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Mercury at elevated levels observed in only some elephant seals, but why?

Marine foraging ecology influences mercury bioaccumulation in deep-diving northern elephant seals, Sarah H. Peterson, Joshua T. Ackerman, Daniel P. Costa.  Proc. R. Soc. B 2015 282 20150710; DOI: 10.1098/rspb.2015.0710. Published 17 June 2015


Mercury in the atmosphere and ocean has been increasing since the late 19th century. Anthropogenic burning of fossil fuels has contributed a significant portion of the mercury measured in soil and sediment. Mercury enters the atmosphere as Hg0. It is then oxidized to HgII when it is deposited into the ocean, where it may be converted to methylmercury and incorporated into the food web.   In the ocean, mercury (total mercury and methylmercury) tends to have highest concentrations at mid depths (mesopelagic, 200-1000m) compared to at the surface (epipelagic, 0-200m), and in deep water (>1000m). Like for humans, mercury is a toxin to marine mammals, and even at low levels, exposure may have detrimental impacts on reproduction, development, behavior, and the nervous system. A group of researchers sought to identify how depth profile of mercury in the ocean is reflected in the mercury levels of large marine mammals.

Researchers focused on the northern elephant seal (Mirounga angustirostris) because it does almost all of its foraging in the mesopelagic and feeds primary on fish and squid. It also has similar behavior to more mystifying predators, such as sharks and tuna. Females specifically were observed because they take two annual trips during which they travel great distances. The longer trip is around 10,000 km and is taken while gestation (the growth of a baby in the womb) occurs. The second trip is only about 5000 km, when they return from this trip they molt. The benefits of the females travel plans is that it enables scientists to compare the mercury in the seals after each trip to see if there are any differences that may be related differences in the path, dives, and depths of the two trips.

Methods, no animals were harmed

Northern elephant seal observations and samples were acquired between 2011 and 2013 from the Ano Nuevo colony in the Ano Nuevo State Reserve, San Mateo County, CA, USA. Seventy-seven adult female northern elephant seals, ranging in age from 4-13 years, were tagged with satellite transmitters, time-depth recorders, and jaw accelerometers.   They were then tracked and samples of tissues and blood were collected, without causing harm to the animals, at specific times: nine and two days after the seals returned from the long and short trips, respectively.

The data were analyzed in numerous ways; follow the link to the paper for specifics.


presented as figure 1 in reviewed article
Figure 1: Short trip (top) vs. Long trip (bottom). The mercury concentrations in the blood is represented by warmer colors are higher levels and cool colors for lower levels. The circle chart illustrates the gestation and molting annual cycle.

Dive depths ranged between 440 and 965 meters, and jaw motion was observed 70-90% of the time during dives deeper than 450m. Mercury was found in all blood and muscle samples after both the short and long trips (figure 1). The seals could be clustered into three groups: 1) northern, near the subarctic gyre and continental shelf, 2) shallow and offshore, and 3) deep and offshore (figure 2).   The deeper offshore cluster is furthest south near the transition zone along the north pacific polar front; the mercury concentrations in seals returning from here were 67% and 40% greater than in seals returning from the north and shallow offshore regions, respectively.

presented as figure 3 in reviewed article
Figure 2: Three clusters of seal behavior and mercury. Northern and near shore, offshore and shallow, and offshore and deep. The deep offshore seals had the highest levels of mercury.


















presented as figure 2 in reviewed article
Figure 3: Blood mercury levels plotted with trip (top left), depth of night dive (top right), age (middle left), latitude (middle right), delta 13C (bottom left), and duration of time in the California Current (bottom right).

Mercury concentration variations measured in the blood and tissue samples are related to foraging trip, age, and foraging behavior (figure 3).   The elevated levels measured after the short trips are attributed to the life cycle of the seal. For instance, mercury can exit the system via hair growth, lactation, or gestation. After the long trip, samples were collected soon after the seals had gestated, potentially off loading some methylmercury.   After the short trip however, prior to molting, mercury levels could be at there highest because the seal has had the most time since her last reproductive cycle and molt. The relationship between elevated mercury levels and deep dives at night has multiple explanations. The favored reason suggests that at greater depths the prey eaten come from deeper places that have higher mercury concentrations. An alternative explanation is that the prey is from a higher trophic level, which would accumulate more mercury over time.  Mercury and age did not share a relationship, suggesting that mesopelagic is a substantial and consistent source of mercury. The negative correlation of blood mercury and latitude is expected based on water column mercury levels. In the north the mercury peak is between 200 and 400m, but in the south it is between 500-800 meters, the same depths that the seals forage. Seals that forage in the south are therefore exposed to a greater amount of mercury over time. The correlation between delta 13C and mercury suggests mercury distribution in the water column may be related to oceanic processes more so than tropic level.  Mercury levels also seem to be negative correlated with exposure to the California current, however scientists think that regardless of the exposure time to the current the food was still coming from depths with elevated mercury, or higher trophic levels.

Overall the researchers learned that the individual travel and diving behavior of each seal influenced their blood and tissue mercury levels. The study determined that 99% of the seventy-seven seals observed had mercury levels greater than the recommended human exposure level of .51 micrograms per gram.   Scientists are not sure what the implications of high mercury are for northern elephant seals specifically. As time passes mercury and its impacts will become better understood and hopefully handled before the impacts become obvious.

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