Moving on from bio-magnification as this post is going to
focus on bioaccumulation in predators at the top of the food chain and the
consequences of having a build-up within an animal or human system. So I’m just
going to jump straight in!
Top of the food chain
For those at the top of the food chain this large
accumulation of mercury, within their system, can be detrimental to their
health. There are two ways which the toxicity of mercury can be assessed in predators.
Firstly the comparison of the total amount of mercury in an animals system to calculated
thresholds. These thresholds are based on a database of known concentrations, calculated
from lab observations. The second approach includes examining animals for mercury biomarkers. Biomarkers
are certain behavioural responses by animals. As Arctic animals are exposed to
many different types of pollutants these markers allow effects of mercury to be
specifically identified. Mercury is often stored within the liver of marine mammals
and the kidney of terrestrial mammals. One of the reasons mercury is so toxic results
from it’s potential to cross the blood brain barrier. This causes a disruption of the nervous
system and symptoms such as numbness and decreased co-ordination in large terrestrial
mammals (ie. polar bears). Furthermore this toxin can be passed from mother to
baby though the placenta or milk, both considerable causes of mercury exposure
in new born animals.
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| Mercury concentrations in Human and polar bears. |
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| Concentration of mercury in pregnant women in the Arctic |
What I found shocking, from this graph (above), was the
high levels of mercury found in humans! However upon further consideration this
isn’t so shocking. Humans are at the top of many food chains within the Arctic,
with seals and fish provide an efficient protein source in this region. Arctic
indigenous communities are particularly prone to mercury exposure due to their traditional cultural
practices of hunting and fishing. Negative impacts of the bioaccumulation,
of large concentrations of mercury within the human system, can affect the reproductive
and cardiovascular
systems, and cause neurotoxicity. These impacts are leading to Arctic residence
having to make a choice between the readily available food source and potential
health risks associated. Like in many animals, mercury can pass through the placenta
to an unborn
human child, because of this many
studies focus on monitoring mercury levels in the blood of pregnant women. Recent
awareness of the impacts of mercury have seen a decrease in mercury concentrations
in blood since the 1990s but the graph still shows high concentrations in pregnant Inuit
women in Greenland (above).
Within academia there seems to be a lot of ambiguity of
results of studies, some establishing bioaccumulation
in food chains were others stating they do
not exist. Inconsistency of findings may be due to the difficulty of
measuring trophic structures and the movement of the element. Publishing bias
must be considered, as journals are unlikely to accept studies which do not
show any notable relationship between mercury. Simply from my own reading I have
found a great consistency between results of studies with most food chains
displaying some form of bio magnification and bioaccumulation. I found this a
very interesting but alarming topic! So if you have any thoughts on this topic
please let me know by commenting.
I wonder if we should be worried at all about mercury accumulation in Non-Arctic regions? Sea birds that migrate to and from the Arctic will inevitably impact and contribute to other food chains...! I'm a little bit scared now.
ReplyDeleteThis is also a big problem in parts of Asia as well particularly in fish. The problem is more pronounced in Asia, and the Arctic, as fish is a major source of dietary protein there. I assume anywhere which experiences extensive coal burning will also have high levels of mercury! Very scary!
DeleteThere's interesting evidence about high levels of mercury in commercially fished tuna also Louis, an interesting point to note is that there appears to be a level of 'mercury saturation' reachable (when studied on humans) suggesting that after a certain level no matter how much more tuna (for example) is consumed with its high mercury content the level of mercury absorbed does not increase - I wonder whether this phenomenon may occur with polar bears and other top arctic predators to some extent also... Let's hope it does for their sake!
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