Assessing trace elements in striped dolphins from the Strait of Gibraltar: Clues to link the bioaccumulation in the westernmost Mediterranean Sea area and nearest Atlantic Ocean

Mediterranean Marine Mammals: Possible Future Trends and Threats Due to Mercury Contamination and Interaction with Other Environmental Stressors

Despite decreasing anthropogenic mercury (Hg) emissions in Europe and the banning and restriction of many persistent organic pollutants (POPs) under the Stockholm Convention, Mediterranean marine mammals still have one of the highest body burdens of persistent pollutants in the world. Moreover, the Mediterranean basin is one of the most sensitive to climate change, with likely changes in the biogeochemical cycle and bioavailability of Hg, primary productivity, and the length and composition of pelagic food webs. The availability of food resources for marine mammals is also affected by widespread overfishing and the increasing number of alien species colonizing the basin. After reporting the most recent findings on the biogeochemical cycle of Hg in the Mediterranean Sea and the physico-chemical and bio-ecological factors determining its exceptional bioaccumulation in odontocetes, this review discusses possible future changes in the bioavailability of the metal. Recent ocean-atmosphere-land models predict that in mid-latitude seas, water warming (which in the Mediterranean is 20% faster than the global average) is likely to decrease the solubility of Hg and favor the escape of the metal to the atmosphere. However, the basin has been affected for thousands of years by natural and anthropogenic inputs of metals and climate change with sea level rise (3.6 ± 0.3 mm year-1 in the last two decades), and the frequency of extreme weather events will likely remobilize a large amount of legacy Hg from soils, riverine, and coastal sediments. Moreover, possible changes in pelagic food webs and food availability could determine dietary shifts and lower growth rates in Mediterranean cetaceans, increasing their Hg body burden. Although, in adulthood, many marine mammals have evolved the ability to detoxify monomethylmercury (MMHg) and store the metal in the liver and other organs as insoluble HgSe crystals, in Mediterranean populations more exposed to the metal, this process can deplete the biological pool of Se, increasing their susceptibility to infectious diseases and autoimmune disorders. Mediterranean mammals are also among the most exposed in the world to legacy POPs, micro- and nanoplastics, and contaminants of emerging interest. Concomitant exposure to these synthetic chemicals may pose a much more serious threat than the Se depletion. Unfortunately, as shown by the literature data summarized in this review, the most exposed populations are those living in the NW basin, the main feeding and reproductive area for most Mediterranean cetaceans, declared a sanctuary for their protection since 2002. Thus, while emphasizing the adoption of all available approaches to mitigate anthropogenic pressure with fishing and maritime traffic, it is recommended to direct future research efforts towards the assessment of possible biological effects, at the individual and population levels, of chronic and simultaneous exposure to Hg, legacy POPs, contaminants of emerging interest, and microplastics.

Metal Levels in Striped Dolphins (Stenella coeruleoalba) and Common Dolphins (Delphinus delphis) Stranded along the Sicilian Coastlines of the Mediterranean Sea

Dolphins, top predators of the aquatic food chain, are used as sentinel species of marine pollution as they are sensitive to environmental changes and able to accumulate a large content of contaminants. Several EU directives promote study of marine mammalians as bio-indicators to evaluate the presence of contaminants in the aquatic environment, such as the Mediterranean Sea, which is rich in environmental pollutants due to its geographic and geo-morphological characteristics. The aim of this study was to evaluate the content of toxic and essential metals and metalloids (Hg, Pb, Cd, As, Se and Zn), through ICP-MS analysis, in organs/tissues (liver, muscle, lung, kidney and skin) of striped dolphins (Stenella coeruleoalba) and common dolphins (Delphinus delphis) stranded along the Sicilian coastlines of the Mediterranean Sea. The results confirm the exposure of dolphins to toxic metals and metalloids, with the highest Hg levels observed in skin and liver, although a low Metal Pollution Index (MPI) was found in all samples of both dolphin species. From a comparative analysis of trace metals and metalloids according to sex and state of development, the highest levels of Cd and As were found in females vs. males and adults vs. juveniles, except for Pb in both species, and significant differences were observed between the two species, size of specimens, and organs/tissues analyzed. The highest Hg levels were correlated to those of essential metals Se and Zn, expressed as molar ratios, to evaluate the potential synergic effect of these detoxifying elements against Hg toxicity. This study confirms the rule of Stenella coeruleoalba and Delphinus delphis as valid sentinel species of the Mediterranean Sea, to verify the trend of metals pollution in this aquatic environment and, consequently, the health of these marine species.

Trace Elements in Stenella coeruleoalba: Assessment of Marine Environmental Pollution and Dolphin Health Status

Heavy metals are environmental contaminants and can easily accumulate and biomagnify in various marine species (fishes and mammalians) at the top of the aquatic food chain. Among marine mammalians, the striped dolphin (Stenella coeruleoalba) is the most abundant cetacean in the Mediterranean Sea and is considered to be a sentinel species to monitor the environmental marine pollution. In this study, the contents of toxic metals and metalloids (Cd, Pb, Hg, and As), micro-elements (Ni, Cr, Cu, Fe, Co, Mn, Se and Zn) and macro-elements (Na, Ca, K, Mg and P) were evaluated by ICP-MS analysis in several organs/tissues (lung, skin, muscle and liver) of Stenella coeruleoalba. The assessment of marine environmental pollution and dolphins health status was carried out through further analysis of the same specific parameters such as the metal pollution index (MPI) and coefficient of condition (K). Finally, the correlation between toxic metals and metalloids and essential micro-elements, expressed as molar ratios, was analyzed to evaluate the detoxifying ability (effectiveness) of Zn, Se and Cu. Data obtained showed the presence of toxic metals and metalloids analyzed in the Stenella coeruleoalba samples but the MPI values suggested a low environmental contamination of the Mediterranean Sea where dolphins lived. The content of micro- and macro-elements was found to be in a normal range for this species and predictive of dolphins good health status, as confirmed by the coefficient of condition K. However, the correlation between toxic and essential metals, expressed as molar ratios, showed that the following toxic metals cannot be detoxified by the essential metals: 66Zn/201Hg, 82Se/201Hg, 63Cu/201Hg and 66Zn/52Cr, 82Se/52Cr, 63Cu/52Cr. Therefore, this study highlights the key role of dolphin Stenella coeruleoalba to assess marine pollution and the importance of analyzing the complete mineral profile to evaluate the animal health status.

Metal and trace element concentrations in cetaceans worldwide: A review

This bibliographical review is a compilation of different scientific publications that reported data on metal concentrations in the muscle tissue of different species of cetaceans from seas and oceans around the world. Forty-nine scientific articles were selected, published over a fifteen-year period (2006-2021) with data on heavy metals and trace elements. The different groups of cetaceans considered in this study generally presented low concentrations of Cd and Pb. The same cannot be said of Hg. The highest concentrations of Hg were found in the groups of false killer whales. Similarly, the use of these groups of cetaceans as bioindicators of metal contamination shows that the Mediterranean Sea is one of the most metallically contaminated areas in the world. This may be due to the closed nature of the Mediterranean Sea and to the fact that it is also a highly populated and industrialized area.

Striped dolphins as trace element biomonitoring tools in oceanic waters: Accounting for health-related variables

In Europe, monitoring contaminant concentrations and their effects in the marine environment is required under the Marine Strategy Framework Directive (MSFD, 2008/56/EC). The striped dolphin (Stenella coeruleoalba) is the most abundant small cetacean species in Portuguese oceanic waters, representing a potential biomonitoring tool of contaminant levels in offshore waters. Concentrations of nine trace elements were evaluated by ICP-MS in kidney, liver and muscle samples of 31 striped dolphins stranded in the Portuguese continental coast. The mean renal Cd concentration was high (19.3 μg.g^-1 wet weight, range 0.1-69.3 μg.g^-1 wet weight) comparing to striped dolphins from other locations. Therefore, the present study reports a possibly concerning level of Cd in the oceanic food chain in Portuguese offshore areas. This study also aimed at evaluating potential relationships between trace element concentrations and striped dolphins' biological and health-related variables. Individual length was related with some of the trace element concentrations detected in striped dolphins. Indeed, Cd, Hg and Se bioaccumulated in larger animals, whereas the reverse was observed for Mn and Zn. Striped dolphins with high parasite burdens showed higher levels of Hg, while animals showing gross pathologies presented higher concentrations of Cd and Se. This study reported relationships between trace element concentrations and health-related variables for the first time in striped dolphins and it also provided information on the relative contamination status of Portuguese oceanic waters in comparison to other regions in the world.

Trace metals in striped dolphins (Stenella coeruleoalba) stranded along the Murcia coastline, Mediterranean Sea, during the period 2009-2015

Mercury (Hg), cadmium (Cd), lead (Pb), selenium (Se) and arsenic (As) concentrations in internal tissues of 72 striped dolphins (Stenella coeruleoalba) from Murcia Region (Mediterranean coastline) have been investigated for the first time. Hg showed the highest concentration, followed by Se, Cd, As and Pb. In general, the levels of metal found in this study were similar to those described in similar studies in the Mediterranean Sea. However, in some adult specimens, Hg liver concentrations were related with toxic effects in cetacean. A significant correlation was observed with age, likewise between Se and Hg and Cd in tissues, which agree with detoxify effect attributed to Se through inert complex formation. Molar ratio Hg:Se in liver was close to 1:1 in some specimens, which would indicate overload of the Hg-Se detoxify function and compromised health. These results could contribute to a better knowledge of the distribution of these persistent pollutants in the Mediterranean Sea.

As, Cr, Hg, Pb, and Cd Concentrations and Bioaccumulation in the Dugong Dugong dugon and Manatee Trichechus manatus: A Review of Body Burdens and Distribution

The death of dozens of manatees Trichechus manatus recently in Tabasco, Mexico, has captured international attention. Speculation about possible causes include water and food contamination by metals. Although federal authorities have ruled out water chemical pollution, the cause of these deaths is still awaiting conclusive laboratory results. Present work seeks to summarize information currently available on non-essential metals and those of great toxicological relevance in Sirenia (dugongs and manatees), highlighting its body distribution, presence in blood, and its relationship with their geographical distribution, gender and age, whenever possible. This paper focuses on the five elements: As, Cr, Hg, Pb and Cd, which are commonly considered as threats for marine mammals and reported in Sirenia. Some of these metals (Cr and Cd) were thought to be related to the recent deaths in Tabasco. All five elements are accumulated by Sirenia at different levels. Metal presence is associated to their diet but does not necessarily imply adverse effects for dugongs and manatees. Toxicological aspects and the human consumption risk in case of any illegal or traditional consumption in some cultures are discussed. Important toxicological research areas that need to be addressed are highlighted.

Using stable isotope analysis to assess trophic relationships between Atlantic bluefin tuna (Thunnus thynnus) and striped dolphin (Stenella coeruleoalba) in the Strait of Gibraltar

Stable isotope analysis (δ¹³C and δ¹⁵N from liver and muscle) was used to assess trophic relationships between Atlantic bluefin tuna (ABFT) (Thunnus thynnus) and striped dolphin (SC) (Stenella coeruleoalba) in the Strait of Gibraltar (SoG). δ¹⁵N values from ABFT muscle and liver tissues were significantly different from those of dolphin samples, but no for δ¹³C values. Diet estimation by MixSIAR models from muscle and liver revealed that ABFT fed mainly on squids (Todaropsis eblanae and Illex coindetii). The shrimp Pasiphaea sp. was estimated to be the most important prey-species in the diet of SC. Trophic positions estimated from muscle and liver isotopic data suggested that ABFT occupy a higher trophic level than SC. Estimations of isotopic niche, as measured by the standard ellipse area, indicated that ABFT show a broader trophic niche than SC; furthermore, SEAc did not show trophic overlap between both predators. The results of this study suggest that resource partitioning occurs between ABFT and SC in the SoG ecosystem.

High status of mercury and selenium in false killer whales (Pseudorca crassidens, Owen 1846) stranded on Southern South America: A possible toxicological concern?

The study was carried out to determine Hg and Se concentrations in false killer whales stranded on the Estrecho de Magallanes, Chile, South America. Tissue samples of five mature specimens were analyzed (two females and three males). Mean Hg concentration in liver 1068 (234) μg g^-1 dry weight (DW) (standard deviation in parenthesis) was markedly higher than those in kidney 272 (152) μg g^-1 DW, lung 423 (325) μg g^-1 DW, spleen 725 (696) μg g^-1 DW, muscle 118 (94) μg g^-1 DW and testicle 18.0 (2.8) μg g^-1 DW. Mean Se concentration in liver, 398 (75) μg g^-1 DW, was higher than those in kidney 162 (69) μg g^-1 DW, lung 128 (84) μg g^-1 DW, spleen 268 (245) μg g^-1 DW, muscle 47 (38) μg g^-1 DW and testicle 25.4 (2.1) μg g^-1 DW. Positive correlations were found between Hg and Se molar concentrations in muscle, lung, spleen and kidney. Molar ratio of Se/Hg in liver, lung and muscle were <1, but those in kidney and testicle were markedly >1 suggesting a Se protection against Hg toxicity. In all the examined specimens Hg values exceeded the toxic thresholds defined for hepatic damage in marine mammals, with Se/Hg molar ratios below 1 implying limited protective action of Se. Generally, our results showed that individuals are carrying a significant burden, reflecting a high exposure to this toxic metal. This constitutes the first report on Hg and Se levels for a large subantarctic odontocete in South America region, providing insights into their contamination status and with information to the understanding of possible impacts on wild populations.