What Drives Mercury in Lake Trout From Katmai and Lake Clark? Early Answers

Fish in some of the most remote and supposedly pristine waters in North America carry elevated mercury. Scientists from the National Park Service and the U.S. Geological Survey are asking why, in lakes in two southwest Alaska parks, and their preliminary answer points to the fish themselves: mercury tends to be higher in older, skinnier lake trout that feed offshore.

Why these fish are watched

The Southwest Alaska Network (SWAN) is part of the Park Service’s Inventory & Monitoring Division, created in 1998 to measure the status and trends of selected indicators called “vital signs.” One of them is resident lake fish, meaning non-migratory freshwater species. Because they are continuously exposed to lake water, which integrates conditions from the surrounding land and air, they are key indicators of ecosystem health. They also matter for subsistence and recreational fishing in Alaska, so the authors call it crucial to understand their contamination levels and sources.

From 2005 to 2015, Park Service researchers sampled more than 300 resident lake fish for contaminants: 9 species from 13 lakes in 2 southwest Alaska parks. Some samples had elevated mercury, most of it methylmercury, a potent neurotoxin and endocrine disrupter. Compared with resident lake fish sampled in 21 parks from 10 monitoring networks in the western United States, mercury in southwest Alaska’s fish was among the highest, though lakes varied considerably.

Lake trout collected for mercury analysis from Katmai National Park and Preserve. NPS / Evan Booher.

How mercury reaches a fish

Mercury is a toxic element with no known essential biological function. It occurs naturally as a solid in minerals and as a gas from volcanic eruptions and from human activities, particularly mining and coal burning. As a gas it can travel through the atmosphere far from its sources, then fall to the ground in rain or snow. In the oxygen-poor conditions typical of wetland sediments, bacteria can convert deposited mercury into methylmercury, which builds up in aquatic organisms through both water and food. In fish it readily crosses biological membranes, is excreted slowly compared with how fast it is taken up, and reaches concentrations vastly above those in surface waters.

The factors behind mercury in fish fall into four groups: loading (how much mercury enters an ecosystem, from distant or local sources), methylation (conversion to methylmercury, set by local ecological conditions), bioaccumulation (the rise of methylmercury in an organism over time, driven by local fish biology) and biomagnification (the rise at higher levels of the food web, driven by differences between food webs). The team hypothesized these candidates for southwest Alaska’s lakes:

The study

To sort these factors out, SWAN partnered with the U.S. Geological Survey’s Mercury Research Laboratory in Wisconsin. The study focused on lake trout, a long-lived fish-eating species, in Katmai and Lake Clark National Parks and Preserves. Its first two years involved sampling water, sediment, plankton and both lake trout and sockeye salmon from 13 lakes with a range of glacier, wetland and salmon influences. Samples were analyzed for mercury, methylmercury and other things, including stable isotopes of mercury, carbon and nitrogen. Two questions drove the work: what causes the relatively high mercury in resident lake fish from southwest Alaska, and what controls the differences among lakes?

The 13 study lakes in Lake Clark and Katmai National Parks and Preserves. Map from the National Park Service article.

What the early results show

• Lake trout mercury varies widely, both among parks and among lakes.
• In 6 of the 13 lakes, the median total mercury in lake trout is above the State of Alaska’s guidance for unlimited fish consumption by at-risk groups, which the source’s chart caption names as women of childbearing age, nursing mothers and children under 12.
• Sockeye salmon, by comparison, have consistently low total mercury with little variability.

Mercury in lake trout and sockeye salmon, lake by lake, north to south. The gray line marks the upper limit for unlimited consumption. Chart from the National Park Service article.

Models that relate lake trout mercury to possible drivers use fish-level variables (recorded once per fish, such as age) and lake-level variables (recorded once per lake, such as wetland cover in the watershed). The fish-level variables that best explain mercury are age, body condition and diet specialization: mercury tends to be higher in older, skinnier lake trout that feed offshore. The lake-level variables, alone or combined, do not improve the models. No significant effect of sockeye salmon on lake trout mercury has shown up so far, so sockeye salmon are unlikely to be a major driver of differences among these lakes.

What comes next

Two manuscripts, one led by USGS on the mercury stable isotopes and one led by SWAN on the quantitative models, were to be submitted to peer-reviewed journals in 2020 or 2021. The team also planned to launch, in 2020, a related project covering five parks in Alaska. It would again use lake trout as the focal species but add associated prey species and primary producers, to assess how mercury builds up through the food web and whether specific energy pathways, such as offshore versus nearshore, promote greater buildup.

Sources

• Krista K. Bartz, Tammy L. Wilson and Daniel B. Young (National Park Service) and Ryan F. Lepak, Jacob M. Ogorek and David P. Krabbenhoft (U.S. Geological Survey), Drivers of Mercury in Top-predator Lake Fish from Southwest Alaska Parklands, Alaska Park Science 19(1), 2020 (Below the Surface: Fish and Our Changing Underwater World); the source says “seven key factors” but its table lists eight, so no count is given; rewritten in hubnx's own words.