Mercury levels in Arctic seals linked to length of summer thaw.
Gaden, A, S H Ferguson, L Harwood, H Melling, and G A Stern. 2009. Mercury trends in ringed seals (Phoca hispida) from the western Canadian Arctic since 1973: associations with length of ice-free season. Environmental Science and Technology doi: 10.1021/es803293z .
Mercury levels in ringed seals living in the Arctic regions of the world fluctuate from year to year, report Canadian researchers who have tied the ups and downs to the number of months seas are ice-free during the summer. They believe the fish the animals eat in the winter contribute to the annual swings.
These kinds of yearly differences are not seen in other marine mammals, such as beluga whales. Instead, their mercury levels have steadily increased during past years.
The results are important because they show how environmental conditions directly impact the accumulation of mercury in the seals. These findings provide a clear example of broad-reaching implications if the ecological balance shifts.
And, as global climate change progresses, and the sea ice melts for longer periods each year, the seals' mercury levels could consistently increase over time. Mercury levels are predicted to increase in fish as coal power plants continue to spew the metal. Mercury is a neurotoxin that has known effects on reproduction, behavior and immune responses.
The Canadian researchers sampled muscle tissue from adult ringed seals harvested by native hunters in the month of June between the years 1973 and 2007. Mercury was then measured in the muscle samples.
They found that levels directly change with the length of time that the waters in the seals’ home range are free of ice in the summer months prior to sampling. Ice melts occur from as early as May through as late as November.
Surprisingly, higher mercury levels were seen after years of both long and short ice-free periods. Yet when the seas thawed for a moderate period of 100 to 140 days, the next year’s mercury levels were lower.
Mercury is a naturally-occuring metal. It enters the environment from coal-fired electic plants, manufacturing and mining. The metal can be transferred around the world by atmospheric air currents where it can contaminate soil, water and widlife.
The metal is found in fish. The long lived, predatory species, such as tuna, shark and tilefish contain the highest amounts. People–especially pregnant women and children–are cautioned to limit the amount of fish eaten to avoid mercury contamination.
Fish, too, are the main source of mercury for the seals. The researchers suspect that the length of the ice-free season affects the populations of Arctic cod, which are seals' main food source in the winter.
The researchers believe that the warmer summers melt more ice and allow more nutrients to grow in the water. The cod population increases. Seals then eat more cod in the winter, resulting in higher mercury levels by the next year.
They hypothesize that the cooler summers may result in smaller cod populations of mostly older cod. The aged fish may have more mercury than younger cod. When the seals eat the older cod during the winter, they take in higher amounts of mercury.
The cod population is balanced only when the summers are neither too hot nor too cold. During those years, the seals are likely to be exposed to less mercury because they will eat less fish in general and fewer older cod.
