\mathrm{s}}$	Si is the evaluation index for a single factor; Ci is the measured value of evaluation factor i (g/kg); Cs is the standard value of evaluation factor i (g/kg). The Cs value for TN is 0.55 g/kg; the Cs value for TP is 0.60 g/kg.	STP < 0.5 STN < 1.0 Uncontaminated 0.5 ≤ STP < 1.0 Low Pollution 1.0 ≤ STN < 1.5 Low Pollution 1.0 ≤ STP < 1.5 Moderate Pollution 1.5 ≤ STN < 2.0 Moderate Pollution STP ≥ 1.5 STN ≥ 2.0 Heavy Pollution	[30]
Comprehensive Pollution Index (FF)	$\mathrm{F}\mathrm{F} = \sqrt{{\displaystyle \frac{{\mathrm{F}}^2 + {\mathrm{F}}_{\mathrm{m}\mathrm{a}\mathrm{x}}^2}{2}}}$	FF represents the comprehensive pollution index; F represents the average value of STN and STP; Fmax represents the maximum value of STN and STP.	FF < 1.0 Uncontaminated 1.0 ≤ FF < 1.5 Low Pollution 1.5 ≤ FF < 2.0 Moderate Pollution FF ≥ 2.0 Heavy Pollution	[31]
Nemerow Integrated Pollution Index (PN)	$\mathrm{P}\mathrm{N} = \sqrt{{\displaystyle \frac{{CF}_{i_{ave}}^2 + {CF}_{i_{max}}^2}{2}}}$	CFiave: The weighted average value of the pollution factor for sampling point i after incorporating the weight value. CFimax: The maximum value of all heavy metal pollution factors at sampling point i.	PN ≤ 0.7: Safe Level 0.7 < PN ≤ 1: Warning Level 1 < PN ≤ 2: Low Pollution 2 < PN ≤ 3: Moderate Pollution PN > 3: Severe Pollution	[32]

.

3. Results

3.1. Particle Size and pH

The Dongzhai Harbor mangrove system occupies an intertidal setting, with wetland sediments exhibiting gray-black to gray-yellow hues and predominantly silt-clay compositions. Sediment core pH (0–100 cm depth) ranges from 3.21 to 8.41 (mean ± SD: 6.95 ± 1.32; CV: 19.02%), indicating limited spatial heterogeneity and minimal environmental influence. A pronounced pH gradient occurs across the study area, transitioning from neutral to weakly acidic conditions with depth, converging regionally toward weak acidity (

Table 2. Physical and Chemical Properties of Sediments.

	TOC (%)	TN (mg/kg)	TP (mg/kg)	pH	Sand (%)	Silt (%)	Clay (%)
Max	3.20	1919	1640	8.41	70.2	78.2	22.6
Min	0.07	154	78	3.21	6.5	27.2	2.6
Average	1.06	548	377	6.95	37.47	52.62	9.91
Median	0.82	417	302	7.53	36	55.2	8.9
CV	66.63%	64.62%	53.47%	19.02%	34.94%	21.31%	39.92%

; Figure 2). Elevated pH values (>7.0) cluster in marine-adjacent zones, while depressed values (<5.5) localize in mangrove-dominated interiors distal from the coastline. The content of sand, silt, and clay ranges from 6.50% to 70.20%, 27.20% to 78.20%, and 2.60% to 22.60%, respectively. The average distribution is silt (52.62%) > sand (37.47%) > clay (9.91%). The wetland sediments are mainly sand and silty, and there is no significant trend in particle size distribution with increasing depth (Table 2, Figure 2). Sand is predominantly found near the coastal areas, especially at point ZK24, where the sand content is the highest, accounting for 58.5% of the total. The distribution of silt and clay is generally the opposite of sand, with the sum of silt and clay content exceeding 80% at most points, particularly in the areas with dense mangrove vegetation near points ZK8, ZK9, ZK12, and ZK18.

3.2. Distribution of Carbon, Nitrogen, and Phosphorus

Sedimentary TOC concentrations range from 0.07% to 3.20% (mean: 1.06%; median: 0.82%), demonstrating surficial enrichment with higher values in upper layers (vertical CV: 6.74%) (Table 2; Figure 3). Spatial maxima occur proximal to forested zones and the wetland park, significantly exceeding peripheral areas. Vertically, TOC peaks at 0–10 cm depth, declining nonlinearly to stabilize below 50 cm (Figure 4). TN content spans 476.93–694.07 mg/kg (mean: 548.22 mg/kg; median: 524.22 mg/kg; CV: 13.87%), exhibiting maximum concentrations (694.07 mg/kg) in surficial sediments. TN decreases parabolically with depth, stabilizing below 80 cm (Table 2; Figure 3). TP concentrations range from 335.25 to 483.27 mg/kg (mean: 377 mg/kg; median: 356.59 mg/kg), mirroring TN’s parabolic vertical decline and stabilization below 80 cm. Comparative assessment against the Ontario Provincial Sediment Guidelines (LEG) [33] reveals: TOC and TN concentrations exceed LEG risk thresholds throughout the wetland; TP exceeds thresholds in high-concentration zones. This indicates regionally significant sedimentary nutrient pollution risks requiring urgent management attention.

4. Discussion

4.1. Factors Influencing Nutrient Enrichment

Sedimentary enrichment of carbon, nitrogen, and phosphorus in Dongzhai Harbor mangrove wetland and adjacent harbor areas is co-modulated by natural processes and anthropogenic drivers. Significant positive correlations exist among TOC, TN, and TP (

Table 3. Correlation coefficients between various indicators of sediment nutrients and particle size (n = 27).

	Sand	Silt	Clay	TOC	TN	TP	pH
Sand	1.000
Silt	−0.980 **	1.000
Clay	−0.690 **	0.582 **	1.000
TOC	−0.390 *	0.408 *	0.229	1.000
TN	−0.552 **	0.561 **	0.335	0.920 **	1.000
TP	−0.611 **	0.591 **	0.413 *	0.578 **	0.771 **	1.000
pH	0.173	−0.174	0.009	−0.678 **	−0.559 **	−0.190	1.000

Notes: ** Correlation is significant at the 0.01 level (2-tailed); * Correlation is significant at the 0.05 level (2-tailed).

), reflecting their coupled spatial distribution. Spatially, nutrient-depleted sites (low TN/TP) localize in unvegetated open-water zones, whereas TOC/TN/TP maxima concentrate in fine-grained sediment accumulation areas (Figure 4)—consistent with surface sediment patterns documented in Bangladesh’s Sundarbans mangroves [34]. Historically, intensive aquaculture (fish/shrimp ponds) and livestock operations have introduced substantial nitrogen-phosphorus loads via wastewater discharge, exacerbating regional eutrophication and environmental degradation [35]. Resultant eutrophic conditions promote proliferative blooms of macroalgae and plankton, which fuel explosive growth of water flea populations. This cascade threatens mangrove ecosystem integrity, as evidenced by the 2013 water flea outbreak triggered by unregulated effluent discharge. That event degraded Dongzhai Harbor water quality to Grade V standards, damaging mangrove root structures. This impairment inhibited sediment organic matter decomposition, driving anomalous carbon-nitrogen accumulation [36,37].

This study demonstrates a statistically significant positive correlation between sedimentary TN and TOC in Dongzhai Harbor mangrove wetland, indicating shared exogenous input pathways and synergistic enrichment behavior within the sediment matrix. Anthropogenic activities—particularly discharge of domestic wastewater and aquaculture effluents near harbor zones—constitute primary drivers of elevated carbon/nitrogen loading. As exemplified by Yanfeng Town, implementation of new wastewater treatment infrastructure and upgraded drainage networks has effectively reduced direct domestic effluent discharge. These interventions improved sediment quality and indirectly modulated organic matter-nitrogen accumulation dynamics.

Mangrove plants exert significant biogeochemical control on nitrogen enrichment through root-mediated assimilation and fixation. Upon senescence, decomposing plant debris releases nitrogen that undergoes transformation and sequestration within sediments [38]. This study documents decreasing TN concentrations with depth (0–50 cm), exhibiting surficial enrichment patterns consistent with TOC and TP vertical profiles. This congruence indicates coupled enrichment dynamics regulated by sedimentary organic matter content and particle size composition [39]. Further analysis establishes that sediment-bound nitrogen predominantly exists in organic forms. Within the 0–40 cm depth interval, TN and TOC spatial distributions demonstrate high coherence, confirming their co-sourcing. Below this depth, however, intensifying anaerobic conditions progressively enhance denitrification and diagenetic mineralization rates. This diagenetic acceleration drives substantial TN depletion, inducing distinct vertical decoupling between TN and TOC [40].

Sediment particle composition critically regulates nutrient enrichment dynamics. Reduced flow velocities resulting from extensive mangrove root systems facilitate sedimentation of coarse particulates and suspended fines, forming silt- and clay-dominated depositional environments. These fine-grained sediments possess elevated nutrient adsorption capacities attributable to high specific surface area and organic content [41]. Hydrodynamic sorting processes indirectly modulate TOC and TN spatial distributions through particle size fractionation [42]. In this study, TOC and TN maxima spatially coincide with fine-grained sediment zones, demonstrating that hydrodynamic processes, plant-mediated effects, and anthropogenic inputs collectively regulate sedimentary nutrient spatial patterns in Dongzhai Harbor mangrove wetland.

Organic matter sources comprise autochthonous inputs (mangrove litter, root detritus) and allochthonous inputs (external hydrologic contributions) [43]. In Dongzhai Harbor mangrove wetlands, organic carbon content and density generally decrease with depth at most sites, though vertical stratification varies among mangrove communities—consistent with patterns documented in Yap (Micronesia), Palau, and Okinawa’s Manko Wetland [44,45]. Surface enrichment of soil organic carbon primarily derives from concentrated litter and root biomass, which enhances the assimilation efficiency of exogenous carbon. Concurrently, fluvially transported organic matter undergoes cyclic tidal deposition in this estuarine system, further amplifying allochthonous contributions to TOC vertical distribution. Relatively stable deep-soil organic carbon concentrations likely attributable to: (1) Minimal pH and salinity fluctuations at depth, reducing microbial activity inhibition and slowing organic matter decomposition [46]; (2) Diagenetic mineralization converting organic carbon to inorganic forms, driving progressive vertical depletion. Discrete high-value zones in deeper strata may reflect sediment adsorption capacity and depositional environment instability [47].

Sedimentary TP concentrations fall within the characteristic global range for mangrove wetlands (0.1–16 mg/g) [48]. The significant positive TP-TOC correlation (Table 3) indicates shared source pathways. Significant TP enrichment occurs above 50 cm depth, with wastewater discharge from regional economic development being a primary driver of surficial and subsurface phosphorus accumulation. Concurrently, TP vertical distribution is mediated by sediment particle size composition [49], where finer textures correlate with elevated phosphorus retention [50]. Sediment pH regulates phosphorus adsorption-release dynamics: acidic conditions promote dissolution of iron/aluminum oxides, releasing adsorbed phosphorus, whereas alkaline environments enhance calcium-phosphorus (Ca-P) stability. In Dongzhai Harbor lagoon sediments, TP exhibits a negative correlation with pH, while iron/aluminum oxide content positively correlates with phosphorus enrichment [50]. Following reserve establishment, modest mangrove expansion enhanced nutrient re-assimilation from decomposing litter, increasing phosphorus capture efficiency and elevating sedimentary phosphorus in restored zones. Studies have shown that sedimentary phosphorus exists mainly in an inorganic form, and its state of existence is closely related to ferromanganese oxides and aluminosilicate minerals.

The Redundancy Analysis (RDA) results (Figure 5) demonstrate that the first axis (Axis 1) explains 85.65% of response variable variation, representing the principal gradient governing major nutrient distribution in sediments; TOC, TN, and TP exhibit strong directional congruence with Axis 1, confirming their significant positive correlation likely co-regulated by organic matter input and retention mechanisms, where clay content shows marked positive association with these nutrients, indicating that finer sediment particles enhance adsorption and preservation of organic matter and nutrients—consistent with mangrove wetland characteristics where low-energy hydrodynamic settings promote organic deposition.

Additionally, pH exhibits significant negative correlations with TOC, TN, and TP, indicating acidic conditions enhance organic carbon and nutrient retention—likely linked to organic acid release from mangrove roots altering sediment pH regimes. Sediment depth demonstrates positive correlations with C/N and C/P ratios, reflecting shifting organic matter sources and decomposition progression with depth: surface sediments receive fresh inputs (e.g., mangrove litter) while deeper layers contain stabilized humified matter. Notably, N/P and C/P ratio variations are better explained by the second RDA axis, suggesting their independence from primary nutrient concentrations, potentially indicating differential sourcing of nutrients including spatial heterogeneity in exogenous nitrogen inputs and sediment phosphorus adsorption–desorption dynamics.

In summary, sedimentary enrichment of carbon, nitrogen, and phosphorus in Dongzhai Harbor mangrove wetland and adjacent harbor areas is co-regulated by natural sedimentation processes and anthropogenic activities; RDA reveals significant positive TOC-TN-TP correlations strongly associated with fine-grained sediments and acidic conditions, demonstrating the governing role of low-energy sedimentary environments and mangrove-mediated biogeochemical processes on nutrient accumulation, while sediment depth critically influences elemental ratios and vertical distribution—with surface layers dominated by fresh organic inputs and deeper horizons exhibiting stable accumulation—further modulated by exogenous pollution sources (aquaculture/domestic wastewater) and mangrove biological functions that collectively dictate spatiotemporal nutrient distribution patterns.

4.2. The Sources of Organic Carbon, Nitrogen, and Phosphorus

Organic carbon in mangrove wetland ecosystems exhibits multi-source inputs—primarily marine-derived, terrestrial, and autochthonous production—wherein mangrove litter decomposition and root turnover constitute the dominant autochthonous source, while allochthonous inputs from marine algae, fungi, and anthropogenic disturbances significantly contribute to sedimentary organic accumulation [51]; this study demonstrates significant spatial heterogeneity in sediment TOC, TN, and TP concentrations (p < 0.05) within Dongzhai Harbor mangrove regions, with mangrove forest zones markedly exceeding harbor water concentrations (Figure 6) and exhibiting strong inter-element correlations, indicating that mangrove-derived autochthonous inputs dominantly regulate sedimentary C-N-P content, as substantial litter and root-derived organic matter accumulates within forest areas sustaining elevated TOC-TN-TP levels.

Furthermore, sediment TOC, TN, and TP concentrations exhibit land-to-sea decreasing gradients, potentially reflecting dynamic migration processes from sediment-water interfaces to seawater; specifically during low-tide periods, prolonged sediment-seawater contact durations and enhanced material exchange frequency facilitate organic carbon and nitrogen leaching-diffusion, a mechanism highly congruent with Qiu Yue et al.’s findings in the Jiulong River estuary [52], further validating mangrove sediments’ critical ecological function in releasing dissolved/particulate organic carbon to adjacent waters.

The carbon-to-nitrogen ratio (C/N) serves as a critical indicator for organic matter source discrimination based on characteristic elemental composition differences; per classical theoretical models [53,54], C/N > 10 indicates exogenous organic matter dominance whereas C/N < 10 signifies predominant endogenous contributions, with C/N ≈ 10 reflecting dynamic endogenous-exogenous equilibrium—specifically, distinct biological sources exhibit characteristic C/N ranges: higher plants (14–23), algae (5–14), plankton (6–13), and aquatic organisms (2.8–3.4); applying a binary mixing model of mangrove detritus (C/N = 30) and marine phytoplankton detritus (C/N = 11) [55], mangrove-derived contributions to sedimentary organic matter range 2–80% (mean 68%), substantially exceeding phytoplankton contributions (20–98%, mean 32%), while spatially, the land-to-sea decreasing TOC/TN gradient validates the transition from mangrove-dominated to phytoplankton-enhanced contributions.

The TOC/TN ratio exhibits distinct vertical differentiation with lower surface values and higher deep-layer values (Figure 7), potentially driven by dual mechanisms: firstly, surficial aquaculture wastewater discharge causes rapid TN decline with depth exceeding TOC reduction rates, elevating deep-layer TOC/TN; secondly, strongly reduced deep-sediment environments facilitate organic nitrogen mineralization to NH₄⁺-N—released into porewater—where oxygen-limited conditions inhibit nitrification, accumulating NH₄⁺-N that diffuses upward along concentration gradients, further increasing subjacent TOC/TN ratios [56]; concurrently, decreasing TOC/TP and TN/TP ratios from deep to surface sediments indicate rising exogenous phosphorus inputs linked to agricultural expansion and intensified mariculture [57], exemplified near Dongzhai Harbor Reserve by Yanfeng’s coastal aquaculture and Sanjiang’s agriculture, which influence sedimentary nutrients through dual pathways: direct loading from excess feed/metabolic products elevating aqueous N-P [58,59], and tidal transport of nutrient-rich effluents into wetlands promoting allochthonous nutrient deposition—these anthropogenic inputs supplement sedimentary organic carbon/nitrogen while shaping land-to-sea decreasing TOC-TN gradients along the east–west axis; notwithstanding, although aquaculture-derived particulate organic matter may contribute to TOC/TN variability, its compositional specificity and flux intensity remain unquantified, thus warranting future targeted quantification to assess sediment nutrient cycling contributions.

In summary, the spatial and vertical distribution characteristics of TOC, TN, and TP in Dongzhai Harbor mangrove wetland sediments indicate significant multi-source inputs and anthropogenic influence. Terrestrial wetland areas exhibit elevated TOC, TN, and TP concentrations, primarily governed by endogenous inputs such as mangrove litter and root turnover. Conversely, in marine-proximal zones, the proportion of exogenous organic matter (e.g., phytoplankton) increases, manifesting a declining concentration trend. The C/N ratio delineates the relative contributions of mangrove detritus and marine phytoplankton to the organic matter composition, while the downward trends of the TOC/TP and TN/TP ratios with depth suggest enhanced exogenous phosphorus inputs in recent years. Aquaculture effluents and agricultural non-point sources constitute key drivers affecting nutrient accumulation and distribution. This study elucidates the sources and migration mechanisms of carbon, nitrogen, and phosphorus in mangrove sediments, providing a scientific basis for ecological protection and pollution control.

4.3. Nutrient Pollution Assessment

As archetypal land–sea ecotones, mangrove wetlands are widely acknowledged as natural bioremediation units for eutrophic wastewater, yet their ecological responses to pollutant inputs demonstrate a distinct dual nature. While filtration through mangrove systems effectively reduces nitrogen and phosphorus in municipal and aquaculture effluents, demonstrating positive ecological functions for wastewater treatment [60,61], impacts on the intrinsic mangrove ecosystems are more complex. Some studies indicate that moderate nutrient inputs can enhance primary productivity and biomass, stimulating plant growth [62]. Nevertheless, mounting evidence indicates that excessive discharges induce ecosystem degradation. Lovelock et al. [63] demonstrated that nutrient enrichment, while stimulating rapid mangrove plant growth, concurrently reduces stress tolerance, increasing susceptibility to degradation. Boehm et al. [60] further revealed that soil hypoxia induced by urban sewage effluents alters sediment physicochemical properties, leading to adverse effects on microbial communities and root architectures. Building on this foundation, Williamson et al. [64] demonstrated that wastewater inputs alter sediment redox conditions and enhance microbial mineralization, thereby releasing previously sequestered organic carbon as greenhouse gases. This process drives a critical transition in ecosystem function—converting mangrove wetlands from carbon sinks to net carbon sources.

As presented in Table 2, the TOC, TN, and TP contents in the study area ranged from 0.07% to 3.20%, 154 to 1919 mg/kg, and 78 to 1640 mg/kg, respectively, with mean values of 1.06%, 548 mg/kg, and 377 mg/kg. The coefficients of variation for TOC, TN, and TP were 66.63%, 64.62%, and 53.47%, respectively, indicating substantial spatial variability for all three nutrients. Based on the content levels, coefficients of variation, and ranges, the distribution of TOC, TN, and TP is strongly influenced by environmental factors, suggesting significant anthropogenic influence in addition to natural conditions.

According to sediment ecotoxicity thresholds established by the Ontario Ministry of the Environment and Energy, Canada (TN: 0.55 g/kg mild effect, 4.80 g/kg severe effect; TP: 0.60 g/kg mild effect, 2.00 g/kg severe effect) [33], significant spatiotemporal pollution variations are observed in Dongzhai Harbor mangrove sediments (Figure 8). Vertical analysis reveals the TN pollution index (STN) ranges from 0.28 to 3.49 (mean = 1.0), exhibiting mild pollution (STN > 0.8) within the 0–40 cm depth and a surface enrichment trend. The TP pollution index (STP) ranges from 0.13 to 2.73 (mean = 0.63), indicating mild pollution (STP > 0.5) throughout the profile, with a significantly higher accumulation rate in surface sediments compared to deeper layers. The organic pollution index displays a bimodal distribution: the organic index ranges from 0.001 to 0.58 (mean = 0.08), generally indicating clean conditions, but exhibiting a notably elevated level (0.58) in the 0–10 cm layer; the organic nitrogen index ranges from 0.01 to 0.18 (mean = 0.05), with pollution concentrated in the 0–20 cm layer, signifying a recent surge in surface organic matter input.

Spatial analysis indicates the comprehensive pollution index (FF) ranges from 0.470 to 2.711 (mean = 1.105), corresponding to mild pollution levels overall, with 63% of monitoring sites falling within the clean category (FF < 1) (Figure 9). Clean sites are predominantly located in nearshore areas, potentially attributable to seawater dilution and algal assimilation. Pollution levels in terrestrial areas significantly exceed those in marine zones. Specifically, mildly polluted sites (ZK1, ZK10) are frequently situated near aquaculture zones characterized by frequent water exchange; moderately polluted sites (ZK9, ZK21) occur within the farmland-mangrove transition zone, influenced by agricultural inputs (fertilizers, pesticides) and mangrove vegetation; severely polluted sites (ZK12, ZK15, ZK18) cluster around Sanjiang Town, where elevated population density likely elevates eutrophication risks in adjacent waters.

Evaluation using the Nemero Index (PN) further corroborates the spatial differentiation pattern (Figure 9), with 59% of the area classified as safe (PN < 1); however, pollution hotspots (PN > 2.0) are identified in southeastern villages and adjacent areas, characterized by significantly higher nitrogen and phosphorus accumulation rates in sediments compared to other regions. Consequently, establishing a pollution control zoning plan based on integrated land–sea management is recommended: prioritizing monitoring of biogeochemical processes in nearshore areas, implementing source reduction measures within terrestrial pollution-sensitive zones, and establishing sediment-water interface flux monitoring in ecologically vulnerable areas.

In summary, while mangroves provide vital nutrient interception and bioremediation functions, chronic wastewater discharge has significantly altered sedimentary environments. The pronounced spatial heterogeneity, vertical enrichment, and spatial differentiation of carbon, nitrogen, and phosphorus within Dongzhai Harbor mangrove sediments underscore the predominant influence of anthropogenic activities in reshaping nutrient distribution patterns. Pollution index quantification further reveals substantially higher terrestrial pressure compared to marine zones, with ecologically vulnerable areas facing elevated risks. Consequently, safeguarding mangrove wetland ecosystem health necessitates not only reliance on natural recovery capacity but also precise identification of pollution sources, optimization of wastewater discharge control strategies, establishment of rigorous monitoring-verification systems, and strengthening protection through ensuring effective blue carbon mechanisms, and implementation of integrated land–sea governance to ensure ecological sustainability.

5. Conclusions

This study focused on the Dongzhai Harbor mangrove wetland in Hainan, investigating the spatial distribution patterns, sources, and ecological risks of sedimentary organic carbon, total nitrogen, and total phosphorus, with the principal findings summarized as follows:

(1)

Sediments within the study area exhibited significant spatial heterogeneity in total organic carbon (TOC), total nitrogen (TN), and total phosphorus (TP) concentrations. Horizontally, concentrations decreased land-to-sea; vertically, they declined with depth—collectively indicating predominant terrestrial control over nutrient distribution. Grain size analysis revealed silt-dominated textures (>52%), while pH displayed a stratified pattern (neutral surface vs. acidic subsurface). This coupled physicochemical regime favors organic matter preservation and stability. The integrated spatial patterns collectively demonstrate high environmental sensitivity of mangrove wetlands to nutrient migration across land–sea ecotones.

(2)

Sediments displayed a mean C/N ratio of 19, and source apportionment indicated that mangrove vegetation and marine phytoplankton contribute 68% and 32%, respectively, to the sedimentary organic matter composition. An average C/P ratio of 29 suggests high phosphorus bioavailability, whereas an average N/P ratio of 1.4 points to predominantly terrestrial phosphorus sources. Significant correlations exist between TOC and TN, TOC and TP, and TN and TP, indicating a common origin for nitrogen and phosphorus. With increasing depth, both the C/P and N/P ratios exhibited a consistent upward trend, suggesting enhanced inputs of allochthonous phosphorus to the sedimentary system. Larger particle sizes weaken the correlations of TOC, TN, and TP, while pH exhibits negative correlations with all indicators.

(3)

The Dongzhai Harbor mangrove wetland exhibits potential organic pollution and nutrient accumulation risks. While nearly half of monitoring sites are classified as “Moderately Clean”, an overall increasing pollution trend is evident, with heavily polluted zones concentrated in areas characterized by minimal tidal influence, dense mangrove vegetation, and frequent anthropogenic activities. Pollution sources are likely driven by combined vegetation metabolic processes and human disturbances. Consequently, enhanced source control measures are recommended to reduce nutrient inputs and promote sustainable ecosystem health.