Pan-oceanic distribution of mercury (Hg) in sea turtles: a review

Abstract

With no known biological function, mercury (Hg) is highly toxic, bio-accumulates, and biomagnifies up the food web. Long-living marine animals, such as sea turtles, can be exposed to Hg in the oceans. The wide distributions of these reptiles and lifespans compatible with Hg residence time in ocean surface waters (approximately 30 yr) makes them reliable biological monitors of the long-term changes in Hg concentrations in the oceans. Taking this into consideration, we conducted a thorough review of studies to compare the concentrations of Hg in the 7 species of sea turtles distributed in different regions of the Atlantic, Pacific, and Indian Oceans and the Mediterranean Sea. Hg concentrations in muscle and scutes of Chelonia mydas were highest in the South Atlantic, whereas the highest concentrations found in Caretta caretta occurred in the Mediterranean Sea. The differences could be associated with the feeding habits of each species and the characteristics of the environment, such as the oligotrophic nature of the water and the lower productivity in the Mediterranean Sea. Unfortunately, few studies exist for the other 5 sea turtles (Dermochelys coriacea, Eretmochelys imbricata, Lepidochelys olivacea, L. kempii, and Natator depressus), which hampers a more detailed regional or ecological comparison among species. The results found in this review reveal information gaps that should be filled through more numerous studies focused on different oceanic regions and species.

Global mercury concentrations in biota: their use as a basis for a global biomonitoring framework

“…Sea turtles have a wide distribution in tropical and subtropical regions, their dietary habits vary according to the species (e.g., herbivores, omnivores, and carnivores), and their lifespan is compatible with the residence time of Hg in the surface layer of the oceans (approximately 30 years) (Aguirre and Lutz 2004 ; Barbieri 2009 ; UNEP 2013 ; Evers 2018 ). These characteristics enable a spectrum of Hg concentrations through different trophic levels and facilitate the comparison among regions (Anan et al 2001 ; Rodriguez et al 2022 ). For instance, when examining the Hg levels in liver samples from juvenile green turtles ( Chelonia mydas ) from two distinct environments – one highly influenced by human activities (Bahia, Brazil) and another with less anthropogenic impact (Ceará, Brazil), the comparison revealed elevated concentrations of Hg in both the green sea turtles and their food items (algae and mollusks) from the highly affected location (Bezerra et al 2015 ).…”

“…Most Hg monitoring studies using sea turtles involved the utilization of internal organs (e.g., liver, kidney, and muscle), proving effective as estimators of environmental Hg concentrations (Bezerra et al 2015 ; Rodriguez et al 2022 ). However, due to their status as endangered species, this type of sampling is not viable for monitoring programs, while the use of non-invasive methods such as blood and keratinized matrices (e.g., scutes and nails) allows periodic monitoring (Sakai et al 2000 ; Day et al 2005 ; Bezerra et al 2012 ; Rodríguez-Gutiérrez et al 2020 ).…”

“…According to Rodriguez et al ( 2022 ), the high concentrations of Hg found for loggerhead sea turtles in the Mediterranean Sea compared to the Atlantic and Pacific Oceans, can be explained by the high density of submarine volcanoes and regional anthropogenic contamination (Selin 2009 ; Cinnirella et al 2019 ; Tseng et al 2021 ). Robust findings for Hg concentrations in fish and marine mammals supports the trend of elevated levels in sea turtles and the tendency to be higher than other areas within the Atlantic Ocean (Gworek et al 2016 ; Kershaw and Hall 2019 ; Rodriguez et al 2022 ).…”

“…This finding holds significant implications for local communities that rely on this species’ eggs as supplemental protein (Ross et al 2016 ; Guzmán et al 2020 ; Tapilatu et al 2020 ). Regions, where sea turtles may need to be monitored more intensively for elevated levels of Hg, include the Caribbean Sea, the Arabian Sea, and especially the Mediterranean Sea (Rodriguez et al 2022 ). Areas with knowledge gaps in Hg exposure information are across the South Pacific (Rodriguez et al 2022 ).…”

“…Regions, where sea turtles may need to be monitored more intensively for elevated levels of Hg, include the Caribbean Sea, the Arabian Sea, and especially the Mediterranean Sea (Rodriguez et al 2022 ). Areas with knowledge gaps in Hg exposure information are across the South Pacific (Rodriguez et al 2022 ).

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