Optimizing Stream Water Mercury Sampling for Calculation of Fish Bioaccumulation Factors

Mercury (Hg) bioaccumulation factors (BAFs) for game fishes are widely employed for monitoring, assessment, and regulatory purposes. Mercury BAFs are calculated as the fish Hg concentration (Hg_fish) divided by the water Hg concentration (Hg_water) and, consequently, are sensitive to sampling and analysis artifacts for fish and water. We evaluated the influence of water sample timing, filtration, and mercury species on the modeled relation between game fish and water mercury concentrations across 11 streams and rivers in five states in order to identify optimum Hg_water sampling approaches. Each model included fish trophic position, to account for a wide range of species collected among sites, and flow-weighted Hg_water estimates. Models were evaluated for parsimony, using Akaike’s Information Criterion. Better models included filtered water methylmercury (FMeHg) or unfiltered water methylmercury (UMeHg), whereas filtered total mercury did not meet parsimony requirements. Models including mean annual FMeHg were superior to those with mean FMeHg calculated over shorter time periods throughout the year. FMeHg models including metrics of high concentrations (80th percentile and above) observed during the year performed better, in general. These higher concentrations occurred most often during the growing season at all sites. Streamflow was significantly related to the probability of achieving higher concentrations during the growing season at six sites, but the direction of influence varied among sites. These findings indicate that streamwater Hg collection can be optimized by evaluating site-specific FMeHg – UMeHg relations, intra-annual temporal variation in their concentrations, and streamflow-Hg dynamics

Box and whisker plots of nitrate uptake velocity (ʋ_f) in the buried and open reaches in Cincinnati, Ohio and Baltimore, Maryland, as reported in Beaulieu et al. [20] and Pennino et al. [21].

<p>Literature data were derived from a recent survey of 72 streams spanning several biomes and land-use conditions [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0132256#pone.0132256.ref016" target="_blank">16</a>]. Plots display 10^th, 25^th, 50^th, 75^th, and 90^th percentiles and individual data points outside the 10^th and 90^th percentiles. Nitrate uptake velocity was 13 times greater in open than buried reaches (p<0.001, paired <i>t</i>-test).</p

Results of simulation scenarios involving an even distribution of burial across the watershed with incremental increases of 5%.

<p>The primary y-axis and solid line represent the average volumetric NO₃^- uptake rate among in the open reaches. The secondary y-axis and dashed line represent total NO₃^- uptake in the open reaches.</p

Percent change in nitrate export in response to stream burial simulation scenarios.

<p>The simulation scenarios involve an even distribution of burial across the watershed with incremental increases of 5% and include: 1) Allowing both uptake rate constants and water velocities to change in response to burial (Combined response); 2) Allowing water velocity to change following burial, but holding uptake rate constants at open reach values; and 3) Allowing uptake rate constants to change following burial, but holding water velocities at open reach values.</p

Stream burial is an extreme, but ubiquitous, consequence of urbanization in stream ecosystems.

<p>The buried stream channels in the cited studies were constructed from various materials including (a) a cement-lined corrugated metal pipe in Baltimore, Maryland (USA), (b) a concrete tunnel in Cincinnati, Ohio (USA), and (c) a corrugated metal pipe in Cincinnati.</p