**1. Introduction**

Mexico is a mega-diverse country with 90,839,521 hectares of protected natural areas, of which Terminos Lagoon, classified as "Flora and Fauna Protection Area"

has 705, 016 hectares that make it one of the largest areas in the country. Within its status as a Protected Natural Area, there are fishing activities and oil and gas extraction-conduction areas. Until a decade ago, the Campeche Sound contributed nearly 95% of the crude oil and 80% of the national natural gas; today, due to recent changes in the use of fossil energy, production has decreased, although it remains one of the most important companies in Mexico.

Campeche Sound in general, and Carmen Island in particular, have been zones of abrupt changes, beginning with the exploitation of shrimp, which in the years of 1969 to 1979, promoted the economic development of the area. Since 1976, a historical production of crude oil began for Mexico, bringing with it important changes in the population, social changes and therefore, environmental changes.

According to Cuellar et al. [1] in 1979 the company "Mexican Petroleum" (PEMEX) had a large number of facilities for the extraction and processing of crude oil and natural gas on the southwestern coasts of the Gulf of Mexico, as well as a total of 200 facilities for different purposes and 185 production platforms. These changes affected the fishing production and the lives of the inhabitants as they went from being a "fishing village" to industrial zones with an increase in the population and the services that were demanded. By 1970, there were more than 800 vessels with capacities ranging from three to fifty tons to process shrimp and more than twenty freezers and packers of the fishing product in the area, as well as four shipyards for the shrimp fleet; at present, all this activity has been in considerable decline, almost disappearing [2].

When the oil boom began, the first oil spills put fishing activity at risk and there have been very few studies in the area to determine the degree of impact of the oil industry on the deterioration of the environment; certain species such as white shrimp (*Litopenaeus setiferus*) are permanently banned to avoid completely depleting the resource; however, recent data and with the current crisis of the SARS-COV2 pandemic indicate that poaching activities have increased in the area, even with the capture of protected species; [https://www.novedadescampeche.com.mx/e stado/campeche/urgente-estrategia-federal-del-control-de-la-pesca-ilegal].

The main fishery resources in this area are shrimp, oyster and scale. The oyster harvest currently has the certification of the Commission for the Protection against Sanitary Risks of Campeche (COPRISCAM, by its acronym in Spanish) in the Atasta lagoon; however, its production has been diminished due to the fishing and poaching of this resource. On the other hand, the clam was the main fishing resource in the Pom lagoon for more than four decades. Currently the catch levels show a notable decrease, which has been attributed to excessive overexploitation; some studies attribute it to pollution and deforestation in the mangrove area. According to Ramos and Villalobos [3], the mangrove ecosystems of the Terminos Lagoon Flora and Fauna Protection Area have registered in recent years, a rapid transformation towards ecosystems with low productivity and biodiversity. The causes of this rapid loss are deforestation, urbanization, industrialization, agricultural, fishing and aquaculture activities; and the alteration of the hydrological regime of the Grijalva-Usumacinta river basin.

The shrimp fishery does not show a better picture. This resource, which was exploited for many years, is now only one fifth of what was obtained in the 1980s. Historical data show that in 1972 the yield of pink shrimp (*Farfantepenaeus duorarum)* was 11,904 tons and in 2000 it was only 1,409 tons [4]. With regard to the seven-bearded shrimp (*Xiphopenaeus kroyeri)* from 1993 due to its overexploitation in the coastal marine strip and with the entry into force of Mexican standards NOM-004-PESC-1993 and NOM-002-PESC-1993 (Diario Oficial de la Federación 1994., Plan de Manejo Pesquero de camarón siete barbas *Xiphopenaeus kroyeri* en las costas de los estados de Campeche y Tabasco) its fishing has been regulated by fishing bans seasons.

#### *Heavy Metal Contamination in a Protected Natural Area from Southeastern Mexico… DOI: http://dx.doi.org/10.5772/intechopen.95591*

Overall, the development of the oil industry, urbanization and overexploitation of marine species have had a strong environmental impact, as well as in the displacement of deep-sea fishing areas. However, very few studies have been conducted in the area that show the overall impact generated on the flora and fauna of this region. Studies have been reported on the impacts on benthic communities and their relation to the presence of hydrocarbons [5]; the studies show the presence and concentration of hydrocarbons in sediments and organisms [5–9]. There are numerous factors to be considered in the deterioration of an ecosystem, among them the great quantity of organic and inorganic substances that are generated not only by oil activity, but also by the entire related industry. In the years 2000–2001 alone, a total of 104,901 tons of sulfur oxides (SOx) and 1,747 tons of nitrogen oxides (NOx) were emitted into the atmosphere [1, 10]. There are currently no recent studies to compare these levels.

Among the inorganic contaminants that cause interest due to the adverse effects they can cause to living beings, heavy metals stand out, some of which have been cataloged as serious threats to human health because of their carcinogenic risk. Regarding the studies carried out to determine the degree of impact on the Campeche Sound, we can cite Vázquez et al. [11] who carried out oceanographic campaigns and comparative studies on the levels of Cd, Cr, Ni and V in marine sediments. In their study, they highlight that oil activity, fishing and marine traffic in the area substantially modify the levels of heavy metals; they also agree that the levels of organic matter have a direct influence on the distribution of metals in sediments; they conclude that metals can interact with organic matter in different ways forming phenomena of adsorption, ion exchange, coprecipitation and complexation.

Other studies have determined the levels of heavy metals in sediments and organisms along the Terminos, Atasta and Pom lagoons and in the Palizada, Candelaria and Chumpan Rivers. Aguilar et al. [12] attributed the levels of Cd, Cr, Cu, Hg and V detected in oysters (*Crassotrea virginica*) to anthropogenic activities; additionally, they calculated the condition index of the oyster (variable that indicates the condition of health) and attributed a decrease in it to the presence of heavy metals; likewise, the levels of Cd, Cr and Cu exceeded the permissible limits established for mollusks and fishery products in the Mexican norms NOM-031- SSA1–1993.

In another study, the concentrations of Cd, Fe, Cu, Pb and Zn were evaluated in oyster (*Crassostrea virginica*), crab (*Callinectes sapidus*) and shrimp (*Litopenaeus setiferus*). The results showed that both oyster and crab are foods that present high levels of Cd, Fe, Cu and Pb in comparison with shrimp; in this study all detected levels were within the permissible limits established by the Mexican Official Standards NOM-031-SSA1–1993 [13].

Regarding sediment studies, Montalvo et al., [14] analyzed the concentration of heavy metals in sediments of the Palizada River; the results showed a high relationship between the levels of metals found with the climatic season and the texture of the sediment. Later, Canedo et al. [15] evaluated the levels of heavy metals in sediments of the Terminos Lagoon; they concluded that the spatial distribution was influenced by river discharges and that the significant correlations found between B, Ba, Co, Mn, Ni and Zn are due to natural biogeochemical inputs; they also found heavy metal levels above background concentrations in sites near the Atasta Lagoon and considered this area vulnerable to heavy metal contamination.

#### **1.1 Effects of heavy metals**

Heavy metals exert a wide range of toxic effects in humans, aquatic and terrestrial life [12]. Different strategies have been developed to study the degree of

contamination of an area, such as the use of organisms called sentinels (oysters, clams) that due to their feeding habits, their little or no mobility, their little capacity to regulate the concentrations of ions in the internal fluids and their high tolerance to the metal ions absorbed above the metabolic requirements [16], make them ideal for studies of contamination by heavy metals; likewise, studies on fish have been of considerable interest to understand the toxic effects and because they are an important source of nutrients for humans and have the potential to bioaccumulate heavy metals in their tissues [17, 18]. Food contamination can come from different sources: from contamination of the aquatic environment, during harvesting, transportation, handling or packaging.

#### *1.1.1 Mercury (Hg)*

Regarding the toxicity of heavy metals, Hg is distinguished because it does not have any biological function; its presence in the environment is due to anthropogenic causes; the natural causes of contamination by this element are not significant. It is an extremely toxic metal; organisms that have been exposed have few biological mechanisms for its elimination and it accumulates progressively through the food chain [19, 20]. The most common form of organic Hg is in the form of methyl mercury (MeHg). Usually levels above tolerance limits can alter the normal functioning of the central nervous system and affect the kidneys and the immune system [21]. Studies show that the toxicity attributed to it is associated with aging and cell death. Bryan and Langston's study [22] study on the oyster *Crassostrea virginica* showed evident embryonic abnormalities at concentrations of 5 to 10 μg/L, while the survival rates of clams, copepods, shrimp and crustaceans were affected by the increase in Hg levels.

#### *1.1.2 Cadmium (Cd)*

Cadmium is an element that has no natural source of generation so its presence in aquatic systems and organisms is entirely anthropogenic [23]. Cd does not have biochemical or nutritional functions; it is highly toxic to plants and animals. The International Agency for Research on Cancer points out the Cd and its compounds as carcinogenic. Cd intake pathways in organisms are gastrointestinal and respiratory; it has severe consequences in the blood by binding to high molecular weight proteins [24]; likewise, it has been reported that it can cause different alterations in the biology of living beings, since it accumulates mainly in the liver and can have a half-life of thirty years [25]. In phytoplankton species, growth inhibition was observed at concentrations as low as 1 μg/L [22]. Other species such as *Galaxias maculatus* exposed to acute concentrations showed deficiencies in metabolic rate and deteriorating oxygen consumption; also, stress parameters and decrease in liver catalase activity were observed [26]. In the *Henanese Sinopotamon* crab, a high deterioration of enzyme activity was found in the stomach, intestines, and hepatopancreas [27]. For *Crassotrea virginica* oyster, hepatological changes of the intestine, digestive gland and other organs were presented when exposed to Cd [28]. Due to its source of origin, the activities by which it can be generated are the industrial processes of fertilizer production, by-product of the smelting of other metals and in electronic devices [24].

#### *1.1.3 Copper (Cu)*

Cu is an essential element for the growth and metabolism of many living beings; when the levels are increased, it becomes a not very tolerable element [12]. This metal can cause harmful effects in fish, showing damage such as histopathological

#### *Heavy Metal Contamination in a Protected Natural Area from Southeastern Mexico… DOI: http://dx.doi.org/10.5772/intechopen.95591*

alteration and accumulation in different organs [29]. Other studies [22] presented experimental evidence that a considerable number of species are sensitive to concentrations of 1 to 10 μg/L of Cu, while at levels of 2 μg/L, the survival rate in young scallops was reduced; likewise, oyster and mussel embryos showed abnormalities in growth and development after exposure to 5 μg/L and the isopod crustacean *Idothea baltica* showed an increase in population mortality. Calabrese et al. [30] studied the acute toxicity of Cu in embryos of *Crassostrea virginica*; the results showed that at certain concentrations there was no development in more than 50% of the individuals under study.

#### *1.1.4 Lead (Pb)*

Pb can be in the environment in particulate form or formed into lead compounds; it can be generated as a result of human activities such as oil combustion, industrial processes and solid waste combustion; there are no natural sources of lead, its presence in the environment is anthropogenic [25]. It has been reported that in humans this metal can cause alterations of the nervous system, kidney problems and is related to the development of cancer. In exposed fish, it has been shown to decrease red and white blood cells and decrease hemoglobin levels [18]. The process of Pb accumulation in fish tissues causes oxidative stress; thus, this stress induces synaptic damage and neurotransmitter malfunction and influences immune responses [31].

#### *1.1.5 Nickel (Ni)*

Ni is a non-essential and toxic metal whose main source of exposure is food, highlighting fish and vegetables that are treated with wastewater. Its introduction to the aquatic environment is anthropogenic. The effects that it causes in different organisms were studied by Martin et al., [32] in embryos of Pacific oyster (*Crassostrea gigas*), embryos of laurel mussel (*Mytilus edulis Linnaeus*) and larvae of Dungeness crab (*Cancer magister Dana*) exposed to ten metals among them Ni; the effects caused in these species are the abnormal development in more than 50% of the studied individuals. In fish such as *Colisa fasciatius*, a freshwater teleoste, exposed to 45 ppm nickel sulfate, the adverse effects observed were leukopenia due to reduced numbers of lymphocytes and polycythemia, as well as a considerable delay in the rate of erythrocyte sedimentation of dying fish [33].

The conditions of the aquatic environment have a great influence on the transport and mobility of metals such as Ni, so Tamzin et al., [34] carried out their studies in saline waters, hoping that these conditions would decrease the impact on marine biota; however, despite the speciation of the metal in these saline environments it was determined that the physiology of the organisms is the main factor in the toxic impact, finding deterioration as inhibition of breathing and promotion of oxidative stress. In other studies, the mortality rate of African catfish, *Clarias gariepinus*, showed a linear trend with increasing concentration; the researchers concluded that the depression observed in hematocrit, hemoglobin and erythrocyte decreases in this hematological study can be used as an indicator of Ni-related stress in fish [35].
