Mercury in tropical and subtropical coastal environments
Introduction
The oceans play a central role in the biogeochemical cycles of essential and non-essential elements through a series of abiotic processes (chemical speciation, inorganic scavenging, sedimentation) and biological processes (bio-alteration, bio-accumulation, and bio-magnification). In the last few centuries (and especially since World War II), anthropogenic perturbations to the global mercury cycle have in many cases overwhelmed the natural processes of mercury cycling (Mason et al., 1994, 2012). For a long time human influence on mercury cycling was detectable only at the local scale, but it is now clear that these influences are global. From land-based sources, natural and anthropogenic mercury releases reach coastal areas and from there gain access to the ocean and its trophic webs. Although changes in Hg concentrations in seawater through direct measurements are not yet fully documented in the tropics, inventories of local sources (e.g., Marins et al., 2004), the increase of mercury levels in the biota (Evers et al., 2008a), temporal trends in geochemical records (Xu et al., 2011) and mercury cycling models, all point towards the existence of a global problem (Doney, 2010), of which tropical coasts are only part.
The tropical and subtropical belt includes countries of every socio-economic level, including very poor countries to some of the ten largest global economies (Fig. 1). It is associated with some of the largest coastal population densities (e.g., India, Bangladesh, China, South-East Asia, Caribbean Islands and Nigeria), and some of the World’s megacities are on the margins of estuaries in tropical regions (e.g., Hong Kong, Rio de Janeiro, Bombay). Global mercury emission patterns (Pacyna et al., 2010) place most of the mercury atmospheric sources in densely populated coastal areas. However, nations in the tropical region are mainly emerging and developing economies, struggling with issues of population welfare, environmental protection and economic growth. In these cases, health and environment policies vary, and (mercury) pollution-oriented policies are not always in place. Therefore, coastal populations and environments are at risk in terms of direct and indirect exposure to mercury (e.g., thorough atmospheric transport-deposition, river discharge and seafood contamination).
Despite efforts to improve the situation, the linkages between river basin and coastal environmental management plans are poorly developed in countries of the tropical and subtropical belts (Barletta et al., 2010). The consequences of poorly integrated basin and estuarine management regarding water quality, and mercury pollution in particular, are transferred to (and amplified) in coastal waters (Costa et al., 2009, Evers et al., 2008a).
Each country has mercury emission characteristics related to their own socio-economic status, but some common features exist. The three major sources of mercury are related to energy generation (industrial and domestic) through coal burning, the sum of small- and large-scale gold mining, production of non-ferrous metals and cement production. Deforestation may be another source of mercury to aquatic and coastal environments (Almeida et al., 2005). In some countries, such as China, mercury mining can also be a contributor to coastal contamination (Li et al., 2009)
Mercury and gold mining remain as major sources of environmental mercury contamination in tropical regions, putting nearly 9 million people at risk at approximately 280 of the 2100 identified contaminated sites all around the world (www.unep.org). Chlor-alkali plants, while historically responsible for significant local and regional mercury contamination, are no longer a major mercury source for the environment since mercury electrodes are being replaced by the membrane-based production process and, where it has not yet been done, effluent treatment is usually in place.
The accumulation of mercury from non-point sources in remote ecosystems, where aquatic resources are extensively used by local populations, is a serious conservation and human health concern in coastal areas (Canuel et al., 2009), as for example in relatively isolated Pacific Islands (Denton et al., 2009, Chouvelon et al., 2009).
Tropical coasts harbour a variety of biomes, ecosystems, and habitats that are home to sand beaches, wetlands and flooded mangrove forests surrounding estuaries which support a rich biodiversity. Coastal reefs and seagrass meadows complete the ecoclines, leading to platform environments. Some of these continuums might disappear, or decline in areal extent. Habitat loss and changes in biological community structure may alter mercury cycling in these regions before we have a chance to fully understand the important mercury cycling processes. The consequences for the remaining elements of the coastal mercury cycle are unpredictable. Simplification of coastal tropical food webs, and consequent loss of biodiversity, may result in new patterns for mercury cycling and contamination in tropical coastal regions. Therefore, we are likely to face new and challenging human health and conservation issues in the near future.
In the absence of significant point sources for mercury contamination, aquatic biota in tropical regions are exposed to mercury contamination derived ultimately from highly seasonal atmospheric deposition (Costa et al., 2009, Barletta et al., 2012). The tropical climate is characterized not only by high air (>18 °C) and water (>25 °C) temperatures, but also by intense, well-defined rainy seasons (type “A” climates, from the Köppen–Geiger classification). Triggered by enhanced nutrient supply to estuaries and coastal waters, eutrophication and concomitant biological dilution tends to reduce mercury bioavailability to consumers, namely vertebrates who prey on estuarine and coastal organisms. On the other hand, mercury bioavailability can increase during the dry season, such that animals towards the top of the food web tend to show higher mercury concentrations (Costa et al., 2009, Barletta et al., 2012).
Mason et al., 1994, Mason et al., 2012 have shown that, in the last five centuries, anthropogenic impacts on mercury biogeochemical cycles have significantly changed both the nature of the processes and the fluxes of mercury on a global scale. Those impacts on tropical coastal ecosystems and habitats are no exception to that pattern. This paper discusses the possible nature and patterns of such changes in the tropics, focusing on coastal food webs from their base (bacteria and plankton), through filter feeders and other consumers, to apex predators (large fish, reptiles, birds and mammals) and finally to humans.
Section snippets
Mercury atmospheric sources to tropical and sub-tropical coasts
In tropical coastal regions, rivers and direct discharges from industrial facilities and sewage can be the dominant sources of mercury to seawater, biota and sediments. Although this is quite often the situation, direct atmospheric deposition can also be very important. Atmospheric sources of mercury might be of a more global nature (coal burning, volcanoes etc.), but others can be quite unexpectedly local, as for instance medical incinerators (Denton et al., 2011) and mishandling of e-wastes (
Mercury inputs to tropical coastal food webs
In many cases, mercury reaches coastal habitats through river discharge into estuaries. From there, a number of abiotic processes influence mercury cycling in tropical coastal waters (Costa and Liss, 1999, Costa and Liss, 2000, Paraquetti et al., 2007, Muresan et al., 2008a). Mercury methylation and uptake by the biota is also a concern since the average mercury levels in biota increased (Evers et al., 2008a), following expected biogeochemical trends (Mason et al., 1994, 2012).
Both primary
The consumers at risk
As the body size of aquatic organisms increases, the direct uptake of dissolved MeHg becomes less important, and trophic transfer becomes the dominant route for uptake and accumulation of MeHg (Mason et al., 2000). In all ecosystems, including tropical and subtropical regions, the biomagnification of methylmercury in the food web of consumers strongly dictates their exposure level. In tropical vertebrates lower mercury concentrations are found in primary consumers such as the manatee (Trichechus
Mercury and human health issues in tropical and sub-tropical coastal regions
On a global scale, the main pathway of Hg exposure to humans is the ingestion of contaminated seafood (2.4 μg MeHg day−1 person−1, WHO), and this is most likely the major pathway of mercury exposure for coastal populations in the tropics and sub-tropics. Local and regional impacts may differ considerably from global averages due to variations in the speciation of mercury emissions, pollutant dispersion, and dietary habits (Fig. 1). Understanding these differences will require more detailed studies
Conclusions and recommendations
Tropical countries facing mercury pollution problems need to be involved in research and monitoring of coastal environments from sources to sinks, and must recognize the risks it poses to human health and ecosystem preservation. Because of an historical focus on mercury pollution in freshwater ecosystems, many marine habitats in developing countries within the tropics remain under-sampled with respect to mercury contamination. More process-oriented research is needed to answer questions such
Acknowledgments
MFC, MB, OM, JCM, SH are CNPq Fellows. This publication was made possible by NIH Grant Number P42 ES007373 from the National Institute of Environmental Health Sciences. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of NIH. Authors listed according to their contribution to the work. Dr. Celia Y. Chen is thanked for her critical reading during different phases along the preparation of the manuscript and careful work as guest editor
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