Chiang Mai Journal of Science

Seasonal Variation of Potentially Harmful Dinoflagellates Across Semi-enclosed and Exposed Coastal Areas in Songkhla, Thailand

1. INTRODUCTION

     Songkhla Province, located along the southeastern coast of the Gulf of Thailand, features a diverse coastal landscape that supports key economic sectors, including tourism, fisheries, aquaculture, and maritime trade. The natural environment encompasses sandy beaches, coral reefs, mangrove forests, and the expansive Songkhla Lagoon System. These ecosystems provide key services and support the livelihoods of local communities, particularly through small-scale fishing and eco-tourism. Stretching about 158 kilometers, the coastline of the province includes estuaries, shallow bays, and lagoons [1-3]. The local climate is classified as humid tropical, influenced by two monsoons: the southwest monsoon (SWM; July–September) and the northeast monsoon (NEM; October–January), the latter delivering most of the 2,076 mm of annual rainfall. Temperatures average around 28°C throughout the year [1]. Strategically positioned, the province also functions as a hub for marine commerce and tourism. Infrastructure includes both a deep-sea port and a major fishing harbor, supporting cargo logistics, aquaculture, and offshore operations [4,5].
     The use of coastal resources in Songkhla is spatially diverse, reflecting distinct environmental characteristics and human pressures. Semi-enclosed zones, especially those surrounding Songkhla Lagoon, have been extensively developed for aquaculture, particularly shrimp and sea bass farming, through both pond-based and cage systems [6-8]. Inputs from these operations, along with domestic wastewater and freshwater inflows, contribute to nutrient accumulation, creating conditions conducive to phytoplankton growth, including harmful dinoflagellates. In contrast, open coastal zones, such as those at Samila and Chalathat beaches, serve primarily recreational purposes. While hydrodynamic flushing tends to prevent prolonged nutrient build-up, these areas are still vulnerable to episodic algal blooms driven by nearshore transport of offshore phytoplankton [9-17].
     Distribution and bloom formation of dinoflagellates are influenced by multiple environmental factors, including temperature, salinity, nutrient concentrations, pH, light availability, and water column structure [16-18]. Nutrient inputs from human activities, such as agriculture, aquaculture, and untreated wastewater, can boost dinoflagellate growth and result in harmful algal blooms (HABs), commonly known as red tides [14,15]. These blooms can harm water quality, disrupt food chains, and reduce DO levels as decaying biomass consumes oxygen [16-18]. Certain genera such as Alexandrium, Karenia, Gymnodinium, Dinophysis, and Tripos are known to produce harmful biotoxins that build up in marine food webs and threaten both animals and human health. These toxins are responsible for various shellfish poisoning syndromes including paralytic (PSP), diarrheic (DSP), and neurotoxic shellfish poisoning (NSP), posing recurring threats to public health and economic stability in coastal regions [14]. Dinophysis species, for instance, release okadaic acid, the agent behind DSP. Meanwhile, dense Tripos blooms have been linked to hypoxia and ecological stress. As nutrient enrichment becomes more common, HABs are occurring more frequently in sensitive coastal zones, complicating resource management. HABs also directly affect aquaculture. Filter-feeding bivalves such as mussels, oysters, and clams accumulate toxins without visible signs, leading to harvest bans and trade limitations [19-21]. Dense blooms can physically obstruct operations by clogging fish gills, damaging gear, and lowering water quality, causing mass die-offs among cultured and wild species [22,23]. Tourism, wild fisheries, and even desalination infrastructure can also suffer from water discoloration, foul odors, and service disruptions.
     HABs have intensified across Southeast Asia due to tropical monsoons, eutrophication, and aquaculture expansion, threatening seafood safety and marine ecosystems [24,25]. Major taxa include Pyrodinium bahamense, Alexandrium spp., and Dinophysis spp., frequently causing PSP and DSP in regional waters [26]. In Thailand, recurrent blooms of Noctiluca scintillans and Alexandrium spp. have led to fish kills and shellfish poisoning [27,28]. Despite advances in remote sensing and molecular tools [29,30], integrated monitoring remains crucial for mitigating HAB impacts. In Thailand, common bloom-forming taxa include Tripos furca, Dinophysis spp., Diplopsalis spp., and N. scintillans, the latter often surfacing during the monsoon season [31-33]. Although N. scintillans is not toxic, its large biomass can deplete oxygen and trigger fish kills [34]. Notably, repeated blooms along the coast of Chonburi Province during 2003–2004 were observed to coincide with phosphate enrichment and freshwater input from the Bangpakong River [31]. Similar blooms involving Tripos spp. and Diplopsalis spp. have been documented in estuaries in Samut Songkhram Province, reflecting the vulnerability of nutrient-rich environments. In Songkhla Lagoon, recurring N. scintillans blooms have been observed, especially during the late dry and SWM seasons. These blooms were associated with phosphate enrichment and fluctuating salinity due to aquaculture and freshwater inflows [33].
     Songkhla Lagoon has experienced water quality degradation driven by cumulative impacts from untreated wastewater, aquaculture discharge, tourism, and limited hydrodynamic exchange [7,8]. These pressures have elevated nitrogen and phosphorus levels, contributing to more frequent HABs, hypoxia, fish kills, and socio-economic disruption [19,35,36]. The semi-enclosed sites such as Songkhla Lagoon exhibit low flushing and higher nutrient retention from aquaculture and urban inputs, creating eutrophic conditions favorable to PHDs. In contrast, open coastal areas are influenced by stronger hydrodynamics and tidal mixing, which reduce nutrient accumulation and bloom persistence. This study examines the spatial and seasonal patterns of PHDs in coastal waters of Songkhla and identifies key environmental drivers. By comparing nutrient-enriched, semi-enclosed zones with open marine sites, the research highlights how human activities influence harmful plankton dynamics.

2. MATERIALS AND METHODS

2.1 Study Site
     The study was conducted across two coastal environments: semi-enclosed areas (Site A) and exposed coastal areas (Site B) (Figure 1). Site A comprises A1 (Haad Kaew) and A2 (Laem Son), both sheltered from wave action but subject to intense anthropogenic influence. A1 is adjacent to aquaculture farms and the Songkhla deep-sea port, while A2, near the lagoon entrance, supports traditional fishing. Site B includes B1 (Samila Beach), B2 (Chalathat Beach), and B3 (Kao Seng Beach), all exposed to open-sea conditions. B1 and B2 are tourism hotspots, whereas B3 is shaped by both coastal tourism and small-scale fisheries.

2.2 Sample Collection and Laboratory Analysis
     Field sampling for PHDs was conducted biweekly from July 2023 to March 2024 across all stations, covering three seasonal periods: SWM (July–September), NEM (October–December), and IMD (January–March). At each station, 100 L of surface seawater were filtered through a 20-µm mesh plankton net, towed obliquely at ~20 cm depth. Sampling was replicated three times to enhance representativeness. Plankton were preserved in 4% formalin for analysis. In the laboratory, samples were examined under a light microscope (200×–400×, Olympus CX21LED) for PHD identification and quantification, using morphological keys [9,10]. Cell densities (cells/L) were determined with a Sedgewick-Rafter counting chamber. Environmental parameters (temperature, pH, salinity, DO, TDS) were measured in situ at 20 cm depth using a ProQuatro multiparameter meter. Nutrients were analyzed using standard colorimetric protocols following [37]. Specifically, nitrite was determined using the Sulfanilamide Method, phosphate via the Ascorbic Acid Method, and ammonium by the Phenate Method. Commercial test kits—Monitor (nitrite), Quantofix (phosphate), and Vunique V-color 9750 (ammonium)—were used in accordance with the respective standard procedures.

2.3 Data Analysis
     The Shannon-Wiener index and evenness index were calculated to compare the diversity among sites [38]. To analyze PHD distribution and community structure across sites and seasons, statistical and graphical methods were applied. Data processing and visualization were performed in RStudio (v4.3.2) [39], while community-level analyses used PC-ORD (v7.11) [40]. One-way ANOVA tested for significant differences in PHD densities among stations and seasons, followed by Tukey’s HSD test for pairwise comparisons. A two-way cluster analysis (Euclidean distance, group average linkage) grouped stations and seasons based on log-transformed abundance data [log(1 + abundance)] to reduce dominance effects. Canonical Correspondence Analysis (CCA) was used to assess the influence of pH, salinity, temperature, DO, TDS, phosphate, nitrite, and ammonium on PHD distribution. Abundance data were log-transformed to improve normality and linearity. The significance of environmental gradients was tested using Monte Carlo permutation (999 iterations).

3. RESULTS AND DISCUSSION

3.1 Coastal Water Quality
     The coastal water parameters measured showed clear spatial and seasonal differences (Table 1), influenced by a combination of monsoonal patterns and land-based activities. At Site A (stations A1 and A2), located near freshwater inflows and anthropogenic sources in the form of active fish cage aquaculture, salinity and TDS exhibited great variability across all three seasons. Salinity values at Site A decreased significantly during the NEM season, particularly at A2 (11.24±12.73 psu), reflecting significant freshwater inflow. Phosphate concentrations peaked during the NEM and IMD seasons (3.00 mg/L), consistent with runoff-driven nutrient inputs. Increased freshwater inflow during these periods may have transported organic matter and nutrients into the coastal system, with contributions from residual feed and waste associated with intensive cage culture in the southern part of the Songkhla Lagoon System, together with dense fish farming activities at Site A1 [7]. Ammonium concentrations were higher during the IMD season (0.23 mg/L at A1; 0.18 mg/L at A2). ANOVA indicated a significant seasonal effect (p = 0.0336), whereas no significant effects of site (p = 0.539) were detected. Ammonium levels were significantly lower during the SWM than the IMD season. Nitrite concentrations remained low (~0.10 mg/L) across seasons, suggesting efficient nitrification or minimal loading. DO at A1 was notably low during the SWM season (3.87 mg/L), likely due to reduced mixing or elevated microbial respiration. ANOVA indicated significant effects of site (p = 3.89 × 10⁻⁵) and season (p = 0.000231), demonstrating pronounced seasonal variability in spatial DO patterns. DO below 5 mg /L is considered stressful for aquatic life [41,42]. The lowest DO recorded (3.87 mg/ L at A1) approaches this threshold, indicating moderate oxygen depletion that may impact sensitive organisms if prolonged. This condition may result from water column stratification driven by freshwater inflows, which inhibit vertical mixing and reduce oxygen exchange between surface and bottom layers. Moreover, inputs of organic matter and nutrients from aquaculture activities, wastewater discharge, and terrestrial runoff likely enhanced microbial decomposition processes, thereby accelerating oxygen consumption and increasing the likelihood of hypoxia [8,33,43,44]. Microbial plankton respiration plays a key role in determining whether organic carbon is stored in the ocean or converted to CO₂, consuming oxygen in the process. Rising sea surface temperatures intensify ocean stratification and reduce deep-water ventilation, prolonging the isolation of deep waters from the atmosphere. These processes delay DO re-equilibration while the microbial degradation of organic matter continues to deplete oxygen and release CO₂ [45].
     In contrast, Site B (stations B1, B2, and B3), located offshore and more influenced by marine conditions, showed relatively stable water quality throughout the study period. Salinity remained consistently high across all seasons, reflecting limited influence from freshwater runoff. Compared with Site A, especially station A2, salinity at Site B was significantly higher (p = 0.000175), highlighting a clear spatial distinction between the semi-enclosed and exposed coastal environments. Seasonal variation of salinity was not statistically significant (p = 0.091126), further emphasizing the physical stability of Site B under open-sea conditions. DO levels remained consistent across seasons (5.17–5.85 mg/L), peaking at station B3 (6.18±0.38 mg/L) during the NEM season, reflecting better ventilation and oxygenation from wave and wind mixing, although slight reductions were observed during the IMD season. Water temperatures tended to be slightly higher during the SWM season. pH levels were also slightly higher at Site B than Site A, especially during the IMD season (e.g., B2 and B3 > 8.5), suggesting stronger buffering capacity or higher photosynthetic activity. Phosphate concentrations increased from initially low levels during the SWM (1.50–2.00 mg/L) to a peak at B3 during the NEM season (3.00 mg/L), suggesting offshore transport of nutrient-rich waters. These results indicate that although offshore environments generally exhibit greater physical stability, they remain susceptible to seasonal variations in land-derived nutrient inputs [46,47]. Nitrite concentrations (~0.10–0.12 mg/L) and ammonium levels (peaking at 0.25 mg/L at B3) showed minor seasonal variation, indicating stronger environmental buffering compared with Site A (Table 1).

3.2 Assemblages of Potential Harmful Dinoflagellates
     A total of 12 PHD species were identified, including N. scintillans, D. caudata, D. miles, Tripos furca, T. fusus, T. humile, T. brevis, T. tripos, T. trichoceros, T. strictum, T. macroceros, and Tripos sp. Among these, seven species, N. scintillans, D. caudata, D. miles, T. furca, T. fusus, T. tripos, and T. trichoceros, were consistently found at all sampling stations (Table 2). PHD diversity was slightly higher at Site B (H′ = 1.45, E = 0.61, S=11) than Site A (H′ = 1.30, E = 0.54, S=11), despite equal species richness. This suggests a more even distribution of species under well-mixed, open-coastal conditions, while Site A exhibited greater dominance, likely driven by nutrient enrichment and reduced flushing [48,49]. Across all seasons and sites, T. furca and N. scintillans were the most abundant and widely distributed species. Both exhibited a non-significant spatial trend and no significant seasonal variation (p > 0.05). These two species showed higher cell densities compared to others, particularly at stations A1 and A2 during the NEM and IMD seasons. For example, T. furca reached high abundances at A1 during the NEM (173 cells/L) and IMD seasons (164 cells/L), while N. scintillans peaked at A2 during the IMD season (150 cells/L) and at A1 (77 cells/L). The observed T. furca concentration was well below internationally recognized HAB thresholds, which typically exceed 10⁴–10⁵ cells/L for bloom designation [22,50]. This represents less than 2% of the lowest bloom threshold, indicating non-bloom conditions with negligible ecological or public health risk [51,52]. The frequent and elevated occurrence of T. furca and N. scintillans in Songkhla coastal waters raises important ecological and environmental concerns. Similar findings were reported by Saengkaew et al. in 2025, who documented the presence of both T. furca and N. scintillans in Songkhla Lagoon [33], further emphasizing the widespread distribution and ecological adaptability of these species across diverse salinity regimes and hydrographic conditions within the region. Both species are recognized as HABs with the capacity to influence marine food webs and water quality, particularly when occurring in high densities [53,54]. Large blooms can clog fish gills, impairing respiration, and their decay can deplete oxygen, leading to hypoxia [23,55,56]. Although T. furca does not typically produce a toxin, its dense blooms can disrupt marine ecosystems. High biomass accumulation can lead to increased turbidity, reducing light penetration and affecting photosynthetic organisms. Moreover, the decay of these blooms can deplete DO levels, potentially causing hypoxic conditions detrimental to marine life. In the Penang Strait, T. furca outbreaks have been linked to significant fish mortality in aquaculture areas [57]. N. scintillans, a mixotrophic dinoflagellate, can form dense red or green water blooms. Although not directly toxic, its blooms can have severe ecological impacts. The species accumulates ammonia, which is released during bloom decay, leading to oxygen depletion. These conditions have been linked to mass mortalities of marine organisms, including fish and corals, as observed in the Gulf of Mannar, India [58].
     Among the identified PHD species, D. caudata and D. miles were occasionally found at both semi-enclosed and exposed sites, with higher cell densities observed during the NEM. The highest abundance of D. caudata was recorded at A1 during the NEM (26 cells/L), while D. miles also peaked at Station A1 in the same season (11 cells/L). Both exhibited a significant seasonal variation (p=0.00508, p=0.033 respectively). D. caudata abundance was lower during the SWM than during the IMD (p = 0.014) and NEM seasons (p = 0.006). D. miles also declined significantly during the SWM compared with the NEM (p = 0.033). Dinophysis species are of particular concern due to their well-documented ability to produce diarrheic shellfish toxins (DSTs), including okadaic acid and its derivatives, which are responsible for risks to fish and human health posed by filter-feeding bivalves [59,60]. Although the observed cell densities in this study were relatively low compared with harmful bloom thresholds, the presence of these species across several stations and seasons suggests that the coastal environment in Songkhla could support their proliferation under favorable conditions. The composition and seasonal dynamics of PHDs in Songkhla Province, dominated by N. scintillans and T. furca, are consistent with regional patterns across Thailand. Key environmental drivers such as salinity, silicate, phosphate, and temperature were also identified in Songkhla Lagoon [33]. Metabarcoding and satellite analyses revealed diverse harmful taxa and linked Noctiluca blooms to monsoonal forcing and river discharge [29,30]. Moreover, niche modeling indicated T. furca’s broader mesohaline tolerance compared to T. fusus, supporting its persistence across variable salinity regimes [61].
     Cluster analysis based on the presence and abundance of 12 PHD species revealed distinct spatial and seasonal groupings among sampling sites in Songkhla coastal waters. During the SWM, stations in different months could be categorized into three subgroups that showed a similarity level of 49.4 %. Group I comprised stations that were characterized by the presence of T. furca, T. fusus, and T. trichoceros in July (A1) and October (A2, B2, and B3). Group II comprised stations that were marked by a high abundance of N. scintillans in August (A1, A2, B1, B2, and B3) and July (B1, B2, and B3). This group harbored the potential to bloom during this time, likely influenced by freshwater inflow and nutrient loading from monsoonal runoff. Previous studies have shown that N. scintillans blooms commonly occur under conditions of elevated nutrient availability and water column stratification [33,34]. Group III comprised only station A1 in October, which was distinguished from the other groups by the presence of all the recorded PHD species, except N. scintillans (Figure 2). Similarly, during the NEM, stations could be classified into three groups at a similarity level of 18.4 %. Group I and Group II presented almost all the studied PHD species with higher diversity than Group III. Group I comprised stations A1 and B2 in November, A1 and B1 in December, and B1 and B3 in January.

     Group II comprised stations B1 and B3 in November, B2 and B3 in December, and A1 and B2 in January. Notably, N. scintillans were found in larger numbers in Group II compared with the other groups. D. caudata, T. furca, T. fusus, T. trichoceros, and D. miles were recorded at low abundances (Figure 3). This pattern may reflect localized impacts of limited water circulation, freshwater inflow, and elevated nitrite concentrations, all of which contribute to lower salinity and potential stratification. Such semi-enclosed environments, while often nutrient-rich, may exhibit reduced plankton diversity due to eutrophication and associated hypoxic conditions, which can limit the proliferation of diverse dinoflagellate assemblages [16].
     During the IMD season, three subgroups were identified at a similarity of 29.4 %. Group I comprised all stations (A1, A2, B1, B2, and B3) in February. Group II and Group III comprised stations in March 2024. Group II comprised stations A1 and A2, while Group III comprised stations B1, B2, and B3. N. scintillans was highly abundant in Groups I and II, whereas it was entirely absent from Group III. This classification highlights seasonal variations in the distribution of PHDs and provides insight into the ecological dynamics of different stations across monsoonal periods (Figure 4). The grouping of stations may also reflect a recovery phase or post-bloom succession, where the phytoplankton community becomes more uniform due to stabilized conditions and internal ecological processes such as nutrient recycling and biological interactions rather than external nutrient loading. Such homogenization patterns following monsoonal transitions have been noted before in tropical estuarine and lagoon systems [62].
     The observed seasonal and spatial clustering of PHDs underscores the need for site-specific and seasonally adaptive monitoring programs, particularly in semi-enclosed areas. Site A may be more at risk of HABs or biodiversity loss due to limited water exchange, stratification, and the cumulative effects of anthropogenic stressors that include aquaculture effluents and nutrient-rich runoff. These characteristics make such environments more vulnerable to rapid ecological changes in response to seasonal forcing. The distinct seasonal groupings, such as the dominance of N. scintillans during the SWM season and reduced species richness at station A2 during the NEM season, revealed critical periods and sites for targeted management. Enhanced monitoring during the SWM season may enable early detection of N. scintillans blooms, while persistent low diversity at A2 indicates the need for localized interventions, such as nutrient reduction or improved water circulation. According to the study of Anderson et al. in 2012, early detection and spatially targeted responses are crucial for minimizing the impacts of HABs. Our findings further support the necessity of enhanced monitoring during high-risk periods in semi-enclosed coastal zones [22].

3.3 Relationships between Potentially Harmful Dinoflagellates and Environmental Variables
     CCA was used to examine the relationships between PHD species distribution and environmental variables across Site A and Site B. The first and second canonical axes accounted for 23.2% and 13.1%, respectively, of the total variance in the species and environment relationship, with a clear differentiation between the two sites (Figure 5A). In terms of seasonal dimensions, the CCA explained 23.2% and 13.1% of the total variance along the first and second axes, respectively (Figure 5B).
     D. miles and D. caudata were positively correlated with DO and nitrite concentration across both spatial (Site B) and seasonal (NEM) dimensions, suggesting that these species prefer well-oxygenated waters with elevated nitrite concentrations. These conditions may result from well-mixed environments or anthropogenic nutrient inputs such as wastewater discharge and agricultural runoff, particularly during transitional periods following freshwater inflow [14,22]. In contrast, T. trichoceros and T. fusus were strongly associated with temperature and pH, indicating a preference for warmer and more alkaline conditions. These environmental features are typically observed during the SWM and IMD periods, when increased solar radiation and evaporation can elevate surface temperatures and pH. T. furca, positioned farther from the environmental vectors in the CCA biplot, appears to be less influenced by the measured environmental parameters, possibly due to a broader ecological tolerance or the influence of unmeasured variables. This observation aligns with findings by Huynh et al. in 2022, which suggest that T. furca and T. fusus exhibit ecological flexibility and can thrive in nutrient-enriched, low-salinity environments [61]. Similar findings have been reported in the Gulf of Mexico and the South-Central Viet Nam, where T. furca persisted under varying salinity and nutrient regimes [61,63]. The consistent presence of N. scintillans across all stations and seasons, along with its central position in the CCA ordination, suggests a generalist ecological role. Its weak association with specific environmental gradients reflects broad tolerance and adaptability to varying conditions. This versatility can be attributed to its mixotrophic nature that allows it to thrive and realize its bloom potential under both nutrient-rich and stratified environments found in diverse coastal systems [64]. Its adaptability also explains its widespread distribution and dominance during certain periods (IMD), as observed in both this and previous studies in the Gulf of Thailand [33,34].
     Samples from Site B clustered more closely with DO and nitrite concentrations, which may reflect physical or anthropogenic differences compared with Site A. These patterns reflect differences in water circulation or marine environmental conditions that are more stable in the exposed coastal zone [16,65]. In contrast, semi-enclosed areas such as Site A are more vulnerable to nutrient accumulation and stratification due to limited flushing and proximity to land-based inputs. These conditions lead to stronger variability in DO and nutrient levels. The observed CCA gradients reinforce the idea that environmental parameters, particularly DO, temperature, and nutrient concentrations play a crucial role in shaping the composition and spatial distribution of PHD communities [61,63-66]. This study of the spatial and seasonal variability of PHDs in the coastal waters of Songkhla Province, Thailand highlights key ecological patterns relevant to coastal management. Semi-enclosed areas (Site A, particularly station A2) exhibited lower species richness and higher variability in DO and salinity, likely due to limited water exchange, stratification, and anthropogenic nutrient inputs from aquaculture and runoff [33]. In contrast, exposed coastal areas (Site B) showed greater physicochemical stability but remained sensitive to nutrient inflows during monsoonal periods. Species-specific responses to environmental gradients were evident. D. caudata and D. miles were positively associated with DO and nitrite, particularly during the NEM season, suggesting their preference for oxygenated, nutrient-rich conditions [22]. T. trichoceros and T. fusus were correlated with higher temperature and pH, indicative of their proliferation during warmer, more alkaline conditions in the SW season [61]. The broad distribution of N. scintillans, found near the centroid in CCA plots, reflects its generalist ecology and bloom potential across varied conditions [64]. These findings emphasize the need for site-specific, seasonally adaptive monitoring strategies. This is consistent with the objectives of Sustainable Development Goals 14: Life Below Water and 13: Climate Action, which aim to reduce marine pollution and enhance ecosystem resilience. Integrating ecological data with coastal management practices will support early warning systems and sustainable aquaculture development in vulnerable regions like Songkhla.

4. CONCLUSIONS

     This study provided an in-depth look at how PHDs are distributed spatially and seasonally in the coastal waters of Songkhla Province, Thailand. A total of 12 PHD species were identified, with T. furca and N. scintillans being the most common and widely distributed across all sites and seasons. The result showed clear spatial heterogeneity between semi-enclosed (Site A) and exposed coastal areas (Site B), with Site A showing greater variability in environmental conditions due to freshwater inflow and anthropogenic pressures, such as aquaculture and land-based runoff. Site B exhibited greater ecological stability, reflected by higher diversity (H′ = 1.45), evenness (E = 0.61), and richness (S = 11) than Site A. Seasonal dynamics, driven by the SWM, NEM, and IMD monsoon periods, significantly influenced species abundance and community composition. Notably, D. caudata and D. miles, species associated with diarrhetic shellfish toxins, were most abundant during nutrient-rich NEM and IMD periods, correlating with elevated nutrient inputs. CCA further demonstrated that temperature, DO, pH, and nitrite were the key environmental gradients structuring dinoflagellate assemblages, representing the highest public health risk. Moderate ecological risks were associated with T. macroceros and T. furca, while N. scintillans, T. fusus, and T. tripos posed lower risks. To support early warning and timely intervention, threshold values should be defined for key indicators. T. furca >10⁴ cells/L may signal HAB risk; ammonium >0.3 mg/L indicates nutrient loading; DO <5 mg/L suggests oxygen stress [51]. Monitoring should intensify during IMD and NEM seasons to detect seasonal variability and prevent ecological degradation. These findings emphasize the ecological complexity of tropical coastal systems and reinforce the need for seasonally responsive, site-specific monitoring programs, with important implications for coastal zone management, public health, and the long-term sustainability of marine ecosystems. These findings enhance the understanding of HAB dynamics in Southeast Asian coastal systems and provide important insights for managing eutrophication and conserving biodiversity. The results offer science-based evidence to support the development of local water quality monitoring programs and early warning systems, particularly in nutrient-enriched, semi-enclosed zones. Practical applications may include seasonal nutrient load management and implementation of adaptive strategies based on monsoonal variations. These approaches can be integrated into provincial coastal management frameworks and align with Sustainable Development Goals 14 (Life Below Water) and 13 (Climate Action), thereby contributing to long-term ecosystem resilience and public health preparedness.

ACKNOWLEDGEMENTS

     The authors gratefully acknowledge the Faculty of Science, the Coastal Oceanography and Climate Change Research Center (COCC), the Marine and Coastal Resources Institute (MACORIN), the Faculty of Environmental Management, and the Faculty of Science and Technology, Prince of Songkla University, for their valuable support and provision of equipment for fieldwork and laboratory analyses.

AUTHOR CONTRIBUTIONS

Supaporn Saengkaew: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Resources, Data Curation, Writing - Original Draft, Writing - Review and Editing, Visualization. Sukree Hajisamae: Conceptualization, Methodology, Validation, Data Curation, Writing - Review and Editing, Supervision. Mathinee Yucharoen: Conceptualization, Methodology, Validation, Data Curation, Writing - Review and Editing, Supervision. Rujinard Sriwoon: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Resources, Data Curation, Writing - Original Draft, Writing - Review and Editing, Visualization, Supervision, Funding acquisition.

CONFLICT OF INTEREST STATEMENT

      The authors declare that they hold no competing interests.

DECLARATION OF GENERATIVE AI IN PREPARATION OF MANUSCRIPT

     The authors used ChatGPT (OpenAI) to assist with the clarity of the English language in the manuscript. All content, interpretations, and conclusions were developed solely by the authors, who take full responsibility for the final version.

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