Mercury Mass Balance and Uptake by Fish in Acid Lake Ecosystems

Abstract

<p>The phenomenon of increasing the mercury body burdens of fish, on a lake to lake basis with decreasing pH₁ is found to exist. Although the interactions between fish, the ecosystem, water chemistry and other variables are extremely complex a simple conceptual solution is established to explain why mercury uptake by fish increases with decreasing pH.</p> <p>A study on mercury in an acid lake watershed ecosystem (near Espanola, Ontario) was done as a precursor exercise. Mercury concentrations of various ecosystem compartments are within the ranges of values found by other workers for other systems. All values found are considered background or below background. For this type of lake-watershed system, the main mercury repository is the watershed soils and overburden materials. The main mercury fluxes are from the atmosphere to the soils and from the soils, via streams and groundwater, to the lake. The net result is that the watershed is being depleted in mercury at the rate of about 30 μg m⁻² yr⁻¹. This mercury is apparently ending up in the lake water, seston or sediments.</p> <p>According to well established thermodynamic equilibria, pH, together with pCI, govern the speciation of inorganic mercury in freshwater inorganic systems. Under certain specific conditions of pH and pCI, the non-toxic mercury species Hg(OH)₂° exists, while under other conditions, most notably depressed pH, the biologically mobile and easily methylated HgCI₂° exists. It is shown here that lakes whose pH and pCl are such that the dominant mercury species will exist as HgCl₂° contain fish with higher mercury concentrations than lakes whose dominant mercury species exists in the form Hg(OH)₂°. This suggests that the more HgCI₂° that is available, the more mercury will either be methylated and taken up by fish, or taken up directly, and probably methylated in vivo.</p>