Biological+Magnification

"Because it has Mercury in it!" you may be thinking (and rightly so).

"From the ocean!" Yes, Mercury is found in the ocean.


 * But this answer is not sufficient because the amount of Mercury in the ocean is significantly less than that found in swordfish . So if the mercury in the ocean alone is not enough to account for the mercury in swordfish, and mercury can only come from pollution in the ocean, than how can a swordfish have so much mercury?**

To fully answer these questions we need to figure out.....
1. How Mercury travels from its source (pollution) to swordfish 2. Why the amount of Mercury in swordfish is greater than the amounts found in the ocean and other marine life



Both of these questions are answered by the concept of Biological Magnification.
Mercury enters the ecosystem of a swordfish by being evaporated into our atmosphere from factories. This Mercury is then carried down by rain and other precipitation into the ocean. Once it enters the ocean, it is then consumed by the smallest organisms in the ocean, producers. Producers such as algae are then consumed by zooplankton which are then consumed by small fish and so on up the food chain to large fish such as swordfish.

Great. So our first question of how mercury travels from its source to swordfish is answered: **Mercury moves from the ocean up the food chain to swordfish via consumption.**

But why is the concentration of Mercury higher in swordfish then it is in the ocean?


 * At every step on the food chain the amount of mercury in each level of organisms increases by a factor of ten.** Looking at the diagram above you will see that producers have a concentration of 1,000, zooplankton has levels of 10,000, small fish 100,000, and our friend the swordfish 1,000,000. This pattern of increasing concentration is called Biological Magnification.

Imagine that every algae in the ocean has 1 unit of mercury in it that it receives from absorbing water in the ocean. Now a zooplankton comes along at eats 5 algae. When digesting these 5 algae the 5 units of mercury are now embedded in the zooplankton. So now we have an ocean of algae with 1 unit mercury and zooplankton with 5 units. A small fish now comes by and eats 10 zooplankton. Since each zooplankton has 5 units of mercury (5 x 10), the small fish now has 50 units of mercury. This process continues onward until we reach our swordfish whom, after eating a few small fish, now has a higher level of mercury then that in the ocean. The lesson here? **Swordfish have higher levels of mercury than the ocean because the food they consume has amplified the amount of mercury it obtained from its food.**
 * Why does this happen? **