Mercury and Ecosystems 1: Bio-magnification

My previous post introduced a new pollutant to the blog. Just to recap, mercury is a toxic element accumulating in the Arctic at a rate of 80 to 140 tonnes, per year. I wish to move on today by looking at some of the major impacts of mercury on Arctic Ecosystems. Unlike the other LRP I have explored mercury does not influence the climate in the Arctic. Mercury has the potential to bio-magnify in food chains, entering at the bottom and passing though the trophic stages to larger predators. Traces of the substance has even been found in human blood! Due to the amount of literature on this topic I have decided to split it into two post; the first discussing bio-magnification and the second bioaccumulation.

Entering the food chain

To enter a food chain mercury has to be in a state which can be readily consumed by Arctic organisms. Most organisms (not including those at the bottom of the food chain eg. Algae) cannot take up inorganic mercury. Upon conversion to methylmercury (by bacteria in oxygen depleted environments) the toxin is easily taken up by animals. Production is dominant around ocean waters which contain layers of concentrated nutrients, but little is known about the rate of conversion by this bacteria.

Moving up the food chain

Un-bear-able Cute!
But these guys often have less mercury in their system
 that predators further down the food chain!

Once entering the food web methylmercury progresses from prey to predator up the web. A “bottom up” process is most common within the Arctic, as focuses on “factors that influence the extent to which methylmercury concentrates at successive tropic levels of the food chain”. This “bottom up” process is also prone to bio-magnification at each tropic step of the food chain. For example, the bio-magnification of an ecosystem in the Gulf of the Farallones was measure to have a trophic magnification factor of 0.32 (this is slightly lower than an average food chain as the Farallones food web is dominated by sea birds, who accumulate mercury differently to other organisms. Furthermore the study was based on the sampling of many tissues including their eggs, so comparison to studies based on muscle tissues may not be representative). As mercury moves through the chain the level of mercury can amplify by approximately 2-7 times, so in the case of a very long food chain, predators at the top can contain methylmercury levels up to a million times greater than those which initially digested the toxic. An expectation of this was found by Atwell et al (2011), who state polar bears demonstrated a lower mean mercury concentration than their prey (ringed seals in this case).

How mercury can enter and leave the system of a polar bear and seal

As mercury is bio-magnifies through a food web it lead to bioaccumulation within predators at the top of the food chain, which can result a series of unwelcome consequences. Stay tuned! My post of bioaccumulation will be coming very soon!