School of Biological Sciences
Applied Marine Biology - Aquaculture, Pollution and Conservation
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School of Biological Sciences |
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Applied Marine Biology - Aquaculture, Pollution and Conservation[ BIOL415 Home ] |
Lecture 1 by Dr Rick Leah
(A copy of the Overheads (as handed out in the Lecture is available here as a Word File
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(Suggested Books for this course
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"1. Basis for action From the UNEP document:
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The metals of Particular interest in the context of this course (because of their toxicity) are :
Hg Mercury
Cd Cadmium
Pb Lead
Cu Copper
Sn Tin
As Arsenic
Metals occur in varying amounts in sea water and since the sea is the final repository of soluble elements from the terrestrial part of the planet all are present. Some (so-called Heavy Metals), such as iron, copper, cobalt and zinc are essential in small quantities for the healthy growth of marine organisms (although they may be toxic when present to excess). Others, such as mercury, lead and cadmium have no known biological role. All of these metals are toxic if present in excess but the most important marine contaminants are generally considered to be those non-essential heavy metals.
(Our own website on Mercury is now available)
Of the heavy metals, most attention is currently paid to Mercury because it still poses the greatest threat to human health from pollution of the marine environment. There are continuing efforts to tighten environmental restrictions on this element as new research reveals that it may be causing neurological damage to unborn and young children.
In marine sediments bacteria may convert the less toxic, inorganic form of the metal to the more toxic, organic form of methyl mercury. This chemical form is relatively mobile in the environment and tends to accumulate in fish. Consumption of marine (or other) food containing methyl mercury over an extended period may lead to the onset of Minimata disease. This disease, first observed in Minimata Bay in Japan where the local fish-eating community received large doses of mercury derived from a local chemical factory manufacturing acetaldehyde and vinyl chloride, has neurological symptoms which include, mental disturbance, ataxia and impairment of gait, speech and hearing. In the original incident a considerable number of fatalities (43) were caused with many more people (>700) being irreversibly affected. Mercury is an important industrial material used, for example, in the chlor-alkali industry and in the manufacture of small batteries.
Mercury is a natural contaminant of the marine system and since methyl mercury moves up the food chain easily (in some circumstances actually biomagnifying) some large predatory fish such as tuna and swordfish have naturally high levels which may approach or even exceed acceptable limits for human consumption in large, older specimens. There are normally no apparent adverse health effects in the fish themselves, the risk is to the next level of consumer in this case humans. (However, wildlife such as birds can be adversely affected directly )
Mercury in tuna has recently been back in the news because of the risk to pregnant women and the unborn foetus (Guardian Article on the risk to pregnant women) This recent, elevated interest in mercury was partly generated by the very large number of US States that have had to issue consumption advisories on wild fish populations (see this map which shows the scale of the problem).
Lead has been widely used for millennia and lead poisoning from non-marine sources is a well known phenomenon. Although use of lead, particularly as an additive to automobile fuel , has led to measurably higher concentrations in remote ocean waters there is no suggestion that harmful effects are occurring from this dispersal. There have been one or two incidents where ships carrying alkyl lead compounds (the chemical form used as fuel additive) have lost some of their cargoes but these have not resulted in any obvious harm and the only known instance of marine lead pollution occurred in 1979/80 in the River Mersey estuary where alkyl lead caused many deaths amongst local seabird populations. This problem was caused by lead tetraethyl being discharged at low concentrations from the manufacturing plant via the Manchester Ship Canal into the estuary at Ellesmere Port. This organo-lead compound was taken up by invertebrates (bioaccumulation) and passed up the food chain to wading birds causing many deaths at various stages of the tide. (This was probably a case of biomagnification although the data to suppot this are sparse)
For humans the amount of dietary lead received from most marine food is trivial compared with other sources such as drinking water.
Cadmium is very toxic and mobile in ecosystems. Cadmium, like mercury, is an industrially important metal which is used directly in electroplating and is also found in processes associated with zinc and phosphorus. Although cadmium has no known biological role it is taken up by marine phytoplankton; apparently by the same mechanism employed for the uptake of the essential nutrient element phosphorus. It has been demonstrated to stimulate phytoplankton growth and photosynthesis up to surprisingly high concentrations. Even in quite heavily contaminated marine systems cadmium is very unlikely to cause acute toxic effects to marine animals but there have been at least two instances where cadmium pollution has apparently been responsible for adverse effects to humans. The best known example of this was the outbreak of itai-itai disease in Japan which affected a village on the Jintsu river in Japan. The use of irrigation water, contaminated with cadmium from a zinc processing plant, in paddy fields was originally identified as the cause of the problem but other factors such as malnutrition and vitamin deficiency have also been cited as major contributory parameters. Itai-itai disease is characterised by brittle bones and considerable pain (the name means 'ouch-ouch') and the symptoms can be alleviated by the administration of large doses of vitamin D. A marine example of cadmium poisoning was caused by the consumption of contaminated Tasmanian oysters which led to nausea and vomiting in the victims.
Recently, Cadmium has become of concern in the Baltic Sea where changing oxygen conditions in deep water caused by eutrophication may be mobilising the metal. Levels in fish liver in the Baltic are increasing as a consequence.
Cadmium is one metal which has never been a significant pollutant of the Mersey Estuary but it was a problem from the fertiliser factory run by Allbright and Wilson at Whitehaven. There, as a consequence of processing phosphate rock, the highest concentrations of cadmium in blue mussels around the UK coastline were measured by Greenpeace. This eventually resulted in the first successful private prosecution for marine pollution by Greenpeace and the regulation of the discharges by Environment Agency. As a consequence, the plant was closed and most phosphate rock now gets processed in the developing world where environmental standards are not necessarily as tight.
Tin (in the form of the organo-tin compound tri-butyl tin known as TBT) has recently been recognised to be quite a serious marine pollutant. The use of organo-tin compounds in anti-fouling paints on boats has resulted in comparatively high concentrations of these chemicals in some enclosed waters (harbours, marinas etc.). Although very effective as anti-fouling agents, organo-tins also affect non-target organisms causing shell deformations in oysters and imposex in the common dogwhelk (Nucella lapillus). The use of organo-tins in cages used for farming fish has also led to concern. Many countries have now banned or severely restricted the use of organo-tin anti-foulants though the chemicals are still used in some other applications such as wood preservation.