Mercury oceanic concentrations

Estimates have been prepared for mercury and other metals for atmospheric inputs to oceanic systems. Table 1 gives these estimates for oceanic waters based on ratios of current urban air values to average crustal material values. Both lead and cadmium have higher enhancement ratios (2,300 and 1,900) than mercury (1,100). When percent increase estimates are made for ocean concentrations, the concentration range for cadmium is 0.02 to 8 jug/1, and for mercury 0.1 to 0.8 /Ltg/1. No probable percent increase is given for lead. 

The transport rate of mercury flowing from the land to the oceans in rivers has been increased by a factor of about three by human activity. While the increased rate is still relatively less important than the total transport of Hg through the atmosphere, it can represent a significant stress on the exposed organisms, particularly since the increased flux is unevenly distributed. That is, human activity has created local environments where the transport of mercury or its concentration in a river or estuary is many tens of times higher than background levels. 

In a report from the U.S. EPA (1980), fish contained between 10,000 and 100,000 times the concentration of methyl mercury present in ambient water. In a study of methyl mercury in fish from different oceans, higher levels were reported in predators than in nonpredators (see Table 8.2). Taken overall, these data suggest that predators have some four- to eightfold higher levels of methyl mercury than do nonpredators, and it appears that there is marked bioaccumulation with transfer from prey to predator. 

Thompson DR, Furness RW, Walsh PM. 1992. Historical changes in mercury concentrations in the marine ecosystem of the north and northeast Atlantic Ocean as indicated by seabird feathers. J Appl Ecol 29 79-84. 

To help prevent this, the U. S. Public Health Service has recommended a maximum limit of 0.5 ppm mercury in any food. If the fish are to have less than this level of methyl mercury and the concentration factor is 3,000, then the surrounding water in which the fish live should have less than 0.16 ppb (parts per billion). Currently the oceans have about 0.1 ppb, but it is not known whether this is in the form of organic or inorganic compounds.8 It is also not known whether fish can convert inorganic mercury into methyl mercury.8 However, a large number of microorganisms can do this, so possibly its usual form is unimportant. 

Reported mercury values in the oceans determined since 1971 span three orders of magnitude, due at least in part to errors induced by incorrect sampling [62-64]. Olasfsson [65] has attempted to establish reliable data on mercury concentrations obtained in cruises in North Atlantic water. 

In many applications, such as the analysis of mercury in open ocean seawater, where the mercury concentrations can be as small as 10 ng/1 [468,472-476], a preconcentration stage is generally necessary. A preliminary concentration step may separate mercury from interfering substances, and the lowered detection limits attained are most desirable when sample quantity is limited. Concentration of mercury prior to measurement has been commonly achieved either by amalgamation on a noble-metal metal [460,467, 469,472], or by dithizone extraction [462,472,475] or extraction with sodium diethyldithiocarbamate [475]. Preconcentration and separation of mercury has also been accomplished using a cold trap at the temperature of liquid nitrogen. 

Fitzgerald [53] used a cold trap to concentrate mercury from large volumes of seawater. Using this technique, he could achieve a detection limit of 0.2 ng Hg, and a coefficient of variation of 15% at the 25 ng 1 1 level. Most oceanic samples contained less than 10 ngl-1, but coastal samples could approach 50 ngl-1. 

Nishimura, M., S. Konishi, K. Matsunaga, K. Hata, and T. Kosuga. 1983. Mercury concentration in the ocean. Jour. Oceanogr. Soc. Japan 39 2951-300. 

On a worldwide basis, toxic concentrations of the heavy metals have thus far been limited to industrialized harbors. The only metals that appear to have accumulated to toxic levels on a regional scale are mercury, cadmium, and lead in the Arctic Ocean. This concentration of mercury and lead has been fecilitated by a natural process, called the grasshopper effect, which acts to transport volatile compoimds poleward. This transport plays a major role in redistributing the volatile organic pollutants, such as the PCBs, and, hence, is discussed at further length in Chapter 26.7. The process responsible for the cadmium enrichment in the Arctic appears to involve low-altitude transport of the fine particles that compose Arctic haze. 

Although the rate of mercury input to the ocean s surface waters has increased as a result of anthropogenic activities, it is not clear that the relatively high MMHg concentrations now seen in the larger fish, e.g., tuna, shark, and swordfish, are the result of pollution. Measurements of mercury levels in preserved fish collected over the past 100 years have proven inconclusive due to small sample sizes and contamination effects. 

Slemr F, Danger E. 1992. Increase in global atmospheric concentration of mercury inferred from measurements over the Atlantic Ocean. Nature 355 434-437. 

Despite its availability and current use, coal is not as widely used today as the other fossil fuels. Coal s major weakness is that it does not burn cleanly. It often contains trace amounts of other elements, including mercury, arsenic, and sulfur, and when it burns, it releases these toxic substances into the air. Over time, coal pollution builds up in the environment. Mercury released during coal combustion, for example, settles in water and builds up in the bodies of fish and shellfish. When these fish and shellfish are eaten by humans and other animals, harmful amounts of mercury can be ingested. In 2008, bluefm tuna served in expensive New York restaurants was found to contain unacceptably high levels of mercury. These fish eat smaller organisms in the ocean, and when these small organisms contain mercury, the toxic element becomes concentrated in the body of the tuna. 

The human-related sources of mercury to the environment are numerous and widespread. Most direct inputs of mercury from point sources to aquatic systems have largely been contained in most developed countries. Inputs of mercury to the environment via the atmosphere are of the greatest concern. These emissions, coupled with long-distance transport of elemental mercury, have resulted in elevated concentrations of mercury in fish from locations that are removed from anthropogenic sources (e.g., open-ocean, and semi-remote regions in the United States, Canada, Scandinavia Wiener et al., 2002). A summary of the fluxes from major sources (for 1995) is shown... 

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