Assessment of Heavy Metal Contamination of Seawater and Sediments Along the Romanian Black Sea Coast: Spatial Distribution and Environmental Implications

Abstract

This study assesses the spatial distribution and contamination levels of some heavy metals (HMs), i.e., cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), and lead (Pb), in seawater and surface sediments along the Romanian Black Sea coast (RBSC). Sampling was conducted at 40 stations across 12 transects during May–June 2021, and the measured levels of HM concentrations were compared with Environmental Quality Standards (EQS), i.e., maximum allowable concentration (MAC) values, for seawater and effects range-low (ERL) thresholds for sediments. HM concentrations were measured using high-resolution continuum source atomic absorption spectrometry (HR-CS AAS). In seawater, the levels of Cd, Cu, and Pb concentrations exceeded the MAC values at three stations located in areas influenced by the Danube River or anthropogenic activities. In sediments, exceedances of ERL thresholds were found for Ni at 11 stations, for Cu at three stations, and for Pb at one station. HM contamination of sediment samples collected from these stations can be caused by both natural and anthropogenic sources, e.g., the Danube River, rock/soil weathering and erosion, agricultural runoff, port and construction activities, maritime and road transport, coastal tourism, petrochemical industry, wastewater discharges, offshore oil and gas extraction. Principal Component Analysis (PCA) provided valuable information about the relationships between relevant variables, including water depth and HM concentrations in seawater and sediments, and potential sources of contamination. The results highlight the influence of fluvial inputs and localized human activities on HM contamination. While the overall chemical status of Romanian Black Sea waters and sediments remains favorable, targeted management strategies are needed to address localized pollution hotspots and mitigate potential ecological risks. These findings provide valuable insights for environmental monitoring and sustainable coastal management.

1. Introduction

The Black Sea, a partially enclosed body of water surrounded by densely industrialized and populated regions, is a dynamic region affected by various environmental pressures [1]. The Romanian coastline is influenced by both natural processes and human activities, making it a focal point for understanding the interplay between anthropogenic impacts and marine ecosystem health [2]. The northern sector is predominantly influenced by the Danube River, the second largest river in Europe, which discharges significant amounts of freshwater, sediments, and pollutants into the northwestern part of the Black Sea [3,4,5]. In contrast, the southern sector is characterized by urbanization, industrial activities, coastal protection works, and port operations, which contribute to localized contamination [6]. Seasonal tourism further intensifies these pressures, creating a complex contamination profile that poses challenges for sustainable coastal management [6,7,8].

HMs, including Cd, Cr, Cu, Ni, and Pb, are persistent environmental pollutants that pose significant ecological and health risks [9]. These metals can originate from a variety of sources, including riverine discharges, rock/soil weathering and erosion, agricultural runoff, industrial and urban wastewater discharges, port and construction activities, maritime and road transport, and offshore oil and gas extraction [10,11,12,13]. Due to their toxic, bioaccumulative, and non-degradable nature, HMs disrupt marine ecosystems and threaten biodiversity [14] and human health [15,16,17,18]. Their concentrations and distributions in seawater and sediments are critical indicators of the environmental status of marine ecosystems [19]. Consequently, monitoring these contaminants is essential for assessing pollution levels, identifying hotspots, and implementing effective management strategies [20].

Marine sediments act as both sinks and potential secondary sources of HMs [21]. Processes such as adsorption, hydrolysis, and co-precipitation lead to the deposition of metals into sediments [22]. However, environmental changes, e.g., shifts in hydrodynamic conditions or sediment resuspension, can remobilize these metals, increasing their bioavailability and prolonging their impact on marine ecosystems [23]. Sediments, therefore, serve as a long-term archive of pollution, offering insights into historical and current contamination levels [24,25]. In contrast, seawater provides a snapshot of ongoing contamination and serves as a medium for pollutant transport [26].

HMs and other pollutants pose severe threats to marine ecosystems and human health. Marine organisms such as fish and bivalves bioaccumulate these contaminants, leading to toxicity, oxidative stress, reproductive issues, and reduced survival [27,28]. This bioaccumulation could impact human health through seafood consumption [18,29,30,31,32,33]. Additional pollutants, e.g., antifouling chemicals and microplastics, exacerbate HM toxicity, further harming marine biodiversity [34].

The Romanian Black Sea coast (RBSC) is a relevant zone for investigating HM contamination levels due to its unique geographic and economic characteristics [35]. The Danube River inputs as well as port activities, urbanization, coastal industrialization, and offshore oil and gas operations contribute significantly to the region’s pollution load [35,36]. While improvements in wastewater treatment and regulatory frameworks have mitigated some pressures, localized hotspots of contamination persist, particularly in areas near major urban centers and ports [8].

Previous studies emphasized the impact of both natural processes and human activities on the distribution of HMs in the RBSC [36,37,38,39]. Comprehensive large-scale studies integrating seawater and sediment analyses are essential for identifying contamination patterns, understanding pollutant sources, and assessing ecological risks [40].

The main aims, novelty, and implications of the research are as follows:

• This study provides a comprehensive spatial assessment of heavy metal (HM) contamination in both seawater and sediments along the RBSC, integrating data from 40 sampling stations across 12 transects;
• Unlike previous studies that focused on either seawater or sediments, this research combines both matrices to offer a more holistic view of contamination patterns and sources;
• The study evaluates compliance with Environmental Quality Standards (EQS), including maximum allowable concentrations (MAC) for seawater and effects range-low (ERL) thresholds for sediments, to assess potential environmental risks;
• PCA was applied to establish relationships between selected variables, i.e., HM concentrations and water depth, and identify contamination sources, distinguishing between natural inputs (e.g., Danube River, soil erosion) and anthropogenic sources (e.g., industrial discharges, maritime transport, wastewater outflows);
• The findings contribute to the understanding of heavy metal contamination dynamics in the northwestern Black Sea and offer scientific support for pollution mitigation and sustainable coastal management strategies.

The findings of this study provide valuable insights into the spatial distribution and potential sources of heavy metal contamination along the RBSC, identifying key pollution hotspots that require targeted management efforts. By integrating Environmental Quality Standards (EQS) assessments and Principal Component Analysis (PCA), this research offers a scientific foundation for improving marine pollution monitoring and mitigation strategies. Furthermore, the results contribute to the implementation of regional and European environmental policies, such as the Marine Strategy Framework Directive (MSFD) and the Water Framework Directive (WFD), supporting efforts to maintain a good environmental status. Overall, this study underscores the need for continuous monitoring and sustainable management practices to protect marine ecosystems and human health in the Black Sea region.

2. Materials and Methods

2.1. Study Area

The RBSC, spanning 244 km and representing approximately 6% of the total length of the Black Sea coastline, experiences a diverse array of natural and anthropogenic pressures [8]. This sector is characterized by the significant impact of the Danube River in the north and the industrialized urban centers to the south, while seasonal peaks in tourism amplify these challenges [35,36].

In the northern sector, the Danube Delta Biosphere Reserve (DDBR) exerts a significant influence on the marine environment [41]. As the largest biosphere reserve in the European Union (spanning 5800 km²), this area receives inputs from the three main distributaries of the Danube, i.e., Chilia, Sulina, and St. Gheorghe [41]. The Danube is the primary supplier of pollutants to the Black Sea, with average discharges of 200 km³/year of water and 25–35 Mt/year of sediments [42]. The influence of its freshwater extends southwards along the coast, due to the longshore current, and in the deep-sea zone [42,43,44].

The southern sector, which includes several urban centers and tourist destinations, e.g., Constanta, Navodari, Eforie, Mangalia, Mamaia, Costinesti, Vama Veche, is subject to complex anthropogenic pressures [45]. Maritime transport, particularly through the major ports of Constanta, Mangalia, and Midia, is a critical economic activity in this region [46]. In the port of Constanta, the largest port on the Black Sea, various commodities, including crude oil, coal, chemicals, and construction materials, are handled [47]. The port of Midia, near Navodari, is a key hub for supplying crude oil to the Petromidia Refinery, the largest oil processing facility in Romania [7,8]. Tourism, agricultural runoff, and untreated or poorly treated wastewater discharges add further pressure [8]. However, improvements in sewage treatment infrastructure have been noted since 2018, following modernization and expansion efforts [48]. The area is also home to offshore oil and gas drilling platforms, contributing to regional economic activities but also acting as sources of marine pollution [37,38,47]. The interplay of pollutants from various sources, including HMs, organic compounds, and nutrients from urban, agricultural, and industrial activities, creates a complex contamination profile.

2.2. Seawater and Sediment Sampling and Characterization

Samples of water (n = 40) (from the sea surface) and marine sediments (n = 40) were collected in May–June 2021 from 40 sampling stations (Figure 1) belonging to the marine monitoring network of National Institute for Marine Research and Development “Grigore Antipa” Constanta, which covers national marine waters (territorial waters). Water depths (h = 5–70 m) and coordinates of the sampling stations are summarized in

Table 1. Water depths and coordinates of sampling stations.

No.	Area	Station	Water Depth, h (m)	Longitude (°)	Latitude (°)
1	Sulina	SU1	10	29.7717	45.1439
2	Sulina	SU2	20	29.7934	45.1411
3	Sulina	SU3	30	29.9242	45.1228
4	Mila 9	ML1	10	29.6630	45.0033
5	Mila 9	ML2	20	29.7333	45.0033
6	Mila 9	ML3	30	29.7928	45.0033
7	Sf. Gheorghe	SG1	10	29.6475	44.8836
8	Sf. Gheorghe	SG2	20	29.6783	44.8836
9	Sf. Gheorghe	SG3	30	29.7017	44.8836
10	Sf. Gheorghe	SG4	40	29.8529	44.8836
11	Portita	PO1	10	29.0812	44.6767
12	Portita	PO2	20	29.2992	44.6767
13	Portita	PO3	30	29.4742	44.6767
14	Portita	PO4	50	29.7500	44.6767
15	Portita	PO5	57	29.9167	44.6767
16	Gura Buhaz	GB1	10	28.7750	44.3897
17	Gura Buhaz	GB2	20	28.8530	44.3897
18	Cazino Mamaia	CZ1	10	28.6686	44.2396
19	Cazino Mamaia	CZ2	20	28.7061	44.2350
20	Cazino Mamaia	CZ3	30	28.8472	44.2347
21	Constanta Nord	CN1	10	28.6607	44.2250
22	Constanta Nord	CN2	20	28.7003	44.2167
23	Constanta Est	EC1	14	28.6833	44.1667
24	Constanta Est	EC2	28	28.7833	44.1667
25	Constanta Est	EC3	36	28.9000	44.1667
26	Constanta Est	EC4	47	29.1333	44.1667
27	Constanta Est	EC5	54	29.3667	44.1667
28	Constanta Sud	CS1	5	28.6561	44.1406
29	Constanta Sud	CS2	20	28.6850	44.1217
30	Eforie Sud	EF2	20	28.6816	44.0187
31	Costinesti	CO1	10	28.6504	43.9450
32	Costinesti	CO2	20	28.6739	43.9450
33	Costinesti	CO3	30	28.7267	43.9450
34	Mangalia	MG1	10	28.6133	43.7989
35	Mangalia	MG2	20	28.6278	43.7989
36	Mangalia	MG3	39	28.7156	43.7986
37	Mangalia	MG4	53	28.8290	43.7921
38	Mangalia	MG5	70	29.3983	43.7573
39	Vama Veche	VV1	10	28.6105	43.7511
40	Vama Veche	VV2	20	28.6211	43.7511

. The stations in the shallow area allow the assessment of the direct impact exerted by land pressures, e.g., the Danube River discharges, agricultural runoff, urban and industrial treatment plant discharges, port and construction activities, road transport, petrochemical industry [35].

Surface water samples were collected using rigorously cleaned Niskin bottles to prevent contamination during collection. Sampling focused on the upper 0–1 m layer, reflecting the most impacted zone by atmospheric deposition, runoff, and human activities. Equipment was pre-cleaned using trace metal clean procedures, including acid washing and ultrapure water rinsing. After collection, samples were transferred to acid-washed polyethylene bottles in a contamination-free environment, sealed, and stored at 4–8 °C to maintain integrity for HM analysis [49].

Surface sediments were collected using a Van Veen grab sampler, specifically chosen for its ability to retrieve undisturbed samples from the top 2 to 5 cm layer, which represents recent pollutant deposition [50]. The grab design minimizes disturbance to the sediment profile, ensuring the integrity of the surface layer for accurate contamination assessment. After collection, sediments were carefully transferred to clean, acid-washed containers, avoiding cross-contamination, and stored at −20 °C until analysis. In the laboratory, the sediment samples were freeze-dried to ensure complete water removal. The lyophilized material was then processed for the analysis of inorganic pollutants. The sediment samples were sieved to remove particles larger than 0.5 mm, ensuring consistency in particle size. The remaining material was homogenized using an electric grinder to achieve a uniform sample for subsequent analyses [50].

HM concentrations in seawater were determined from unfiltered samples acidified to pH 2 using ultrapure nitric acid (HNO₃) [51]. This approach targets the total recoverable fraction of metals, which includes both dissolved metals and acid-leachable particulate metals. The addition of HNO₃ also served two key purposes: preserving the samples by inhibiting microbial activity and preventing metal adsorption onto container walls and acting as a matrix modifier to mitigate matrix interferences caused by salts during atomic absorption spectrometry (AAS) analysis [52]. For sediment samples, 0.05–0.5 g of dried material was digested with 10 mL of concentrated HNO₃ in sealed Teflon vessels and heated at 120 °C on a hot plate (Cole-Parmer HP-200D-L-C, Chicago, IL, USA) to ensure complete dissolution of metal-containing components. The digested solutions were then diluted to 100 mL with ultrapure deionized water (18.2 MΩ∙cm, Millipore, Burlington, MA, USA) [35].

HM concentrations in seawater and surface sediments were determined using a High-Resolution Continuum Source Atomic Absorption Spectrometer (HR-CS ContrAA 800 G, Analytik Jena, Jena, Germany). Calibration standards for Cd (0–10 μg/L), Cr (0–50 μg/L), Cu (0–50 μg/L), Ni (0–50 μg/L), and Pb (0–25 μg/L) were prepared from Merck stock solutions. Each sample was analyzed in triplicate, and the relative standard deviation (RSD) was maintained below 10%, consistent with AAS precision standards [35]. The method detection limits, close to instrument detection limits, ranged between 0.001 and 0.01 μg/L, depending on the element [35]. All analytical procedures adhered to rigorous quality control standards to ensure reliable and reproducible results [49,50,53]. Results were reported as μg/L for seawater and μg/g dry weight (dw) for sediments.

2.3. Data Processing

Characteristic variables of water and surface sediments were processed using PCA. The Pearson correlation coefficient (r) was used to evaluate the strength of the linear correlations between different variables. Statistical analysis was performed using XLSTAT ver. 2019.1 (Addinsoft, New York, NY, USA). Spatial distributions of metal concentrations in water and surface sediments were visualized using the Ocean Data View (ODV) version 5.1.7 [54].

3. Results

3.1. Metal Concentrations in Seawater

In this study, EQS in accordance with Directive 2013/39/EU, i.e., MAC values, were used to assess HM contamination of seawater. Directive 2013/39/EU amends the Water Framework Directive (WFD, Directive 2000/60/EC) by establishing EQS for priority substances in surface waters [55,56]. The legislation targets pollutants, including HMs such as Cd, Ni, and Pb, to mitigate ecological risks and harmonize water quality objectives across freshwater and marine environments. By comparing HM concentrations to the MAC values specified by the directive, this research evaluates short-term contamination risks and supports the achievement of Good Environmental Status goals under the WFD and Marine Strategy Framework Directive (MSFD, Directive 2008/56/EC) [57]. MAC values from national legislation were used to assess Cr and Cu contamination [58].

The levels of Cd, Cr, Cu, Ni, and Pb concentrations in water samples (Cd = 0.011–3.320 μg/L, Cr = 1.174–26.60 μg/L, Cu = 1.120–41.62 μg/L, Ni = 0.027–32.48 μg/L, and Pb = 0.001–23.58 μg/L) are summarized in Table S1, HM spatial distributions are shown in Figure 2, and related relevant statistics and MAC values are specified in Table 2. Data presented in Table S1, Figure 2 and Table 2 highlight the following aspects:

(i)

Higher levels of Cd (0.495–3.320 μg/L) at stations ML1, SG2, PO2, PO5, and EC5; maximum level of Cd (found at PO5) was 2.2 times higher than MAC_Cd (1.5 μg/L);

(ii)

Higher levels of Cr (13.12–26.60 μg/L) at stations SU3, SG2, PO2, MG3, MG5, and VV1; no level of Cr exceeded MAC_Cr (100 μg/L);

(iii)

Higher levels of Cu (9.415–41.62 μg/L) at stations ML1, ML3, SG2, PO2, GB2, CZ3, MG1, and MG3; maximum level of Cu (found at PO2) was 1.4 times higher than MAC_Cu (30 μg/L);

(iv)

Higher levels of Ni (14.20 μg/L and 32.48 μg/L) at stations MG1 and MG3; no level of Ni exceeded MAC_Ni (34 μg/L);

(v)

Higher levels of Pb (7.751–23.58 μg/L) at stations ML1, PO2, GB2, and EC4; levels of Pb found at ML1 (23.58 μg/L) and PO2 (21.05 μg/L) were up to 1.7 times higher than MAC_Pb (14 μg/L);

(vi)

Significant variability of Cr, Cu, Ni, and Pb (84.98% ≤ RSD ≤ 203.13%).

Table 2. Relevant statistics of metal concentration levels in water for 40 sampling stations and maximum allowable concentration values.

Statistics	Metal Concentration in Water (μg/L)
	Cd	Cr	Cu	Ni	Pb
Minimum (MIN)	0.011	1.174	1.120	0.027	0.001
Maximum (MAX)	3.320	26.60	41.62	32.48	23.58
Mean (m)	0.249	6.899	7.300	3.115	2.554
Median	0.082	4.704	5.016	1.782	0.763
Quartile 25	0.051	3.821	3.713	0.621	0.391
Quartile 75	0.192	7.173	6.757	3.336	1.974
Standard deviation (SD)	0.061	5.863	7.819	5.428	5.188
Relative standard deviation (RSD) (%)	0.543	84.98	107.1	174.2	203.1
Maximum allowable concentration (MAC)	1.5 *	100 **	30 **	34 *	14 *

* Directive 2013/39/EU [56]; ** Ord. 161/2006 [58].

Table 2. Relevant statistics of metal concentration levels in water for 40 sampling stations and maximum allowable concentration values.

Statistics	Metal Concentration in Water (μg/L)
	Cd	Cr	Cu	Ni	Pb
Minimum (MIN)	0.011	1.174	1.120	0.027	0.001
Maximum (MAX)	3.320	26.60	41.62	32.48	23.58
Mean (m)	0.249	6.899	7.300	3.115	2.554
Median	0.082	4.704	5.016	1.782	0.763
Quartile 25	0.051	3.821	3.713	0.621	0.391
Quartile 75	0.192	7.173	6.757	3.336	1.974
Standard deviation (SD)	0.061	5.863	7.819	5.428	5.188
Relative standard deviation (RSD) (%)	0.543	84.98	107.1	174.2	203.1
Maximum allowable concentration (MAC)	1.5 *	100 **	30 **	34 *	14 *

* Directive 2013/39/EU [56]; ** Ord. 161/2006 [58].

Figure 2. Distribution of the levels of cadmium, chromium, copper, nickel, and lead concentrations in marine waters along the coast of Romania in 2021; metal concentration levels at sampling stations highlighted with red circles were higher than maximum allowable concentration (MAC) values [56,58].

Figure 2. Distribution of the levels of cadmium, chromium, copper, nickel, and lead concentrations in marine waters along the coast of Romania in 2021; metal concentration levels at sampling stations highlighted with red circles were higher than maximum allowable concentration (MAC) values [56,58].

Higher levels of metal concentrations at sampling stations SU3 (the last station along the Sulina–SE transect), ML1 and ML3 (along the Mila 9–E transect), SG2 (along the Sf. Gheorghe–E transect), PO2 and PO5 (along the Portita–E transect), GB2 (along the Gura Buhaz–E transect), CZ3 (along the Cazino Mamaia–E transect), EC4 and EC5 (the last stations along the Est-Constanta–E transect), MG1, MG3, and MG5 (along the Mangalia–SE transect), and VV1 (near Vama Veche) can be caused by both natural and anthropogenic sources, e.g., the Danube River, rock/soil weathering and erosion, agricultural runoff, port and construction activities, maritime and road transport, coastal tourism, petrochemical industry, wastewater discharges, offshore oil and gas extraction) [35,36].

Distributions of Cd and Pb levels (Figure 2) reveal clear negative gradients in the north–south direction, emphasizing the significant contribution of the Danube River in the northwestern part of the Black Sea [59]. Higher levels of Cu were found in seawater samples collected from stations under the influence of the Danube River, i.e., ML1, ML3, SG2, and PO2. Additionally, localized metal enrichments were observed near specific hotspots, likely influenced by local anthropogenic activities such as wastewater treatment, port operations, and maritime transport. Accordingly, higher levels of Cu and Pb were found near Gura Buhaz (an area receiving treated wastewater from petrochemical plant), of Cd, Cu, and Pb in the Constanta area, of Cr, Cu, and Ni near port of Mangalia.

The Danube River remains the primary source of nutrients and pollutants to the northwestern Black Sea, despite recent declining trends [3,43,48]. Monitoring data from the International Commission for the Protection of the Danube River (ICPDR) indicate that the Danube delivers approximately 14.2 tonnes of Pb and around 1 tonne of Cd per year, with most of these metals bound to suspended particulates rather than dissolved in water [60]. Industrial, agricultural, and urban runoff contribute to elevated metal concentrations along its course [60]. Although the Danube Delta acts as a natural buffer, reducing metal fluxes before reaching the Black Sea [61], the north–south negative gradient of metal concentrations reflects the influence of the Danube, with dilution and sediment deposition playing key roles in distribution patterns [4]. Assessments from ICPDR confirm that regulatory measures and improved wastewater treatment have significantly reduced metal inputs [60]. However, cadmium remains a persistent pollutant in the lower Danube [61], and the river continues to shape HM concentrations in Black Sea coastal waters. While riverine input dominates regional metal budgets, localized sources from ports, industry, and urban runoff also contribute to pollution hotspots [61].

Previous studies confirm that port activities and wastewater discharges are major contributors to metal contamination in the coastal waters. A research conducted in 2019 on the coastal hotspots proved that, in comparison to the surrounding areas, the highest concentrations of HMs were recorded inside Constanta Port basin for Cu (16.08 µg/L), Cd (1.35 µg/L), Pb (11.14 µg/L), and Cr (6.87 µg/L), while Ni (6.36 µg/L) had its highest value in Midia Port basin. Additionally, slightly elevated concentrations of Cd (1.26 µg/L) and Pb (5.57 µg/L) were recorded in front of the Eforie South WWTP discharge [6]. A similar study in Turkey reported high levels of HM concentrations in the Samsun hot spot, particularly in areas impacted by industrial activities, wastewater discharges, and port operations (Cd = 1.33 µg/L, Ni = 1.08 µg/L, and Pb = 2.31 µg/L) [6].

Consequently, the assessment of marine water quality during May–June 2021, conducted in accordance with MAC values outlined in Directive 2013/39/EU [56] and national regulations (Ord. 161/2006) [58], generally revealed a good chemical status of the Romanian Black Sea marine waters. Most levels of HM concentrations were within acceptable limits. Only four levels of Cd, Cu, and Pb exceeded the MAC values at three stations, i.e., PO5 (for Cd), PO2 (for Cu and Pb), and ML1 (for Pb), located in areas influenced by anthropogenic activities or significant river inputs. These isolated exceedances indicate localized contamination rather than widespread pollution across the study area. Moreover, Cr and Ni did not exceed the MAC values. These findings underscore the effectiveness of current pollution control measures and regulatory frameworks in maintaining marine water quality in compliance with European standards [62]. However, the occurrence of localized exceedances highlights the need for continued monitoring and targeted management strategies, particularly in hotspots associated with port activities, wastewater discharge, and other anthropogenic sources [63].

3.2. Metal Concentrations in Surface Sediments

The quality of sediments in the present study was evaluated against specific thresholds that define Good Environmental Status (GES) under the Marine Strategy Framework Directive (MSFD) [64]. Effects range-low (ERL) values [65] are commonly applied to assess the quality of the marine environment and the ecological significance of the levels of hazardous substance concentrations in sediments. Numerous studies have shown that adverse effects on organisms are rarely observed when the levels of contaminant concentrations remain below the ERL threshold [66,67,68].

The levels of Cd, Cr, Cu, Ni, and Pb concentrations in surface sediments (Cd_S, Cr_S, Cu_S, Ni_S, and Pb_S) are summarized in Table S1, HM spatial distributions are shown in Figure 3, and related relevant statistics and ERL values are specified in Table 3. Data presented in Table S1, Figure 3 and Table 3 reveal the following aspects:

(i)

All levels of Cd_S (0.059–1.124 μg/g) and Cr_S (9.760–69.77 μg/g) were lower than ERL_Cd (1.2 μg/g) and ERL_Cr (81 μg/g);

(ii)

The levels of Cu_S (2.362–79.14 μg/g) exceeded ERL_Cu (40 μg/g) at stations SU1 (by 4%), SG4 (by 19%), and CS1 (by 98%);

(iii)

The levels of Ni_S (9.693–59.35 μg/g) exceeded ERL_Ni (35 μg/g) at 11 stations (SU1, SU2, ML1, ML3, SG4, PO5, CZ2, EC2–4, and MG4) (by 1.0–70%), the values being higher by more than 20% at stations SU2, ML1, SG4, PO5, CZ2, and EC3;

(iv)

The levels of Pb_S (5.042–95.63 μg/g) exceeded ERL_Pb (47 μg/g) only at station CS2;

(v)

Cd_S, Cr_S, Cu_S, Ni_S, and Pb_S had a significant variability (45.91% ≤ RSD ≤ 72.80%).

Table 3. Relevant statistics of metal concentration levels in surface sediments for 40 sampling stations and effects range-low values.

Statistics	Metal Concentration in Surface Sediments (μg/g)
	CdS	CrS	CuS	NiS	PbS
Minimum (MIN)	0.059	9.760	2.362	9.693	5.042
Maximum (MAX)	1.124	69.77	79.14	59.35	95.63
Mean (m)	0.266	31.72	22.90	28.99	22.62
Median	0.226	29.85	21.81	28.15	20.34
Quartile 25	0.137	20.04	11.00	16.39	10.24
Quartile 75	0.342	45.67	32.74	38.74	30.42
Standard deviation (SD)	0.188	14.98	15.61	13.31	16.47
Relative standard deviation (RSD) (%)	70.76	47.22	68.18	45.91	72.80
Effects range-low (ERL)	1.2 **	81 **	40 *	35 *	47 **

* Ord. 161/2006 [58]; ** ERL [66,67,68].

Table 3. Relevant statistics of metal concentration levels in surface sediments for 40 sampling stations and effects range-low values.

Statistics	Metal Concentration in Surface Sediments (μg/g)
	CdS	CrS	CuS	NiS	PbS
Minimum (MIN)	0.059	9.760	2.362	9.693	5.042
Maximum (MAX)	1.124	69.77	79.14	59.35	95.63
Mean (m)	0.266	31.72	22.90	28.99	22.62
Median	0.226	29.85	21.81	28.15	20.34
Quartile 25	0.137	20.04	11.00	16.39	10.24
Quartile 75	0.342	45.67	32.74	38.74	30.42
Standard deviation (SD)	0.188	14.98	15.61	13.31	16.47
Relative standard deviation (RSD) (%)	70.76	47.22	68.18	45.91	72.80
Effects range-low (ERL)	1.2 **	81 **	40 *	35 *	47 **

* Ord. 161/2006 [58]; ** ERL [66,67,68].

Figure 3. Distribution of the levels of cadmium, chromium, copper, nickel, and lead concentrations in surface sediments along the coast of Romania in 2021; metal concentration levels at sampling stations highlighted with red circles were higher than effects range-low (ERL) values [66,67,68].

Figure 3. Distribution of the levels of cadmium, chromium, copper, nickel, and lead concentrations in surface sediments along the coast of Romania in 2021; metal concentration levels at sampling stations highlighted with red circles were higher than effects range-low (ERL) values [66,67,68].

The highest levels of Cd_S (1.124 μg/g) and Cu_S (79.14 μg/g) were found at station CS1, whereas the highest level of Pb_S (95.63 μg/g) was recorded at station CS2. Moreover, high levels of Cr_S, Ni_S, and Pb_S were found at stations EC2 and EC3. In addition to these hotspots in the area of the port of Constanta, high concentrations of HMs were generally found in the sediments near the Danube River mouths (Sulina–SE, Mila 9–E, and Sf. Gheorghe–E transects) and in deeper offshore areas (h ≥ 50 m) along the Portița–E transect. High values of Cr_S and Ni_S were also detected in the sediments collected from station CZ2, possibly associated with the coastal works related to the beach expansion project in this area, involving dredging and sediment relocation [69].

Several levels of Cu_S, Ni_S, and Pb_S exceeded the ERL thresholds at 7.5%, 27.5%, and 2.5% of the total number of sampling stations, respectively. Nickel concentration varied spatially, with exceedances of the ERL threshold of 35 µg/g recorded at several stations, particularly in areas influenced by the Danube River and deeper offshore zones (Figure 3). The highest values, observed at SU1, SU2, ML1, ML3, SG4, PO5, CZ2, EC2–4, and MG4, surpassed ERL threshold by 20–70%, consistent with previous reports of Ni_S levels in north-western Black Sea ranging from 1 to 207 µg/g, with an average of 66.4 µg/g [37,70,71]. Conversely, lower Ni_S levels were recorded in the coastal areas of the central sector (e.g., CZ1, CN1, CN2, EC1, EF2), where values remained below or close to ERL threshold (Figure 3). This pattern aligns with numerous studies indicating that Ni tends to accumulate in fine-grained sediments rich in organic matter, while coarser nearshore sediments exhibit lower concentrations [37,71,72]. Some research has indicated that Ni shows a strong geochemical association with Fe₂O₃, reflecting its natural origin in regional lithology [37,70]. While these elevated Ni_S levels exceeded the threshold, they were consistent with the naturally high background levels of the Black Sea, as demonstrated through normalization using elements such as Rb or Fe [37,70], reflecting natural geochemical inputs rather than anthropogenic contamination [37,71,72]. Further studies are recommended to better differentiate between natural and anthropogenic sources of heavy metals in sediments [73,74,75,76].

The levels of Cd_S, Cr_S, Cu_S, and Pb_S were below ERL thresholds at most stations, indicating limited ecological risk, although localized exceedances were noted, highlighting potential hotspots of concern. Previous research focused on coastal pressures evinced that port basins such as Midia, Constanta, and Mangalia exhibited significantly elevated metal concentrations compared to surrounding areas, with localized hotspots likely influenced by industrial activities, shipping, and urban runoff. The highest values of HM concentrations recorded in 2019 were as follows: 0.95 µg/g for Cd, 55.06 µg/g for Cr (in the area of Midia Port), 47.10 µg/g for Cu, 57.84 µg/g for Ni, and 36.84 µg/g for Pb (in the area of Constanta Port) [6]. A similar study in Turkey evidenced that the Samsun hotspot showed the highest heavy metal concentrations in sediments, particularly near copper production facilities and WWTP discharge areas (207.08 µg/g for Cr, 464.33 µg/g for Cu, 20.75 µg/g for Hg, and 158.07 µg/g for Ni). Metal concentrations generally decreased with distance from industrial sources. Notably, Ni and Cr concentrations were higher in deeper waters (>40 m) due to fine sediment deposition, geochemical binding, and redox conditions, which favor their accumulation [6].

3.3. PCA Results

A data matrix with 40 rows (number of samples) and 11 columns (number of variables, i.e., Cd, Cr, Cu, Ni, Pb, Cd_S, Cr_S, Cu_S, Ni_S, Pb_S, and h) was used in PCA. The eigenvalues corresponding to PC1 (3.94), PC2 (2.03), PC3 (1.28), and PC4 (1.11) were greater than 1. These first four principal components (PCs), which explained 76.00% (35.80% + 18.45% + 11.62% + 10.13%) of the total variance, were further used in PCA. The results presented in

Table 4. Factor loadings.

No.	Variable		PC1	PC2	PC3	PC4
	Name	Symbol
1	Cd concentration in water	Cd	0.28	0.10	0.51	0.63
2	Cr concentration in water	Cr	−0.11	0.59	0.20	−0.42
3	Cu concentration in water	Cu	−0.06	0.87	−0.20	0.16
4	Ni concentration in water	Ni	−0.13	0.66	0.10	−0.41
5	Pb concentration in water	Pb	0.17	0.67	−0.23	0.52
6	Cd concentration in surface sediments	CdS	0.83	−0.02	−0.31	−0.09
7	Cr concentration in surface sediments	CrS	0.92	−0.05	−0.08	0.01
8	Cu concentration in surface sediments	CuS	0.89	0.11	−0.17	−0.18
9	Ni concentration in surface sediments	NiS	0.88	0.04	0.16	0.02
10	Pb concentration in surface sediments	PbS	0.79	−0.04	0.05	−0.15
11	Water depth	h	0.27	0.11	0.84	−0.12

Significant levels of factor loadings are highlighted in bold.

(factor loadings) highlight that the most important variables were Cd_S, Cr_S, Cu_S, Ni_S, and Pb_S for PC1, Cr, Cu, Ni, and Pb for PC2, h and to some extent Cd for PC3, Cd and to some extent Pb for PC4. According to the data specified in

Table 5. Correlation matrix.

Variable	Cd	Cr	Cu	Ni	Pb	CdS	CrS	CuS	NiS	PbS	h
Cd	1	−0.07	0.04	−0.08	0.17	0.12	0.15	0.15	0.28	0.10	0.28
Cr	−0.07	1	0.32	0.31	0.15	−0.09	−0.20	0.01	−0.05	−0.04	0.15
Cu	0.04	0.32	1	0.47	0.59	−0.03	−0.04	0.02	−0.07	−0.07	−0.05
Ni	−0.08	0.31	0.47	1	0.11	−0.14	−0.15	0.04	−0.07	−0.10	0.11
Pb	0.17	0.15	0.59	0.11	1	0.10	0.13	0.13	0.18	0.06	−0.05
CdS	0.12	−0.09	−0.03	−0.14	0.10	1	0.68	0.93	0.57	0.53	−0.01
CrS	0.15	−0.20	−0.04	−0.15	0.13	0.68	1	0.74	0.86	0.72	0.18
CuS	0.15	0.01	0.02	0.04	0.13	0.93	0.74	1	0.68	0.60	0.14
NiS	0.28	−0.05	−0.07	−0.07	0.18	0.57	0.86	0.68	1	0.63	0.33
PbS	0.10	−0.04	−0.07	−0.10	0.06	0.53	0.72	0.60	0.63	1	0.24
h	0.28	0.15	−0.05	0.11	−0.05	−0.01	0.18	0.14	0.33	0.24	1

Values of Pearson correlation coefficient (r) highlighted in bold are different from 0 with a significance level α = 0.05.

(correlation matrix), Cd_S, Cr_S, Cu_S, Ni_S, and Pb_S were directly correlated with each other, as well as Ni_S with h, Cu with Cr, Cu with Ni, and Cu with Pb, their corresponding Pearson correlation coefficients (0.32 ≤ r ≤ 0.93) being different from 0 with a significance level α = 0.05.

The significant correlations between Cr_S, Cu_S, Ni_S, and Pb_S could suggest common pollution sources such as industrial discharges, port activities, and urban runoff. Cu and Pb are associated with shipping and anti-fouling paints, while Cr and Ni are linked to metallurgical and petrochemical industries [77,78]. Beyond source similarities, these metals exhibit similar geochemical behaviors, binding to fine sediments, organic matter, and iron-manganese oxides, which explains their co-deposition [79]. Their transport in suspended particles and accumulation in low-energy depositional areas further reinforce these correlations [80]. The PCA results confirm that these metals share input pathways and environmental accumulation processes, highlighting both anthropogenic influence and natural geochemical affinity in sediment composition.

The PCA biplot for PC1 and PC2 (Figure 4) indicates discrimination in the PC1 direction between the group I of stations highlighted in yellow (in the right quadrants) and group II of stations highlighted in green (in the left quadrants). For samples from group I of 14 stations, including SU1 and SU2 (near the mouth of the Sulina arm), ML1 and ML3 (along the Mila 9–E transect), SG3 and SG4 (along the Sf. Gheorghe–E transect), PO5 (the last station along the Portita–E transect), CZ2 (along the Cazino Mamaia–E transect), EC2–4 (along the Est-Constanta–E transect), CS1 and CS2 (near Constanta Sud), and MG4 (along the Mangalia–SE transect), the values of Cd_S (0.213–1.124 μg/g), Cr_S (28.78–69.77 μg/g), Cu_S (24.59–79.14 μg/g), Ni_S (26.11–59.35 μg/g), and Pb_S (27.21–95.63 μg/g) (Figure 5) were generally higher than those for samples from group II of 18 stations, i.e., SG2, PO1 (near Portita), PO3, GB1 (near Gura Buhaz), CZ1 (near Cazino Mamaia), CZ3, CN1 and CN2 (near Constanta Nord), EC1 (near Est-Constanta), EC5 (the last station along the Est-Constanta–E transect), EF2 (near Eforie Sud), CO1–3 (near Costinesti), MG1 and MG2 (near Mangalia), VV1 and VV2 (near Vama Veche), where Cd_S = 0.059–0.235 μg/g, Cr_S = 9.760–33.51 μg/g, Cu_S = 2.362–36.45 μg/g, Ni_S = 9.693–30.90 μg/g, and Pb_S = 5.042–18.38 μg/g (Figure 6). The spatial distribution of sampling stations in groups I and II is shown in Figure S1.

The PCA biplot shown in Figure 4 also indicates discrimination in the PC2 direction between the two groups of stations, i.e., group III (in the upper quadrants) and group IV (in the lower quadrants). Each station in groups III and IV is highlighted in bold, and the groups are delineated in Figure 4 using blue ellipses. For samples from group III of four stations, including ML1 (near Mila 9), PO2, GB2, and MG3, the values of Cr (5.125–18.11 μg/L), Cu (10.21–41.62 μg/L), Ni (2.249–32.48 μg/L), and Pb (2.616–23.58 μg/L) were generally higher than those for samples from group IV of three stations, i.e., ML2, SG1 (near Sf. Gheorghe), and CN1 (near Constanta Nord), where Cr = 1.174–6.821 μg/L, Cu = 1.390–3.257 μg/L, Ni = 0.374–1.153 μg/L, and Pb = 0.081–1.183 μg/L (Figure 7). The spatial distribution of sampling stations in groups III and IV is shown in Figure S2.

The PCA biplot for PC3 and PC4 (Figure 8) indicates discrimination in the PC4 direction between the two groups of stations, i.e., group V (in the upper quadrants) and group VI (in the lower quadrants), which are delineated using blue ellipses. For samples from group V of three stations, including ML1, PO2, and PO5, the values of Cd (0.594–3.320 μg/L) were higher than those for samples from group VI of three stations, i.e., SU03, MG3, and MG5 (Cd = 0.035–0.259 μg/L) (Figure 9). Moreover, the values of Pb for stations ML1 (23.58 μg/L) and PO2 (21.05 μg/L) were significantly higher than those for samples from group VI of stations (Pb = 0.564–2.616 μg/L).

The PCA biplot shown in Figure 8 also indicates discrimination in the PC3 direction between the two groups of stations delineated using green ellipses. Samples collected from group VII of two deeper stations PO5 (h = 57 m) and EC5 (h = 54 m), located in the right quadrants, had significantly higher values of Cd (3.3205 μg/L and 0.749 μg/L) compared to samples taken from group VIII of two shallow stations SU1 (h = 10 m, Cd = 0.340 μg/L) and CS1 (h = 5 m, Cd = 0.147 μg/L), located in the left quadrants (Figure 9). The spatial distribution of sampling stations in groups V–VIII is shown in Figure S3.

4. Discussion

The distribution patterns of HMs in both seawater and sediments observed along the RBSC during May–June 2021 highlight the interaction of multiple natural and anthropogenic factors. The negative north–south gradient of Cd and Pb concentrations in seawater emphasizes the dominant influence of the Danube River as a major source of pollutants in the northwestern Black Sea [43,81]. These findings are consistent with previous studies that have identified rivers as key contributors of HMs and other pollutants in coastal areas, particularly in semi-enclosed seas where water exchange is limited [4,48,82]. This study confirms that Cd and Pb concentrations follow a decreasing gradient due to the Danube’s influence, supported by discharge and metal flux data [60,61]. While riverine transport is the dominant source of these metals, localized anthropogenic contributions remain relevant, particularly in areas with industrial and port activity [4,6,61].

Localized hotspots with high levels of Cu and/or Pb concentrations in seawater near urban centers and port areas reflect direct inputs from wastewater discharges, industrial activities, and maritime operations [35,48]. The higher levels of Cu concentration in these areas can be attributed to antifouling paints used for ships, while the higher levels of Pb concentration are likely related to legacy pollution from older infrastructure as well as runoff from roads and industrial effluents [83]. These results emphasize the role of urban and industrial centers located along the RBSC as significant contributors to localized marine pollution, highlighting the need for targeted mitigation measures in these areas [84].

PCA results suggest the following relevant aspects:

(i)

Discrimination on PC1 and significant positive correlation coefficients between Cd_S, Cr_S, Cu_S, Ni_S, and Pb_S indicate that HMs in surface sediments likely came from common natural and anthropogenic sources, e.g., Danube River discharges, rock/soil weathering and erosion, agricultural runoff, port and construction activities, maritime and road transport, coastal tourism, petrochemical industry, wastewater discharges, offshore oil and gas extraction [36,38,39];

(ii)

The significant positive correlation coefficient between Ni_S and h highlights that Ni associated with finer carriers, including silt, clay, and organic matter, was transported offshore by currents and waves [36];

(iii)

Discrimination on PC2 and significant positive correlation coefficients between Cu and Cr, Cu and Ni, Cu and Pb emphasize common sources of Cr, Cu, Ni, and Pb in seawater at stations ML1, GB2, PO2, and MG3, e.g., port activities, maritime and road transport, urban and industrial discharges;

(iv)

Discrimination on PC3 indicates higher levels of Cd in seawater samples collected from deeper stations PO5 (h = 57 m) and EC5 (h = 54 m); a possible pathway of Cd contamination is the atmospheric deposition of airborne particles from industrial emissions, fossil fuel burning, and waste incineration [85];

(v)

Discrimination on PC4 reveals higher levels of Cd and Pb in seawater samples collected from stations ML1 and PO2, probably caused by the Danube discharges [4].

The observed exceedances of MAC values in seawater for Cd, Cu, and Pb (at 7.5% of sampling stations) and ERL thresholds for Cu, Ni, and Pb (at 7.5%, 27.5%, and 2.5% of stations, respectively) in sediments have important ecological implications. Although these exceedances are limited to specific locations, they suggest potential risks to marine organisms, e.g., plankton, bivalves, and fish, through bioaccumulation along the food web [30,31,86]. This bioaccumulation not only causes toxicity in these organisms, leading to oxidative stress, reduced growth, and impaired reproduction [27] but also poses risks to higher trophic levels, including predators and humans [29,87]. Sediments with high levels of HM concentrations represent long-term sources of contamination, especially under conditions that favor remobilization, e.g., storms and dredging activities [88]. Hotspots located in port areas, wastewater discharge areas [6], and regions under the influence of the Danube [4], where contamination is most severe, require closer monitoring and targeted mitigation to protect marine biodiversity and ensure the safety of seafood consumers [89].

A comparison of the data with the dynamics of previous years (2018–2020) in the studied area reveals fluctuating trends rather than consistent increases or decreases (Figure S4). Copper concentration showed a decrease in median values over time, from around 13–17 µg/L in 2018–2019 to ~5 µg/L in 2021, but with notable variability and outliers, including a maximum value of 41.62 µg/L in 2021, indicating sporadic contamination events. Cadmium concentration followed a similar pattern, with stable median values in 2018–2019 (~1 µg/L), a drop in 2020 (~0.07 µg/L), and a slight increase in 2021 (~0.08 µg/L). Despite this rise, the main concern in 2021 is the increased spread of values, with occasional spikes suggesting localized pollution. Lead concentration exhibited a clear decreasing trend, with median values dropping from ~14 µg/L in 2018 to ~1 µg/L in 2021, along with a reduction in extreme values, indicating improved pollution control or reduced emissions. Nickel concentration followed a similar pattern, showing lower median values in 2020 and 2021 (~2 µg/L) compared to earlier years (~8 µg/L in 2018). Chromium concentration remained relatively stable, with minor fluctuations and no significantly increasing trend. Overall, while Cd and Cu concentrations exhibited high variability and outliers in 2021, they did not show a general upward trend in their median values. The decrease in Ni and Pb concentrations suggests improvements in water quality, while Cr concentration remained relatively constant. These findings highlight the need for continued monitoring to assess both long-term trends and episodic pollution events that may temporarily impact water quality.

The levels of Cd, Cr, Cu, Ni, and Pb concentrations in seawater from the RBSC are generally similar to those reported in studies of other semi-enclosed seas, including the Baltic Sea and Mediterranean Sea, and estuarine areas [90,91,92]. For example, studies have reported hotspots of HMs located near urban and industrial areas of the Baltic Sea [93], similar to the contamination patterns observed near the port of Constanța and wastewater discharge points in this study. Due to lower hydrodynamic activity, the Baltic Sea often exhibits slower dilution rates, leading to higher average levels of HM concentrations [94,95,96,97]. Coastal areas of the Mediterranean, such as those influenced by the Po River (Italy) and Nile River (Egypt), show comparable HM concentration gradients, indicating significant riverine contributions [98,99]. Industrial hubs and maritime activities often result in higher levels of Cu and Pb concentrations in Mediterranean water [100,101,102].

Previous studies on the biogeochemical processes and HM distribution in the northwestern Black Sea have highlighted the importance of riverine metal and nutrient inputs, alongside localized sources, and the influence of redox cycles involving manganese (Mn) and iron (Fe) complexes [35,36,103]. For example, higher concentrations of Cu and Ni were reported on the Black Sea continental shelf compared to the deep basin, emphasizing the significant impact of riverine discharges and anthropogenic activities on this semi-enclosed sea [36]. Elevated dissolved Pb concentrations occasionally observed in offshore surface waters were attributed to atmospheric deposition combined with reduced scavenging efficiency in particle-poor waters [103].

Fertilizers and pesticides from agricultural runoff are recognized as significant contributors to high concentrations of Cd, Cr, Cu, Ni, and Pb in marine environments [78,104,105]. Fuel combustion by ships and boats was also identified as a major source of Ni and Pb contamination [106,107,108,109]. Additionally, antifouling paints used for ships represent an important anthropogenic source of Cu [78,106]. In the coastal areas of Constanța, Eforie, and Mangalia, wastewater treatment plants could discharge significant amounts of organic matter and pollutants, including Cd, Cr, Cu, Ni, and Pb into the marine environment [8,39,48]. Offshore oil and gas drilling platforms near stations PO4 and PO5, as well as crude oil refineries and construction activities represent additional anthropogenic sources of metals such as Cd, Cr, Cu, Ni, and Pb [110]. These contaminants can enter marine systems through direct wastewater discharges or via atmospheric deposition, including both dry and wet deposition of airborne particles from regional industrial emissions, construction activities, and maritime and road traffic [78,109,111].

HMs are persistent pollutants that can disrupt the natural biogeochemical balance in marine environments [112]. In seawater, metals are removed through both biological uptake, where ionic forms may enter cells via specific membrane transport mechanisms, and passive deposition processes, involving adsorption onto particulate matter followed by sedimentation [23,113]. Marine sediments act as both sinks and sources of metals, releasing them back into the water column through processes such as resuspension, advection, upwelling/downwelling, diagenesis, and diffusion [114]. These remobilization processes can extend the environmental impact of metal pollution, even with ongoing restoration efforts [115,116]. Through combined processes such as adsorption, hydrolysis, and co-precipitation, a significant proportion of HMs is deposited in sediments [117]. On the other hand, changes in environmental conditions can transform sediments from a sink of HMs into a source, releasing contaminants back into the water column [118]. Thus, monitoring HM concentrations in sediments provides critical insights for environmental risk assessments [119,120]. The distribution of HMs in sediments is influenced by both natural and anthropogenic sources and is closely linked to the mineralogical and granulometric characteristics of sediments [121]. Finer-textured sediments with higher organic matter content tend to accumulate higher amounts of HMs compared to coarser sediments found in shallow zones [25].

The analysis of dynamics of concentrations in sediments from the studied area during 2018–2021 shows fluctuations across years, with some elements exhibiting increased variability in 2021 (Figure S5). Copper concentration presented relatively stable median values over time, with 21.81 µg/g in 2021, similar to 2019 and slightly higher than in 2020 (17.37 µg/g). However, these values were lower than the median value found in 2018 (24.43 µg/g). Cadmium concentration followed a similar pattern, with a median value of 0.226 µg/g in 2021, slightly higher than in 2020 (0.191 µg/g) but remaining within the range of previous years. The presence of outliers in 2021 suggests localized pollution events rather than a steady upward trend. Lead concentration exhibited a decrease in its median values from 22.98 µg/g in 2019 to 20.34 µg/g in 2021, continuing a downward trend after peaking in 2019. Nickel concentration showed a declining trend, with the median values decreasing from 38.16 µg/g in 2020 to 28.15 µg/g in 2021, while chromium concentration also decreased, with its median values varying from 38.16 µg/g in 2020 to 29.85 µg/g in 2021. Accordingly, although Cu and Cd concentrations showed increased variability in 2021, their median values remained within historical ranges. In contrast, Pb, Ni, and Cr concentrations exhibited decreasing trends.

The comparison of heavy metal trends in seawater and sediments from the studied area during 2018–2021 highlights key differences in their temporal evolution. Lead and nickel exhibited a consistent decreasing trend across both matrices, suggesting reduced input over time, possibly due to improved regulatory measures. Copper and cadmium showed increased variability in seawater, with more frequent outliers, but remained relatively stable in sediments, indicating long-term accumulation processes. Chromium concentration maintained a stable trend in seawater, while in sediments, it showed a slight decline. These findings emphasize the role of sediments as persistent sinks for heavy metals and the need for continued monitoring of both seawater and sediments to assess contamination dynamics and potential pollution events.

The role of marine sediments as reservoirs for HMs is well documented in global marine systems [21], and the patterns observed in this study have similarities with findings from other regions. High levels of HM concentrations in sediments under the influence of the Danube are similar to those observed in other river-dominated coastal areas, such as the Mississippi Delta (USA) or Ganges Delta (India), where fluvial inputs contribute significantly to sediment contamination [122,123,124,125]. Similar to the contamination patterns near the port of Constanța, high levels of Cu and Pb concentrations in sediments are often reported for Mediterranean ports (e.g., Piraeus, Greece) and industrialized Asian coasts, as effects of port activities, shipping, and use of antifouling agents [126,127,128].

Relatively low levels of HM contamination in seawater and sediments, apart from localized hotspots, indicate a generally favorable chemical status for the RBSC. This contrasts with more heavily industrialized or urbanized coastal systems, such as those in Southeast Asia or densely populated parts of the Mediterranean, where risks of large-scale ecological degradation and bioaccumulation are prevalent [129,130,131,132]. However, localized HM exceedances in sediments deserve attention, as similar trends have been linked to sub-lethal toxicity in benthic organisms in globally comparable coastal areas [66,67,68,88].

The comparison of HM concentrations in seawater and sediments across different Black Sea regions (Ukraine, Romania, Bulgaria, and Turkey) during the ANEMONE Joint Cruise in 2019 reveals notable regional variations [133]. In seawater, copper concentrations ranged from 1.29 to 11.70 µg/L in Ukrainian waters, 0.45 to 5.26 µg/L in Romania, 0.29 to 3.14 µg/L in Bulgaria, and 0.31 to 2.90 µg/L in Turkey, showing a negative gradient from north to south. Cadmium concentrations followed a similar trend, with the highest values in Ukrainian waters (0.21–1.26 µg/L) compared to lower levels in Romania (0.01–0.37 µg/L), Bulgaria (0.001–0.17 µg/L), and Turkey (0.01–0.09 µg/L). Lead concentrations were also elevated in Ukrainian waters (1.14–8.72 µg/L) compared to Romania (0.31–4.86 µg/L), Bulgaria (0.14–3.15 µg/L), and Turkey (0.12–2.90 µg/L). Nickel and chromium displayed a more uniform distribution across the basin, with Ni concentration levels ranging from 1.21 to 8.26 µg/L and those of Cr from 0.05 to 3.36 µg/L. None of the measured concentrations exceeded MAC-EQS thresholds from Directive 2013/39/EU, indicating that while spatial variations exist, the overall chemical status of Black Sea waters remained within acceptable limits [133]. In sediments, regional differences were more pronounced. Nickel concentrations were highest in Ukrainian and Romanian waters, ranging from 30.2 to 70.4 µg/g in Ukraine and 28.5 to 65.1 µg/g in Romania. In Bulgaria and Turkey, Ni concentrations were lower, ranging from 9.2 to 40.3 µg/g and 11.3 to 38.7 µg/g, respectively. Copper concentrations showed an increasing trend toward the southwestern Black Sea, i.e., 8.81–32.75 µg/g in Ukraine, 10.25–42.63 µg/g in Romania, 15.10–50.10 µg/g in Bulgaria, and 20.25–55.07 µg/g in Turkey. Chromium concentrations followed a similar pattern, with values increasing from Ukrainian waters (7.67–54.36 µg/g) to the southwestern areas (12.23–97.88 µg/g in Turkey). Lead and cadmium exhibited higher concentrations in western areas, i.e., 5.37–57.74 µg/g for Pb and 0.024–0.610 µg/g for Cd, reflecting possible anthropogenic influences [133]. The comparison with previous studies, such as the MISIS Joint Cruise in 2013 [70], indicates that heavy metal concentrations in sediments have remained within similar ranges over the years (2013–2019), with slight regional variations. These findings highlight the importance of continued monitoring to assess both natural and anthropogenic influences on heavy metal distribution in the Black Sea.

This study provides a snapshot of HM contamination in seawater and sediments along the RBSC during May–June 2021. While the findings provide valuable insights into spatial distribution patterns, the temporal variability of metal concentrations remains unaddressed. Seasonal changes, influenced by variations in freshwater input from the Danube River, agricultural activities, and atmospheric deposition, could significantly alter metal concentrations in both seawater and sediments [134,135]. Long-term monitoring over multiple seasons and years is needed to capture these temporal dynamics and to establish reliable trends in HM pollution [136,137]. Temporal studies can identify trends linked to natural processes (e.g., river inputs and seasonal hydrodynamics) and anthropogenic activities (e.g., port operations, maritime and road transport, industrial and urban wastewater discharges) [138]. Monitoring in different seasons will provide a comprehensive picture of how contamination levels fluctuate throughout the year, while multi-year datasets can help assess the effectiveness of pollution control measures and regulatory frameworks [139].

The study covers a wide spatial area along the RBSC, comprising 40 stations across 12 transects. However, the sampling resolution may not fully capture small-scale variations, especially in hotspots located near industrial zones, ports, or wastewater discharge points. Increasing the density of sampling stations in future studies, particularly in known contamination hotspots, would improve the accuracy of spatial assessments [140].

To better understand the ecological and human health risks, future research should focus on analyzing HM concentrations in marine organisms such as fish, bivalves, and crustaceans. These species are key indicators of marine pollution due to their ability to accumulate metals through direct exposure or trophic transfer [141,142]. Assessment of bioaccumulation levels will provide information on the potential of HMs entering the food web and impact higher trophic levels, including humans [18,143].

Also, investigating the vertical distribution of HMs along the water column is essential for understanding their transport mechanisms and interactions with sediments [144,145]. Such studies can reveal how metals move between dissolved and particulate phases and how hydrodynamic processes, e.g., upwelling, stratification and resuspension, influence their behavior [146]. Analysis of vertical profiles in shallow and deeper zones will clarify the relationship between water column contamination and sediment metal deposition [147], improving the understanding of HM dynamics in the Black Sea.

By integrating bioaccumulation studies, vertical distribution analysis, and long-term monitoring, future research can provide a more robust understanding of HM dynamics [148], enabling targeted management strategies to protect marine ecosystems and human health in the Black Sea region. This study highlights the need for targeted management strategies to mitigate localized contamination hotspots along the RBSC. Key actions include upgrading wastewater treatment systems, implementing stricter controls on industrial pollution, promoting sustainable agricultural practices to reduce runoff, and addressing pollution from port and maritime activities [149]. For areas with significant sediment contamination, remediation techniques such as dredging or stabilization should be carefully evaluated [150]. Long-term monitoring programs are crucial for identifying trends and measuring the effectiveness of mitigation efforts [151]. Future research should expand its scope to include additional pollutants, e.g., hydrocarbons, and persistent organic pollutants (POPs), while using advanced tools such as geochemical modeling to improve source identification and pollution management [152].

These findings align with the objectives of the Marine Strategy Framework Directive (MSFD) and Water Framework Directive (WFD), providing a scientific basis for updated regional management plans [153]. Policymakers and stakeholders must prioritize intervention in pollution hotspots, integrate sustainable practices, and foster collaboration to preserve the ecological integrity and economic value of the RBSC [154]. In conclusion, this study highlights the importance of integrated approaches for monitoring and managing HM contamination in marine environments. By addressing localized sources of pollution and adopting sustainable practices, stakeholders can work to preserve the ecological integrity of the RBSC while supporting regional economic development [154].

5. Conclusions

This study provides a comprehensive assessment of HM contamination in seawater and sediments along the RBSC, revealing significant spatial variations influenced by both natural and anthropogenic factors and highlighting localized exceedances of regulatory thresholds. The observed differences indicate the interplay of pressures from riverine inputs, urbanization, industrial activities, and maritime operations.

The distribution of Cd and Pb in seawater showed a pronounced negative north–south gradient, underlining the dominant role of the Danube River as a source of contaminants in the northwestern Black Sea. Moreover, higher levels of Cu concentration were found in seawater samples collected from stations under the influence of the Danube River, i.e., ML1, ML3, SG2, and PO2. Hotspots located near wastewater discharge points, ports, and industrial areas, including GB2, CZ3, EC4, MG1, and MG3, further contributed to higher levels of Cu and/or Pb concentrations in seawater. The overall chemical status of marine waters in 2021 was classified as good, with exceedances of MAC values for Cd, Cu, and Pb at three stations (ML1 for Pb, PO2 for Cu and Pb, and PO5 for Cd) located in areas influenced by the Danube River or anthropogenic activities.

Marine sediments, acting as both sinks and potential secondary sources of HMs, showed greater variability in contamination levels. High anthropogenic loads were observed in the Constanta and Mangalia areas as well as in the sediments influenced by the discharges of the Danube River. Exceedances of ERL thresholds in sediments were found for Ni at 11 stations (SU1, SU2, ML1, ML3, SG4, PO5, CZ2, EC2–4, and MG4), for Cu at three stations (SU1, SG4, and CS1), and for Pb at one station (CS2), highlighting both natural and anthropogenic contributions to contamination.

This study emphasizes the importance of integrating water and sediment analyses to obtain a comprehensive understanding of HM distribution and sources. The application of PCA provided valuable information on the relationships between variables and identified key sources of contamination, e.g., river discharges, urban wastewater, agricultural runoff, industrial discharges, and atmospheric deposition.

In conclusion, the findings generally highlighted a favorable environmental status of the Romanian marine ecosystem, at the same time underlining the need for continuous monitoring and targeted management strategies to mitigate pollution. Efforts should focus on addressing localized pollution hotspots, including upgrading wastewater treatment facilities, enforcing stricter industrial and port activity regulations, and promoting sustainable agricultural practices to minimize runoff. These actions are critical for maintaining the ecological integrity of the region and ensuring sustainable development along the RBSC. By addressing these challenges, the study provides valuable insights for sustainable coastal management, helping to protect the ecological health and chemical quality of the RBSC.