I continue my 3-part series on water pollution in the Ocean environment. Last week I introduced the types of toxins found in the marine environment from marine pollution and mentioned a few of the physiological effects that they have on living organisms.
This week’s post will delve into the feeding and reproductive ecology of bioaccumulation, in addition to discussing the interesting adaptations that marine mammals have evolved to cope with bioaccumulation effects in their environment.
In the North Pacific, you can find two different groups of orcas, “resident” type and “transient” type. The diet of the residential type consists of fish, typically salmon. The diet of the transient type, though, is mostly other marine mammals like dolphins and seals.
A study found that organochlorine levels, particularly DDT, was 25 times higher in transient types than in resident types. So even within the same species, we see differences in toxin accumulation levels because of a change in one ecological component: diet.
The reason for this is because fish are lower on the food web than marine mammals. In apex predators like marine mammals, or even predatory fish like tuna, we see higher levels of metal and organochlorine accumulation than in animals lower on the food web like krill or small fish such as sardines.
One of the biggest problems with bioaccumulation, especially with regards to organochlorines and mercury, is the effect that it has on the young.
Marine mammals all have a thick layer of fat, which we call blubber. It is used for a variety of purposes, but one of the main functions is energy storage. Because of the lipophilic nature of organochlorine molecules, all of these toxins ultimately get stored in the blubber.
During lactation, when the female dolphin is providing milk for her calf, this blubber is what is used to produce milk. However, not only does the fat from the blubber end up in the milk, so do the toxins.
A study was done in Sarasota, Florida, which showed that mortality (dolphins dying) of bottlenose dolphin first born calves was all the way up to 50%. However, subsequent calf mortalities dropped down to 30%. Essentially, all of the mother’s lifetime organochlorine burdens (most marine mammals do not reach sexual maturity until around 8-10 years of age) were offloaded into her tiny first born calf’s mere few weeks of life. As a result, many first borns died. But when the same mom gave birth to calves after that first calf, the calves were much healthier, since they didn’t have to carry the lifetime accumulation of toxic burdens of the female the way the first calf did.
The arctic demonstrates particularly high levels of marine pollutants–especially heavy metals like lead, cadmium, and mercury. Although there has been a recent influx of these metals due to man-made industry, levels in this part of the world has historically been high. Therefore, marine mammals that live in these parts have developed strategies to cope with this and survive.
A study was done analyzing bowhead whale internal organs, and it was found that even in spite of near-kidney failure, female bowhead whales had lived well into their old age and even successfully reproduced and reared young. This has been shown with several other types of mammals and also Arctic birds. With regards to the metabolization of mercury, it was found that marine mammals evolved a physiologically mechanism to enhance survival.
By eating foods high in selenium (eg certain kinds of fish), these animals provide their body with an element that binds to the mercury during the metabolization process, which then detoxifies the metal, so it can be safely be excreted.
Next week, in the final post covering bioaccumulation in the marine environment, we will talk more about the implications of marine pollution and what can be done to help ameliorate the problem.