Spatiotemporal patterns of mercury accumulation in lake sediments of western North America.

For the Western North America Mercury Synthesis, we compiled mercury records from 165 dated sediment cores from 138 natural lakes across western North America. Lake sediments are accepted as faithful recorders of historical mercury accumulation rates, and regional and sub-regional temporal and spatial trends were analyzed with descriptive and inferential statistics. Mercury accumulation rates in sediments have increased, on average, four times (4×) from 1850 to 2000 and continue to increase by approximately 0.2µg/m(2) per year. Lakes with the greatest increases were influenced by the Flin Flon smelter, followed by lakes directly affected by mining and wastewater discharges. Of lakes not directly affected by point sources, there is a clear separation in mercury accumulation rates between lakes with no/little watershed development and lakes with extensive watershed development for agricultural and/or residential purposes. Lakes in the latter group exhibited a sharp increase in mercury accumulation rates with human settlement, stabilizing after 1950 at five times (5×) 1850 rates. Mercury accumulation rates in lakes with no/little watershed development were controlled primarily by relative watershed size prior to 1850, and since have exhibited modest increases (in absolute terms and compared to that described above) associated with (regional and global) industrialization. A sub-regional analysis highlighted that in the ecoregion Northwestern Forest Mountains, <1% of mercury deposited to watersheds is delivered to lakes. Research is warranted to understand whether mountainous watersheds act as permanent sinks for mercury or if export of "legacy" mercury (deposited in years past) will delay recovery when/if emissions reductions are achieved.

Spatial trends and factors affecting mercury bioaccumulation in freshwater fishes of Washington State, USA.

Twenty-four lakes in Washington State, United States, were sampled for largemouth and smallmouth bass as well as water chemistry parameters during 2005 to 2009 to evaluate trends in mercury (Hg) concentrations. We analyzed spatial patterns in bass Hg levels across a gradient of land and climate types, lake chemistry parameters, and physical watershed characteristics to identify factors influencing Hg bioaccumulation. Across the state, bass Hg levels followed rainfall patterns and were statistically greater on the coastal west side of the state and lowest in the drier eastern region. Lake and watershed variables with the strongest correlations to Hg bioaccumulation in bass were annual watershed precipitation (+) and lake alkalinity (-). Principal component analysis (PCA) explaining 50.3 % of the variance in the dataset indicated that wet, forested landscapes were more likely to contain lakes with greater fish Hg levels than alkaline lakes in drier agriculture-dominated or open space areas. The PCA did not show wetland abundance and lake DOC levels as variables influencing bass Hg levels, but these were generally associated with small, shallow lakes containing greater chlorophyll levels. The effect of in-lake productivity may have counteracted the role of wetlands in Hg bioaccumulation among this study's lakes.