A Dataset of Environmental Toxins for Water Monitoring in Coastal Waters of Southern Centre, Vietnam: Case of Nha Trang Bay

Abstract

This study presents a comprehensive dataset developed to monitor coastal water quality in the south-central region of Vietnam, focusing on Nha Trang Bay. Environmental data were collected from four research cruises conducted between 2013 and 2024. Water samples were taken at two depths: surface samples at approximately 0.5–1.0 m below the water surface, and bottom samples 1.0 to 2.0 m above the seabed, depending on site-specific bathymetry. These samples were analyzed for key water quality parameters, including biological oxygen demand (BOD₅), dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and Chlorophyll-a (Chl-a). The data establish a valuable baseline for assessing both spatial and temporal patterns of water quality, and for calculating eutrophication index to evaluate potential environmental degradation. Importantly, it also demonstrates practical applications for environmental management. The dataset can support assessments of how seasonal tourism peaks contribute to nutrient enrichment, how aquaculture expansion affects dissolved oxygen dynamics, and how water quality trends evolve under increasing anthropogenic pressure. These applications make it a useful resource for evaluating pollution control efforts and for guiding sustainable development in coastal areas. By promoting open access, the dataset not only supports scientific research but also strengthens evidence-based management strategies to protect ecosystem health and socio-economic resilience in Nha Trang Bay.

Dataset: 10.17632/nv7nsddk3p.1

Dataset License: CC-BY-4.0

1. Introduction

In Vietnam, marine environmental monitoring has been recognized as a key tool to assess the impacts of economic development and inform integrated coastal zone management [1,2,3,4,5,6]. Despite these efforts, many monitoring programs remain limited in duration, narrow in scope, and fragmented in spatial coverage. Vinh and Tam [7] investigated sediment quality in the southern part of Nha Trang Bay, but the study was restricted to a single survey and a small number of sites. Similarly, Vietnam has established marine environmental monitoring systems at both the national and provincial levels. In the national system, only one monitoring station exists in Nha Trang Bay/South Central Coast, whereas the provincial system comprises seven stations located primarily near the coastline in the Bay. Both systems are designed to provide early warnings about environmental pollution, and their monitored parameters have shifted periodically in response to changing development priorities [1,8,9]. Therefore, these systems cannot meet the requirements of long-term water quality monitoring or coastal water/onshore sampling. The previous programs often involved fewer than ten sampling stations, and focused on isolated parameters rather than integrated assessments, thereby underscoring the need for a more comprehensive and sustained dataset.

To address this critical gap, the objective of this initiative is to develop a dedicated and scientifically robust dataset on environmental toxins and water quality parameters in Nha Trang Bay. The dataset focuses on collecting and integrating key indicators, including dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), chlorophyll-a (Chl-a), BOD, DO, and eutrophication index. These variables are essential for detecting early signs of eutrophication and organic pollution, especially in areas influenced by aquaculture effluents and tourism-related discharges [10]. Furthermore, the dataset will serve as a foundation for applying integrated eutrophication indices, which enable multi-parameter assessments of water quality and trophic status [11,12]. By enhancing spatial-temporal analysis and enabling long-term trend evaluation, this approach strengthens evidence-based decision-making for adaptive coastal governance.

This dataset provides critical support for sustainable resource management, benefiting policymakers, researchers, environmental managers, and local stakeholders. It ensures that Nha Trang Bay’s economic development is aligned with the preservation of its ecological integrity [13,14].

This paper provides a dataset of environmental toxins and eutrophication index in Nha Trang Bay, Viet Nam, collected from 2013 to 2024 which is openly available in the Mendeley Data repository (https://data.mendeley.com/preview/nv7nsddk3p?a=0ab3af81-1abc-46ed-b627-0f073392a093 (doi: 10.17632/nv7nsddk3p.1), accessed on 19 August 2025) under the Creative Commons Attribution 4.0 International license.

2. Data Description

2.1. Study Area

Nha Trang Bay (12.24° N, 109.28° E), located along Vietnam’s south-central coast, is renowned for its stunning natural beauty and ecological richness. The bay features a diverse array of coastal habitats, including islands, coral reefs, sandy beaches, and seagrass beds, all of which contribute to its high marine biodiversity. Historically, the local economy was based on agriculture and small-scale fisheries [15]. However, over the past three decades, the region has experienced a major shift toward marine aquaculture and tourism [16]. By the 2000s, aquaculture had become a key economic sector, and in more recent years, mass tourism has emerged as the primary driver of growth. This transition has brought economic benefits, such as improved infrastructure and increased job opportunities. Yet, it has also resulted in growing environmental pressures.

Rapid and largely unregulated development has led to extensive habitat degradation, increased waste discharge, and declining water quality [17,18,19]. Coral reefs and seagrass ecosystems, which are critical to fisheries and biodiversity, have been damaged by sedimentation, pollution, and marine construction [19]. Meanwhile, the expansion of aquaculture in the 2000s–2010s had contributed to organic pollution and habitat modification in localized areas [10,18]. Although Nha Trang Bay was declared a marine protected area to harmonize conservation and sustainable development, enforcement of environmental regulations remains weak, and integrated coastal zone management strategies have not been effectively implemented [14]. Going forward, reconciling economic ambitions with ecological sustainability will require more robust governance, stricter regulation, and the adoption of adaptive management practices. Protecting the bay’s ecological integrity is essential not only for biodiversity conservation but also for ensuring the long-term viability of the tourism sector that now dominates its economy.

2.2. Environmental Dataset

A total of 16 monitoring stations were established within Nha Trang Bay, along with two additional stations located in the adjacent rivers, to support the assessment of eutrophication in the bay (Figure 1). Data were collected through four separate surveys conducted under the framework of three Vietnamese research projects, as summarized in

Table 1. Information on surveys.

Period	Project Code	Time of Survey	Number Stations	Code
2013–2014	VAST.ĐLT.01/13–14	July–August 2013	12	2013
		January 2014	12	2014
2018–2020	VT-UD.12/18–20	August 2019	12	2019
2024–2026	TĐĐTMT.01/24–26	November 2024	15	2024

. The resulting dataset is organized in an Excel file (.xlsx), comprising 1974 records structured into 95 rows and 21 columns (including the first label row). Each row represents a seawater sample collected from both the surface and bottom layers at a specific station. The columns include temporal data (year, month, day), spatial coordinates (station ID, latitude, longitude, water depth, and sampling depth), and a range of environmental parameters, as well as calculating the eutrophication index. These parameters, analyzed in the laboratory of Institute of Oceanography, include dissolved oxygen concentration (DO, mg L⁻¹), biological oxygen demand over five days (BOD₅, mg L⁻¹), chlorophyll-a concentration (Chl-a, mg m⁻³), and nutrient levels such as nitrite (NO₂⁻, mg m⁻³), nitrate (NO₃⁻, mg m⁻³), ammonium (NH₄⁺, mg m⁻³), total dissolved inorganic nitrogen (DIN = NO₂⁻ + NO₃⁻ + NH₄⁺, mg m⁻³), and dissolved inorganic phosphate (DIP, mg m⁻³).

3. Methods

3.1. Sampling Collection

Four surveys were conducted in Nha Trang Bay in four periods of July–August 2013, January 2014, August 2019, and November 2024 on the NCB95 ship for the first two surveys and other ships for the others. Water samples were collected using 5 L Niskin Bottles-non-metallic free-flushing sampling bottles (General Oceanics, Miami, Florida, USA). Sampling was conducted at two depths: (i) the surface layer at approximately 0.5–1.0 m below the water surface and (ii) the near-bottom layer at 1.0–2.0 m above the seabed, depending on site-specific bathymetry. Prior to each survey, the Niskin bottles underwent a standardized inspection and calibration procedure to ensure sampling reliability and reproducibility. The calibration involved (1) visual inspection of the bottle chambers, O-rings, and seals for signs of wear or leakage; and (2) testing the triggering mechanism to confirm proper closure. The bottles were rinsed three times with site water at each station before sampling to minimize contamination and cross-sample interference. Following retrieval, collected water samples were transferred into DO glass bottles for DO and BOD₅ and plastic cans and bottles for other parameters.

Water samples for DO samples were collected in the 125 mL glass bottles and prepared immediately on the ship with MnCl₂ and KI/NaOH and analyzed by the Winkler method [20] later in the laboratory whereas ones for biological oxygen demand (BOD₅) were also collected in the 125 mL dark glass bottles, prepared at the same time as DO, and kept in the dark but analyzed the DO after 5 days (called DO₅). This approach was selected to ensure high accuracy and consistency across all surveys (2013–2024), as it is less affected by calibration drift or equipment malfunction compared to oxygen meters. The use of the Winkler method also enabled a reliable comparison of results over the long-term monitoring period.

Other water samples were collected using pre-cleaned polyethylene containers. Samples for chlorophyll-a (Chl-a) analysis were collected in 5 L gray plastic cans to protect samples from light, whereas nutrient samples were collected in 0.5 L plastic bottles. Nutrient bottles were prepared with a few drops of carbon tetrachloride (CCl₄) to inhibit biological activity. After collection and preparation, all samples were stored at approximately 4 ± 2 °C in 160 L insulated plastic ice boxes filled with ice to maintain low temperatures and minimize biological or chemical changes during transport. Each parameter had a specific holding time to ensure the accuracy of analytical results. If the maximum holding time (in hours or days) was exceeded, data could not be guaranteed. For example, seawater samples for Chl-a were maintained at ≤6 °C, without freezing, until filtration, which began as soon as possible and no later than 48 h after collection. In contrast, nutrient samples preserved with CCl₄ had a holding time of up to 28 days at ≤6 °C. To comply with these requirements, all samples were shipped to the laboratory on the same day of collection and never exceeded a 6 h transport time. At the laboratory, Chl-a samples were filtered immediately, with filters wrapped in foil, sealed in airtight bags, and frozen at −20 °C for up to 24 days prior to analysis. Nutrient water bottles were also stored at −20 °C and analyzed within 4 weeks of collection.

3.2. Laboratory Analysis

After acidification by H₂SO₄, DO and DO₅ concentration were measured with Na₂S₂O₃ 0.01N by the Winkler method [20] with a measurement in the range of 0.1–10.0 mg L⁻¹ [21] and uncertainty of ±0.51 mg L⁻¹ [22]. BOD₅ is the difference between the DO and DO₅ concentration [20].

Nitrogen (NO₂⁻ + NO₃⁻ + NH₄⁺) and phosphorus (DIP/PO₄⁻³) inorganic concentration were measured by the spectrophotometer of the U2900 double-beam Hitachi. NH₄⁻ concentrations were measured by the indophenol method with a measurement in the range of 0.1–10.0 μg-at L⁻¹ [20,21,23,24]. NO₂⁻ concentrations were measured by a highly azo dye color due to react with sulfanilamide and N-(l-naphthyl)-ethylenediamine [20,21] with a measurement in the range of 0.01–2.50 μg-at L⁻¹. NO₃⁻ concentrations in seawater, with a measurement in the range of 0.05–45.0 mg-at L⁻¹, were reduced to NO₂⁻ in a column containing cadmium filings coated with metallic copper, and then measured by the NO₂⁻ method with the correction for nitrite initially present in the sample [20,21]. PO₄³⁻ concentrations were measured by blue color due to the reaction of molybdic acid, ascorbic acid, and trivalent antimony [20,21] with a measurement in the range of 0.03–5.00 mg-at L⁻¹.

Water samples for measuring the Chl-a concentration were filtered using glass microfiber filters (GF/F membranes with a pore size of 0.45 μm) within six hours from their collection time. Pigments are extracted from the filter in 90% acetone within 24 h at 0 °C, and their concentration is estimated spectrophotometrically with the U2900 double-beam Hitachi [25].

3.3. Quality Control and Assurance

Comprehensive quality control (QC) measures were implemented to ensure the reliability and reproducibility of the dataset. During each research cruise, duplicate water samples were collected at approximately 10% of all sampling stations to assess sampling precision and analytical repeatability. Field blanks (using deionized water) were taken at the beginning and end of each sampling day to check for potential contamination during handling, while laboratory blanks were included in each analytical batch to verify the absence of cross-contamination during laboratory procedures.

Analytical quality was maintained using calibration standards and certified reference materials for nutrient (DIN, DIP) and Chl-a analyses. Instruments, such as spectrophotometers, were calibrated daily following manufacturer-recommended procedures and verified with reference standards at least once per week. All reagents were prepared fresh and stored under recommended conditions to preserve their integrity.

In addition, all field and laboratory staff followed standardized protocols [20,26,27] to ensure consistency between surveys. Data from duplicate and blank samples were routinely evaluated to calculate relative percent differences and detect outliers. Samples or datasets not meeting the predefined QC criteria were flagged and excluded from the final dataset. These QC measures collectively ensure that the compiled dataset meets international standards for environmental monitoring research.

3.4. Eutrophication Estimation

Eutrophication, driven by excess nitrogen and phosphorus, leads to increased algal growth, water quality decline, oxygen depletion, and ecosystem imbalance. The Eutrophication Index (EI) is a quantitative tool used to assess and monitor this process, especially in coastal and estuarine waters. It integrates key parameters-nitrate, phosphate, Chl-a, dissolved oxygen, and organic matter-to evaluate trophic status and the risk of algal blooms, oxygen depletion, and biodiversity loss. Elevated EI values indicate a higher risk of eutrophication, reduced water quality, and biodiversity loss. Therefore, the Eutrophication Index is essential for managing aquatic health and mitigating the adverse impacts of nutrient over-enrichment [28,29]. According to Minh-Thu et al. [30], EI was calculated as the Formula (1).

$$EI={\displaystyle \sum W_i(U_i/U_i)}$$

(1)

where EI: eutrophication index (unitless); W_i: weight coefficient for each environmental variable i in such a way that the sum of all weight coefficients is 1; U_i is the measured concentration of environmental variable i; and U_Si is the criterion or threshold concentration of environmental variable i. The weight coefficients (W_i) are determined as follows, Formula (2).

$$W_i=F_i/{\displaystyle \sum F_i}$$

(2)

where F_i represents the first principal component analysis (PCA1) obtained from the principal component analysis of variable i [31]. The sum of all F_i values equals the sum of all PCA1 values of the environmental variables used for EI calculation. In the case of Nha Trang Bay, W_i for Chl-a, BOD, DIN, and DIP are 0.266, 0.237, 0.206, and 0.291, respectively [30]. The U_Si is presented in

Table 2. Criterion concentration of parameters.

Parameter	Criteria
	Minimum	Medium	Maximum
Chl (mg m−3)	1	5	10
DIN (mg m−3)	200	250	300
DIP (mg m−3)	15	15	15
BOD (mg L−1)	1	3	10

.

4. Scientific Relevance and Considerations

This dataset represents a significant contribution to the long-term monitoring and protection of coastal ecosystems in Vietnam’s south-central region, with a specific focus on Nha Trang Bay, to safeguard its sensitive ecosystems and guarantee sustainable coastal management [32]. Recognized as one of the Vietnamese marine protected areas and a biodiversity hotspot, Nha Trang Bay hosts ecologically sensitive habitats, including coral reefs, seagrass beds, and diverse marine fauna, that underpin both ecological integrity and socio-economic activities. Historically, the bay was designated for integrated marine economic development, combining maritime transport, aquaculture, fisheries, tourism, and marine services. In recent years, however, its economic structure has shifted toward services and marine ecotourism.

These ecosystems are increasingly threatened by anthropogenic pressures such as coastal development, nutrient inputs from land-based sources, and the impacts of climate change [32,33,34]. Such pressures have heightened concerns over declining water quality and ecosystem health. The dataset directly addresses these issues by providing systematic, spatially distributed, and long-term measurements of key environmental parameters that are directly linked to eutrophication and pollution.

Scientifically, the inclusion of BOD₅, DIN, DIP, and Chl-a ensures robust assessment of nutrient status, oxygen depletion risks, and changes in primary productivity. These metrics are globally recognized for their relevance in evaluating eutrophication status, ecosystem functioning, and compliance with water quality guidelines. The dual-depth sampling strategy—near-surface and near-bottom—offers a more comprehensive understanding of vertical variability in nutrient concentrations and oxygen demand, which is critical in stratified or semi-enclosed coastal systems. This approach facilitates deeper insights into nutrient regeneration, phytoplankton dynamics, and hypoxia formation.

Covering more than a decade (2013–2024) and consolidated from four separate research cruises (

Table 3. Mean (±SD) concentrations of key environmental parameters in surface and bottom waters of Nha Trang Bay from 2013 to 2024. Paired t-tests were applied to assess vertical differences between surface and bottom layers; t- and p-values are reported for each parameter.

Parameter	Unit	Surface (Mean ± SD)	Bottom (Mean ± SD)	t-Value	p-Value
Dissolved Oxygen	mg L−1	6.44 ± 0.39	6.32 ± 0.33	1.620	0.109
BOD5	mg L−1	0.53 ± 0.24	0.47 ± 0.21	1.367	0.175
Chlorophyll-a	mg m−3	1.17 ± 1.39	1.05 ± 0.59	0.528	0.599
NH4+	mg m−3	3.68 ± 3.71	4.67 ± 3.39	−1.350	0.180
NO2−	mg m−3	34.68 ± 36.23	29.54 ± 16.51	0.857	0.394
NO3−	mg m−3	45.61 ± 28.63	38.06 ± 24.23	1.366	0.175
DIN	mg m−3	83.97 ± 57.23	72.27 ± 31.78	1.193	0.236
DIP	mg m−3	16.02 ± 15.19	14.12 ± 11.57	0.670	0.504
EImin	-	0.83 ± 0.66	0.74 ± 0.29	0.885	0.378
EImed	-	0.48 ± 0.38	0.43 ± 0.24	0.865	0.389
EImax	-	0.41 ± 0.35	0.36 ± 0.24	0.804	0.423

and Figure 2), the dataset captures both temporal and spatial variability. Table 3 shows that most key parameters exhibited small vertical gradients between surface and bottom waters, with paired t-tests indicating no statistically significant differences for dissolved oxygen, BOD₅, chlorophyll-a, DIN, DIP, and eutrophication indices across the 2013–2024 surveys. This vertical homogeneity reflects the shallow bathymetry and well-mixed conditions typical of Nha Trang Bay, which likely minimize strong stratification and benthic nutrient regeneration.

Figure 2 further reveals clear temporal variations in several parameters. BOD₅ displayed significant differences across time periods, rising sharply in 2014–2019 and again after 2019 (p < 0.05), suggesting increased organic inputs during these intervals. Chl-a concentrations showed significant declines between 2013 and 2024 (p = 0.001–0.009), indicating a possible reduction in phytoplankton biomass or improved wastewater controls after 2019. DIN and DIP also exhibited marked temporal shifts: DIN decreased significantly between 2013 and 2019 (p < 0.001) and between 2014 and 2019 (p < 0.001), while DIP declined across most intervals (p < 0.001), consistent with gradual nutrient management improvements and changing aquaculture practices.

These patterns collectively suggest that water quality in Nha Trang Bay has undergone dynamic changes over the decade. Periods of higher BOD₅ and nutrients coincide with intensive aquaculture and peak tourist activity (2014–2019), whereas later reductions in chlorophyll-a and DIN/DIP after 2019 align with the implementation of stricter environmental regulations and wastewater treatment upgrades. The dataset therefore captures not only spatial and vertical conditions but also long-term trends driven by anthropogenic pressures and policy interventions.

While only four measurement series are available, their distribution over multiple five-year periods allows for the detection of long-term trends and the evaluation of management interventions. This integration of fragmented and project-based monitoring data into a harmonized dataset establishes a reliable reference baseline for future assessments. It is particularly valuable for identifying both gradual degradation and episodic disturbances—such as harmful algal blooms and hypoxic events—that can have significant ecological and socio-economic impacts.

Temporal and spatial variations in the EI closely mirror aquaculture and tourism intensity in Nha Trang Bay [30]. EI values consistently peaked near the Cai and Tac River mouths, reflecting nutrient and sediment inflows from domestic waste, riverine runoff, and aquaculture effluents. The highest EI values were recorded in areas of intensive lobster cage culture at Mieu Island, where large numbers of cages, high feed inputs, and limited flushing intensified localized nutrient enrichment [10]. Tourism hotspots along the coast and around Tre Island also showed elevated Chl-a and BOD compared to offshore areas, indicating that wastewater discharges from tourist facilities significantly contribute to nutrient loading.

Periods of strengthened environmental regulations and expanded wastewater treatment (post-2019) corresponded with declining EI values toward mesotrophic conditions, highlighting the effectiveness of management interventions [35]. By combining EI values with supporting parameters—DIN, DIP, BOD₅, and Chl-a—this dataset allows for a more nuanced interpretation of nutrient enrichment and phytoplankton blooms under changing socio-economic conditions [36].

The patterns demonstrate that aquaculture intensity (number of cages, stocking density, and effluent release) and tourist arrivals are primary drivers of nutrient loading and eutrophication risk [30]. This linkage reinforces the need for adaptive zoning, stricter effluent standards, and seasonal management of tourism wastewater to mitigate eutrophication in Nha Trang Bay. Moreover, integrating EI trends with socio-economic indicators such as aquaculture output, tourist numbers, and wastewater treatment capacity can provide a forward-looking tool for scenario modeling, early warning, and policy evaluation. This combined approach advances beyond simple water quality monitoring toward a full socio-ecological framework capable of tracking environmental responses and management effectiveness over time, thereby supporting evidence-based governance and sustainable development of the bay’s coastal resources.

In addition to its direct scientific value, the dataset enables calculation of a eutrophication index, aligning with regional and international environmental assessment frameworks, including those promoted under the United Nations Sustainable Development Goal 14 (Life Below Water). By linking nutrient concentrations with biological responses through Chl-a measurements, the dataset supports integrated ecosystem assessments essential for evidence-based policymaking and adaptive management strategies [37,38]. Beyond its core parameters, the dataset’s consistency across four surveys over a decade makes it suitable for developing predictive models, stressor–response analyses, and early-warning indicators of ecosystem degradation [36].

A key strength of this work lies in its open-access nature, which fosters collaboration among scientists, policymakers, and stakeholders. By making the dataset publicly available, the study promotes transparency, encourages peer review, and supports interdisciplinary research [39]. The lack of standardized, comparable data has long hindered the effective management of eutrophication in coastal waters [40], whereas recent advances in water quality indicator development underscore the potential of high-quality datasets to drive environmental improvements [41]. These elements underline the transformative power of open-access datasets to reach sustainable coastal ecosystems [42,43]. Furthermore, beyond eutrophication assessment, these data can be integrated into diverse applications, including ecological modeling at multiple spatial scales, remote sensing calibration, and cross-regional comparative analyses. Such versatility amplifies the dataset’s scientific and practical significance, enhancing understanding of coastal water quality dynamics across Southeast Asia.

These combined vertical and temporal patterns demonstrate that the dataset captures not only spatial heterogeneity but also long-term trends and episodic changes driven by anthropogenic pressures and management responses in Nha Trang Bay. Far more than an archive of environmental measurements, this dataset constitutes a strategic scientific resource that bridges the gap between marine environmental research and practical management. By integrating eutrophication indices with key parameters such as DIN, DIP, BOD₅, and chlorophyll-a, it provides an evidentiary foundation for evaluating ecological risks, identifying high-pressure zones, and measuring the effectiveness of regulatory interventions.

5. Conclusions

This dataset represents one of the most comprehensive long-term records of environmental toxins and eutrophication indicators in Nha Trang Bay, compiled from four coordinated surveys between 2013 and 2024. It provides essential insights into the spatial and temporal dynamics of nutrient enrichment, organic loading, and biological responses, offering a robust scientific foundation for assessing ecological risks. Standardized methodologies—including depth-specific sampling, strict preservation protocols, and validated laboratory techniques—ensure reliable and comparable results over time.

Beyond its scientific value, the dataset serves as a baseline for identifying eutrophication trends, high-risk zones, and evaluating the effectiveness of environmental protection measures such as wastewater treatment upgrades and aquaculture zoning initiatives. By integrating eutrophication indices with key parameters (DIN, DIP, BOD₅, and Chl-a), it links ecological patterns to socio-economic drivers such as aquaculture intensity and tourism.

Aligned with regional and global initiatives, including the United Nations Sustainable Development Goal 14, the dataset supports integrated, evidence-based approaches to marine conservation and adaptive management. It provides a critical reference for forecasting future ecosystem changes, evaluating climate resilience, and guiding investments in pollution control, restoration, and sustainable development, thereby strengthening stewardship of Nha Trang Bay and similar vulnerable coastal ecosystems.