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Article

Can a 16th Century Shipwreck Be Considered a Mercury Source in the 21st Century?—A Case Study in the Azores Archipelago (Portugal)

by
Hugo C. Vieira
1,*,
Maria D. Bordalo
1,
Jaime Rendón-von Osten
2,
Amadeu M. V. M. Soares
1,
Sizenando N. Abreu
1 and
Fernando Morgado
1
1
CESAM—Centre for Environmental and Marine Studies, Department of Biology, University of Aveiro, 3810-193 Aveiro, Portugal
2
EPOMEX Institute, Universidad Autónoma de Campeche, Campus VI, Av. Héroe de Nacozari 480, Campeche 24070, Mexico
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2023, 11(2), 276; https://doi.org/10.3390/jmse11020276
Submission received: 6 December 2022 / Revised: 15 January 2023 / Accepted: 16 January 2023 / Published: 25 January 2023
(This article belongs to the Section Marine Pollution)
Browse Figures
Versions Notes

Abstract

:
During the Spanish colonial era, ships frequently transported mercury across the Atlantic to the New World to be used in gold mining. As many of those eventually sank, shipwrecks may represent a local source of mercury contamination in the marine environment. In this context, evaluating mercury contamination in coastal sediments and mercury magnification in marine food webs is crucial for understanding mercury dynamics and estimating exposure to marine life in locations where shipwrecks occurred. This study investigated mercury concentration present in coastal sediments and biota from three distinct groups: macroalgae (Asparagopsis armata and Ulva lactuca), gastropods (Littorina striata and Patella candei gomesii) and crustaceans (Palaemon elegans and Pachygrapsus marmoratus) collected in the Azores Archipelago, Portugal (one site near a 16th-century shipwreck and others in locations further away). Mercury analyses indicated that the sediments and species from the shipwreck area had significantly higher mercury levels than the other areas. Fine sediments showed values above those established in sediment quality guidelines; however, considering the mercury concentration of the total sediment fraction, adverse biological effects are not expected to occur. Moreover, increased mercury concentration from primary producers to consumers reinforced the biomagnification potential of this metal.

1. Introduction

Over the centuries, mercury has been used in gold and silver mining processes [1] to separate the metal particles from sediments (aquatic and terrestrial) through a process called amalgamation [2,3]. The amalgamation process was brought to South America (mainly Mexico, Peru and Bolivia) by Spanish colonizers in the 16th century [4,5], becoming increasingly popular after depleting gold and silver resources in the West Indies [6]. It is estimated that between the years 1550 and 1880, nearly 200,000 metric tonnes of mercury were released into the environment [7].
In the Spanish colonial era, the mercury necessary for the mining process was carried across the Atlantic in galleons, from Spain to the New World [8]. However, some of these vessels occasionally sank due to storms or collisions with reefs [6], becoming a serious source of pollution in the marine environment [9]. Over the years, some archaeological evidence of the transport of mercury on Spanish ships has been found. In 1992, at Pensacola, Florida, the Emanuel Point Ship was identified as a galleon of the fleet of Tristan de Luna, whose fleet was wrecked during a hurricane in 1559, and more than 250 mL of mercury has been collected from the ship’s bilge [10]. Other wrecks have been discovered, such as the galleons of Nuestra Senora de Guadalupe and Conde de Tolosa, caught in a hurricane and sunk in 1724 in Sarnana Bay, Dominican Republic, with a large cargo of mercury used in the process of precious metal extraction [6]. According to Monteiro [11], at least 96 ships crossed the Azorean waters between 1522 and 1998, the majority being Portuguese and Spanish ships coming back from India or the New World. In 1996, during an archaeological survey of Angra Bay in Terceira Island (Azores archipelago), two wrecks were found (Figure 1A). Wood samples taken from both wrecks provided evidence that they dated from the last quarter of the fifteenth century or the first quarter of the sixteenth century. In one of these shipwrecks (Figure 1B), more than 150 mL of elemental mercury was recovered from the ship’s hull. The presence of mercury in the ship’s bilge suggests that the vessel was carrying, or had carried in the past, a cargo with great quantities of this metal, which was used mainly as a component for the extraction of silver and gold in Central and South America [12].
Mercury is considered one of the most dangerous elements found in the environment [13] and considered by the Convention for the Protection of the Marine Environment of the North-East Atlantic (OSPAR Convention) as a pollutant of primary importance [14]. In the environment, mercury can be found in three distinct forms: elemental mercury (Hg0), inorganic mercury (IHg) and organic mercury (OHg) [15]. Elemental mercury can exist in gaseous and liquid state [16]. Liquid elemental mercury is not bioavailable since it is poorly absorbed through the gastrointestinal tract (less than 0.01%) [17,18]. However, in aquatic environments, under certain circumstances, elemental mercury can be sorbed—desorbed to sediment particles, onto suspended particulate matter or interact with dissolved organic matter—and can be transformed into other forms of mercury through chemical and photochemical reactions in aquatic systems [16]. Thus, elemental mercury can be oxidized and transformed into inorganic mercury [19] and the inorganic mercury can be methylated and converted to organic mercury (e.g., methylmercury) by the action of bacterial processes [20]. Unlike liquid elemental mercury, both ingested inorganic (approximately 7% to 15%) and organic mercury (∼95%) are absorbed through the gastrointestinal tract [18,21]. Mercury has no known biological functions and is extremely toxic for living organisms even at low concentrations [22], affecting essential life processes such as feeding, locomotion, development, respiration and physiological processes [23]. Once in the environment, mercury (especially the organic forms) has the tendency to bioaccumulate and biomagnify along aquatic food webs [24] due to a strong affinity for sulfhydryl groups (thiol; -SH) [25]. As a consequence, mercury concentration found in organisms occupying high trophic positions can be many times higher than those observed in the surrounding environment [22]. For this reason, the primary cause of mercury poisoning is the consumption of fish and seafood [26].
In the aquatic environment, mercury tends to bind to fine sediment particles [27], since they have a higher specific surface area that increases the association of heavy/trace metals and have higher levels of organic matter, which increases the binding sites between mercury and sediment particles [28]. In this way, sediments can act as mercury repositories and as a potential source of mercury contamination [29]. Due to the ecological risk resulting from the exposure of aquatic organisms to mercury, sediment quality guidelines (SQG) have been published by Canada, the European Union and the United States. These SQGs include two distinct concepts, the threshold effect level (TEL), which represents the concentration below which toxic effects are rarely or never observed, and the probable effect level (PEL), which represents the concentration above which toxic effects on living organisms are usually or frequently observed [27].
In the last two decades, interest in evaluating the environmental impact caused by shipwrecks has increased significantly [30]; however, most of the studies regarding shipwrecks focus on the impact of oil spills on surrounding environments [31,32,33,34]. In this context, this 16th century shipwreck provides an opportunity to assess the influence of the shipwreck presence on the mercury contamination in the surrounding environment (sediments and biota). Thus, this study aims to evaluate the mercury levels in different areas: surface sediment fractions and three different groups of marine organisms (algae, gastropods and crustacea) in the intertidal rocky shores of two Azorean islands (Terceira—where the 16th century shipwreck was found— and Graciosa), providing information regarding possible hazards to marine life and humans.

2. Materials and Methods

2.1. Sampling and Sample Preparation

This study was performed in the Azores archipelago, located in the North Atlantic Ocean (Figure 2). Surface sediment samples were randomly collected at 0–2 cm depth in five different intertidal sampling sites during low tide: Shipwreck Bay (S1—38°39′13.1″ N 27°13′13.1″ W) and other bays without evidence of shipwrecks on Terceira Island (S2—38°40′48.5″ N 27°03′19.0″ W, S3—38°48′04.4″ N 27°14′49.5″ W, S4—38°39′25.0″ N 27°14′56.2″ W) and Graciosa Island (S5—39°00′38.6″ N 27°57′48.3″ W). It should be noted that fine sediments were very scarce (beaches mainly had bed rock and igneous rock fragments >2 mm), and sediment was collected mostly from small puddles among igneous rocks.
Biotic samples were also collected in the same locations: the red alga Asparagopsis armata, the green alga Ulva lactuca, the gastropods Littorina striata and Patella candei gomesii and the crustaceans Palaemon elegans (shrimp) and Pachygrapsus marmoratus (crab). All samples were placed in clean zip bags, properly identified and stored at −20 °C for later mercury determination. Once in the laboratory, all samples were defrosted and prepared for mercury quantification. Algae were carefully cleaned from eventual particulate matter; L. striata shells were cracked in a bench vice and animals carefully removed using plastic tweezers, P. candei gomesii were separated from their shells with a stainless steel scalpel and P. elegans and P. marmoratus were dissected in order to remove the muscle tissue. Gastropods and crustaceans were also measured for their length before removal of the soft tissues.
Sediment samples were oven-dried at 35 °C for 3 days and subsequently homogenized and sieved (shaken continuously at amplitude 8) with stainless steel sieves with meshes sizes of 2 mm, 1 mm and 63 μm. Sediment fractions between 1 mm and 63 μm (sand fraction) and the fraction <63 μm (silt fraction) were used for mercury quantification.

2.2. Total Mercury Quantification

Total mercury quantification of the biotic and sediment samples was performed with the Advanced Mercury Analyzer (AMA-254, made by ALTEC and distributed by LECO). The procedure is based on a pyrolysis process of the sample using a combustion tube heated at 750 °C in an oxygen atmosphere, and the released mercury is trapped in a gold amalgamator and subsequently detected and quantified using atomic absorption spectrometry [35].
Sample analyses were quintupled to check the reproducibility of the results and three blank analyses (analysis without sample) were performed between samples to verify that mercury was not being accumulated over the samples. In this study, blank readings typically correspond to values of < 0.02 ng of mercury. Based on three times the standard deviation of the blank readings, the limit of detection was calculated to be 0.012 ng g−1 of mercury.
Analytical quality of the procedure was validated using the biological reference material TORT-2 (Lobster Hepatopancreas Reference Material for Trace Metals, National Research Council of Canada) and the inorganic reference material PACS-3 (Marine Sediment Reference Material for Trace Metals and other Constituents, National Research Council of Canada). The obtained data (0.28 ± 0.004 μg g−1 of mercury for TORT-3 and 2.88 ± 0.06 μg g−1 of mercury for PACS-3) and reference values of mercury (0.29 ± 0.02 μg g−1 of Hg for TORT-3 and 2.98 ± 0.36 μg g−1 of Hg for PACS-3) were not statistically different (p > 0.05).
The organic matter content in sediments was determined as a Loss on Ignition fraction (LOI) according to [27], where the sediment sample was re-weighted immediately after the mercury analysis. The sample weight loss after mercury analysis pyrolysis corresponds to the amount of organic matter present in the sediment.

2.3. Assessment of Hg Pollution

2.3.1. Geoaccumulation Index (Igeo)

The geoaccumulation index (Igeo) assesses the mercury concentration ([Hg]) of sediments by comparing present elemental concentrations with background levels. The present calculation enables the assessment of contamination by comparing the present and crustal average values of elemental concentrations [36]. The Igeo can be calculated by using the following formula:
I geo = log 2 ( [ Hg ] sample 1.5   [ Hg ] baseline )
Here, [Hg]sample is the measured concentration of the metal in the sediment, [Hg]baseline is the geochemical background value for the metal and the factor 1.5 is introduced to include possible differences in the background values due to lithological variations. The Igeo index was calculated using a background value of 0.05 µg g−1 established by the International Council for the Exploration of the Sea [37]. The values of Igeo were interpreted as suggested by [27], where Igeo < 0: practically unpolluted, 0–1: unpolluted to moderately polluted, 1–2: moderately polluted, 2–3: moderately to strongly polluted, 3–4: strongly polluted, 4–5: strongly to very strongly polluted and >5: very strongly polluted.

2.3.2. Contamination Factor (CF)

The contamination factor is an indicator used to evaluate the contamination level of an element in sediments [38] and can be determined using the equation [39]:
CF = [ Hg ] sample [ Hg ] baseline  
where [Hg]sample is the measured concentration of the mercury in the sediment sample and [Hg]baseline is the geochemical background value for mercury. The CF was calculated using the same background value used in the Igeo calculation (0.05 µg g−1) and the CF values were interpreted according to Ben Mna et al. [40]: low contamination (CF < 1); moderate contamination (1 < CF < 3); considerable contamination (3 < CF < 6) and very high contamination (CF > 6).

2.3.3. Potential Ecological Risk Index (PERI)

Potential ecological risk index is used to assess the potential impact of mercury (and other contaminants) on the environment [41]. PERI is calculated using the contamination factor (CF) of elements and toxic response coefficient (Tr) [39] through the formula:
PERI = CF × T r
The coefficient used for mercury was 40 [41] and the PERI values have been interpreted according to [40]: low-risk (PERI < 150); moderate (150 < PERI < 300); considerable (300 < PERI < 600) and high-risk condition (>600).

2.4. Statistical Analysis

After checking data normality (Shapiro–Wilk test) and homoscedasticity, parametric statistical one-way ANOVA was used to compare mercury concentration in sediments fractions and intertidal species of each sampling site. Tukey’s test was used to determine significant differences in mercury concentration between the shipwreck bay and the other bays. One-way ANOVA was also used to compare the mercury concentration between producers (macroalgae) and consumers (crustaceans and gastropods) from the same sampling site.
Statistical analyses were performed using GraphPad Prism version 9.00 for Windows (GraphPad Software, La Jolla, CA, USA) and statistically significant differences were considered when p < 0.05. Mercury data are presented as mean value ± standard error value (mean ± SE).

3. Results and Discussion

Metallic mercury (in liquid phase) had been directly observed during a previous archaeological survey, when about 2 kg of metallic mercury (150 mL of metallic mercury or 2.029 Kg of metallic mercury) had been collected from the ship wreckage [12]. It should be noted that this amount (~2 kg) was probably a small visible (and removeable) portion of the potentially huge amount of metallic mercury still present in the shipwreck. The mercury concentration in the sediments from the shipwreck area were very high compared to the other areas a few kilometers away, which can be explained due to the very high density (13.5) of the metallic mercury (trapping it in the restricted area for over 500 years), helped by a relatively low temperature in the aquatic environment, together with this form of mercury’s lack of bioavailability also restricting it from biodispersing. Mercury volatilization and mercury dissolution with subsequent transfer to the atmosphere, despite being very slow processes starting from underwater metallic mercury, might have overcome the oxidation of elemental mercury (which would have led to bioaccumulation and biodispersion) as the main process of mercury dispersion from the shipwreck, thus absolving, at least to some extent, the 16th century shipwreck from being considered a mercury contamination source in the 21st century, except at the shipwreck site.

3.1. Mercury in Surface Sediments

Regarding the mercury content determined in the different sediment fractions (sand and fine sediments) of the surface sediments, the mercury levels found in S1 were very high in both sediment fractions when compared with the mercury content in the sediment fractions from the other sampling sites (Figure 3). On average, the mercury concentration found in the sand fraction (Figure 3A) was 0.21 ± 0.01 µg g−1 in S1, 0.002 ± 0.0003 µg g−1 in S2, 0.002 ± 0.0008 µg g−1 in S3, 0.005 ± 0.003 µg g−1 in S4 and 0.002 ± 0.0003 µg g−1 in S5. At the same time, the mercury concentration in the fine fraction follows the same order as the concentration found in the sand fraction: S1 > S4 > S3 > S5 > S4 (Figure 3B). However, this mercury concentration is much higher than that observed in the sand fraction. The average mercury concentration found in the fine sediment fraction (Figure 3B) was 1.04 ± 0.004 µg g−1, 0.03 ± 0.001 µg g−1, 0.04 ± 0.002 µg g−1, 0.06 ± 0.001 µg g−1 and 0.03 ± 0.001 µg g−1 at S1, S2, S3, S4 and S5, respectively. The increase in mercury concentration in the fine sediment fraction was statistically significant (p < 0.05) in all sampling sites when compared to the mercury concentration present in the sand fraction of the same sampling site.
The mercury content present in marine sediments is often linked to the fine fraction of the sediments [27]. For this reason, the fine sediment fraction (<63 μm) is commonly used for mercury-monitoring studies [27,42,43,44]. The higher levels of mercury found in the fine sediment fraction in relation to the sand fraction can be related to the fact that the fine sediment particles (<63 μm) have a greater capacity to adsorb mercury [45,46], given their greater surface area per unit mass ratio and their higher organic matter content compared to other sediment fractions [47].
A significant difference (p < 0.05) was found in mercury concentration between S1 (shipwreck site) and the sites further from the wreck area (S2, S3, S4 and S5) in both sediment fractions (Figure 3A,B). This pattern was also observed in other studies that relate mercury contamination of marine sediments to the presence of shipwrecks. For example, Ndungu et al. [48] found higher mercury concentrations in the sediments adjacent to different wrecks in relation to the sediments from more distant areas. Leino et al. (2011) also state that metals such as mercury found in shipwrecks may influence the surrounding environment and Sbriz et al. [49] suggest that mercury concentrations found in some sediment samples collected along the Dominican Republic coast may have been influenced by the shipwrecks of two 270-year-old Spanish ships with 400 tonnes of pure mercury.
Mercury particles have a strong affinity with organic matter, which plays a fundamental role in the mercury accumulation dynamics in marine sediments [28,50]. This relationship may explain the differences (p < 0.05) in the mercury concentration found between the sand and fine sediment fractions (Figure 4), since smaller sediment particles often trap more organic matter than larger particles, as they have more surface area per unit weight [51].
Although some authors report a strong correlation between organic matter and mercury concentration in marine sediments [28,52], this correlation was not observed in this work. According to Vieira et al. [27], this lack of correlation between organic matter and mercury concentration may be due to the heterogeneity of sampling sites (urban areas vs. rural areas). Human activity in urban centers can lead to a greater enrichment of organic matter in sediments [53]. On the other hand, in remote areas, the amount of organic matter present in the sediments can be influenced by rivers [54] or continental runoffs [55]. In addition, mercury quantification in this study was performed “as Hg”, meaning that it was quantified as the volatile mercury being released after pyrolysis of each sample. When considering sediment particles, either lithogenic or organic, elemental mercury could be found adsorbed to the fine sediment particle surface, although other forms of mercury also could be incorporated (assimilated) into the organic particle.
Considering the SQG values (PEL and TEL) established by the different guidelines (Table 1) and the mercury concentration obtained in the fine sediment fraction, it is possible to observe that only the shipwreck site (S1) presents higher values than the PEL and TEL established by the different guidelines.

3.2. Assessment of Mercury Pollution

When considering the assessment of mercury pollution based on the mercury concentrations found in fine sediments (Table 2), only S1 (shipwreck area) can be classified as strongly polluted according to the Igeo classification. According to CF, these sediments are considered very highly contaminated with mercury. Furthermore, PERI in the shipwreck area was 834, indicating a high ecological risk. The remain sampling sites presented Igeo values ≤ 0 and CF values < 150, which means that the sediments of S2, S3, S4 and S5 can be considered uncontaminated, indicating a low ecological risk. In the Azores archipelago, the main economic activities are fisheries and agriculture [59]. At the same time, large industries are scarce, which means that there are no significant discharges of anthropogenic mercury into the environment [60]. This fact, combined with the difference in the classification of Igeo, CF and PERI between S1 and the other sites, reinforces the theory of the presence of an anthropogenic source of mercury at this site.
Although the fine sediment fraction collected in S1 presents a high ecological risk and exceeds the values established for the PEL and TEL in the different guidelines, the granulometric composition of the surface sediments collected in all sampling sites shows that fine sediment fraction represents less than 1% of the total sediment fraction (Table 3). These results are in line with those obtained by Quartau [61] in a study carried out on Faial Island (Azores archipelago), where the author found a very insignificant percentage of finer material (normally less than 1% of silt and clay) in coastal sediments.
According to Vieira et al. [27], the total concentration of mercury present in the sediments can be determined by considering the concentration of mercury present in the different fractions and the percentage of these fractions in the total amount of the sediments. Thus, considering the total fraction of the sediments, it is possible to observe that S1 continues to present significantly higher mercury values (p < 0.05) than the other sites (0.209 ± 0.009 µg g−1 in S1, 0.0018 ± 0.0002 µg g−1 in S2, 0.0022 ± 0.0002 µg g−1 in S3, 0.0046 ± 0.0002 µg g−1 in S4 and 0.0016 ± 0.0002 µg g−1 in S5). However, this concentration of mercury is much lower than the concentration present in the fine fraction of the sediments, which means that if we look at the concentration of mercury present in the total fraction of the sediments, the shipwreck area (S1) can be classified according to Igeo classification as moderately polluted (Igeo = 2), with considerable contamination according to CF (CF = 4). In terms of PERI, the value obtained for the total fraction of sediments was 167 (instead of the 834 observed in the fine fraction of sediments), which means that the classification changes from high ecological risk to moderate ecological risk. Regarding the other locations, their classification remained unchanged. Finally, considering the mercury concentration present in the total sediment fraction, it is possible to verify that none of the sampling sites exceed the values established for the PEL and TEL in the different guidelines.
The Portuguese legal guidelines for sediment quality (Portaria n.° 1450/2007 de 12 de Novembro) classify marine sediment quality into five classes. Marine sediments with mercury concentration <0.5 µg g−1 are classified as class 1 (clean dredged material) and concentrations in the range 0.5–1.5 µg g−1 are classified as class 2 (dredged material with trace contamination). On the other hand, mercury concentrations between 1.5–3 µg g−1 are classified as class 3 (lightly contaminated dredged material), mercury concentrations in the range 3–10 µg g−1 are classified as class 4 (contaminated dredged material) and marine sediments having a mercury concentration >10 µg g−1 are classified as class 5 (highly contaminated material). Therefore, considering the mercury concentration present in the total sediment fraction, all sampling sites in this study are classified as class 1.

3.3. Mercury Accumulation in Intertidal Species

Concerning the mercury levels present in intertidal species of the different sampling areas, the species A. armata [62], U. lactuca [63,64], P. candei [65], P. marmoratus [66,67] and P. elegans [68] are considered as good bioindicators of metal contamination in other studies, since they have the capacity to reflect the environmental bioavailability of elements where the organisms live. The mercury concentration present in both species of algae ranged between 0.001 µg g−1 and 0.020 µg g−1 and between 0.001 µg g−1 and 0.015 µg g−1 for A. armata and U. lactuca, respectively. In the gastropod and crustacean species, the levels of mercury ranged between 0.015 µg g−1 and 0.054 µg g−1 in L. striata, between 0.004 µg g−1 and 0.045 µg g−1 in the species P. candei, between 0.007 µg g−1 and 0.045 µg g−1 in P. elegans and between 0.002 µg g−1 and 0.027 µg g−1 in the crab species P. marmoratus. Mercury data in the available literature for these organisms are scarce and the few existing studies show only the mercury concentration in dry weight, and do not provide the percentage of moisture after drying, which makes it difficult to compare the mercury levels obtained in this study with those already published. Nevertheless, a study by Shiber and Washburn [69] reported mercury levels between 0.00 and 8.00 µg g−1 (dw) in U. lactuca collected in nine locations along the rocky coast of Ras Beirut, Lebanon, and Green-Ruiz et al. [70] found a mercury concentration of 0.058 ± 0.029 µg g−1 (dw) in Ulva lactuca from Guaymas Bay, Gulf of California. The mercury levels found by Cunha et al. [71] in different populations of P. candei gomesii collected on São Miguel Island, Azores, ranged between 0.01 and 0.03 µg g−1 (dw) and Álvaro et al. [66] reported mercury concentration in P. marmoratus muscle between 0.07 and 0.09 µg g−1 (dw), also in the Azores.
All species collected at S1 presented higher levels of mercury (p < 0.05) than the ones collected at the other sampling sites (Figure 5). However, none of these concentrations exceeded the maximum mercury concentration (0.5 μg g−1) established through the ecotoxicological assessment criteria (EACs—concentration of contaminants in the environment below which chronic effects on marine species are not expected to occur) created by the OSPAR Convention [57].
Despite this, the mercury content in the biotic samples followed the same trend found in the sediments from the sites where they were collected, indicating that, along with the influence of the mercury concentration in the sediments, the shipwreck also has the ability to influence the mercury bioaccumulation in marine trophic chains. This assumption is corroborated by Rua-Ibarz et al. [72], who suggest that mercury concentration found in the brown tissue of the crab Cancer pagurus was affected by the mercury released from the German submarine U-864 sunk at the end of World War II. Rogowska et al. [9] also state that the presence of shipwrecks can be considered an important source of pollution for marine ecosystems, thereby influencing the surrounding environment (e.g., metal contamination).
Comparing the mercury concentration found among the species from the same sampling site, it was possible to observe significant interspecific differences (p < 0.05) in the average mercury concentrations between producers (macroalgae) and consumers (crustaceans and gastropods). In all sampling sites, no significant differences (p > 0.05) were found between the A. armata and U. lactuca collected. However, when we compared the levels of mercury present in these species with the levels of mercury in the remaining species, it was observed that consumers had significantly higher mercury concentrations (p < 0.05) in all sampling sites. These results suggest mercury biomagnification in the trophic chain, reinforcing the importance of the role that macroalgae may play in metals’ transfer cycle throughout the food chain [73].

4. Conclusions

In summary, this study suggests that the presence of a 16th century shipwreck in Angra Bay, Terceira Island, Azores archipelago, may be the responsible for increased mercury levels in the surrounding environment. From the absence of mercury sources in the study area, combined with the results obtained both in the biotic and abiotic matrices, it is possible to conclude that the 16th century shipwreck can be considered a source of mercury in this area.
The results show that mercury present in surface sediments is especially associated with the fraction of fine sediments. In the shipwreck area, the fine sediment fraction is considered extremely contaminated and presents a high ecological risk. However, this sedimentary fraction represents only 1% of the total sediment fraction, which means that the levels of mercury present in the total sediment fraction are significantly lower. As a result of the decrease in mercury concentration in the total sediment fraction compared to the fine sediment fraction, the contamination status of the sediments drops from extremely contaminated to considerably contaminated and the ecological risk changes from high to considerable. On the other hand, all sediment fractions from more distant areas present a low ecological risk and can be considered unpolluted.
As in the sediments, the mercury concentrations found in all the intertidal species also reflect the proximity to the shipwreck and the differences in mercury concentrations found between producers (A. armata and U. lactuca) and consumers (L. striata, P. candei gomesii, P. elegans and P. marmoratus) indicate a potential biomagnification through this marine food web. The mercury levels in biotic samples are indicative of the bioavailabilility of this metal, particularly in the shipwreck area.
Despite the increase in mercury concentration in both sediment and intertidal species with increased proximity to the shipwreck, the measured values are far from being considered dangerous and, therefore, do not have a significant impact on either marine food webs or human welfare.

Author Contributions

Conceptualization, H.C.V. and S.N.A.; methodology, J.R.-v.O., S.N.A. and F.M.; formal analysis, H.C.V. and M.D.B.; investigation, H.C.V. and M.D.B.; resources, J.R.-v.O. and A.M.V.M.S.; writing—original draft preparation, H.C.V.; writing—review and editing, M.D.B., J.R.-v.O., A.M.V.M.S., S.N.A. and F.M.; supervision, J.R.-v.O., S.N.A. and F.M.; project administration, F.M., S.N.A. and A.M.V.M.S.; funding acquisition, F.M. and A.M.V.M.S. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded through CESAM (UIDP/50017/2020+UIDB/50017/2020+LA/P/0094/2020), with financial support from FCT/MCTES through national funds. We are thankful for the financial support by the project INSIDER (PTDC/CTA-AMB/30495/2017 and POCI-01-0145-FEDER-030495) funded by FEDER, through COMPETE2020—Programa Operacional Competitividade e Internacionalização (POCI), and by national funds (OE), through FCT/MCTES. The authors also thank FCT and POPH/FSE (Programa Operacional Potencial Humano/Fundo Social Europeu) for the doctoral grant of H.C.V. (PD/BD/127808/2016). M.D.B. is funded by national funds (OE), through FCT, I.P., in the scope of the framework contract foreseen in the numbers 4, 5 and 6 of article 23, of the Decree-Law 57/2016, of 29 August, changed by Law 57/2017, of 19 July.

Institutional Review Board Statement

The current study was led by accredited researchers of the Portuguese National Authority for Animal Health (DGAV—Direção Geral de Alimentação e Veterinária) for the practice of animal experimentation (FELASA Category B), following Portuguese Law number 113/2013 and the European Directive 2010/63/EU of the European Parliament and European Union Council on the protection of animals used for scientific purposes.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available in the current manuscript and raw data are available on request from the corresponding author.

Acknowledgments

The authors would like to thank the Direção Regional de Cultura do Governo Regional dos Açores for providing and authorizing the use of photographs related to the shipwreck.

Conflicts of Interest

The authors declare that they have no conflict of interest (financial or non-financial).

References

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Jmse 11 00276 g001 550
Figure 1. Images taken during the archaeological work survey of Angra Bay in Terceira Island, Azores archipelago: (A) underwater work during an archaeological survey in 1996 and (B) traces of elemental mercury found in the ship’s hull. Elemental mercury droplets are indicated with white circles. (Photo credit: Direção Regional de Cultura do Governo Regional dos Açores).
Figure 1. Images taken during the archaeological work survey of Angra Bay in Terceira Island, Azores archipelago: (A) underwater work during an archaeological survey in 1996 and (B) traces of elemental mercury found in the ship’s hull. Elemental mercury droplets are indicated with white circles. (Photo credit: Direção Regional de Cultura do Governo Regional dos Açores).
Jmse 11 00276 g001
Jmse 11 00276 g002 550
Figure 2. Location of the sampling sites in the Azores archipelago: S1, S2, S3, S4 (Terceira Island); S5 (Graciosa Island).
Figure 2. Location of the sampling sites in the Azores archipelago: S1, S2, S3, S4 (Terceira Island); S5 (Graciosa Island).
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Jmse 11 00276 g003 550
Figure 3. Mercury concentration (μg g−1) in the sand fraction (A) and fine sediment fraction (B) from the different sampling sites. Asterisks (*) denote statistically significant (p < 0.05) differences when compared with the wreck site (S1).
Figure 3. Mercury concentration (μg g−1) in the sand fraction (A) and fine sediment fraction (B) from the different sampling sites. Asterisks (*) denote statistically significant (p < 0.05) differences when compared with the wreck site (S1).
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Jmse 11 00276 g004 550
Figure 4. Organic matter content (%) in sand and fine sediment fractions from the different sampling sites in the Azores archipelago: S1, S2, S3, S4 (Terceira Island); S5 (Graciosa Island).
Figure 4. Organic matter content (%) in sand and fine sediment fractions from the different sampling sites in the Azores archipelago: S1, S2, S3, S4 (Terceira Island); S5 (Graciosa Island).
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Figure 5. Mercury concentration (µg g−1) in different intertidal species. N/A means that species were not analyzed in these sampling sites. Asterisks (*) denote statistically significant (p < 0.05) differences when compared with the wreck site (S1).
Figure 5. Mercury concentration (µg g−1) in different intertidal species. N/A means that species were not analyzed in these sampling sites. Asterisks (*) denote statistically significant (p < 0.05) differences when compared with the wreck site (S1).
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Table
Table 1. TEL and PEL values (μg g−1) established by the different guidelines.
Table 1. TEL and PEL values (μg g−1) established by the different guidelines.
GuidelinePELTEL
NOOA SQuiRTs 10.130.7
EU-WFD 20.05–0.5▬
Canadian SQG 3▬0.7
1 Buchman [56]; 2 Bignert et al. [57]; 3 CCME [58].
Table
Table 2. Igeo (class and value), contamination factor (class and value) and potential ecological risk index (class and value) for each sampling site.
Table 2. Igeo (class and value), contamination factor (class and value) and potential ecological risk index (class and value) for each sampling site.
Sampling SitesAssessment of Mercury Pollution
Igeo
(Obtained Value)		CF
(Obtained Value)		PERI
(Obtained Value)
S1Strongly polluted
(4)		Very high contamination (21)High-risk condition
(834)
S2Practically unpolluted
(−1)		Low contamination
(0.5)		Low risk
(21)
S3Practically unpolluted
(0)		Low contamination
(0.9)		Low risk
(34)
S4Practically unpolluted
(0)		Low contamination
(1)		Low risk
(45)
S5Practically unpolluted
(−1)		Low contamination
(0.6)		Low risk
(26)
Table
Table 3. Granulometric composition (%) of the surface sediments in sampling sites according to the Wentworth scale.
Table 3. Granulometric composition (%) of the surface sediments in sampling sites according to the Wentworth scale.
Sampling SitesGravelSandFine Sediments (Silt)
Weight (g)%Weight (g)%Weight (g)%
S100320.3899.950.150.05
S200426.8999.940.240.06
S300297.7599.581.260.42
S400499.8599.970.170.03
S500313.7899.960.130.04
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Vieira, H.C.; Bordalo, M.D.; Osten, J.R.-v.; Soares, A.M.V.M.; Abreu, S.N.; Morgado, F. Can a 16th Century Shipwreck Be Considered a Mercury Source in the 21st Century?—A Case Study in the Azores Archipelago (Portugal). J. Mar. Sci. Eng. 2023, 11, 276. https://doi.org/10.3390/jmse11020276

AMA Style

Vieira HC, Bordalo MD, Osten JR-v, Soares AMVM, Abreu SN, Morgado F. Can a 16th Century Shipwreck Be Considered a Mercury Source in the 21st Century?—A Case Study in the Azores Archipelago (Portugal). Journal of Marine Science and Engineering. 2023; 11(2):276. https://doi.org/10.3390/jmse11020276

Chicago/Turabian Style

Vieira, Hugo C., Maria D. Bordalo, Jaime Rendón-von Osten, Amadeu M. V. M. Soares, Sizenando N. Abreu, and Fernando Morgado. 2023. "Can a 16th Century Shipwreck Be Considered a Mercury Source in the 21st Century?—A Case Study in the Azores Archipelago (Portugal)" Journal of Marine Science and Engineering 11, no. 2: 276. https://doi.org/10.3390/jmse11020276

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