PROXY MEASURES OF MERCURY AND CARBON QUALITY IN FORESTED UPLAND STREAMS

In eastern USA undeveloped forested catchment streams, we observe strong relations of dissolved mercury (Hg) to dissolved organic carbon (DOC) and particulate Hg to particulate organic carbon (POC). When we isolate the hydrophobic acid fraction of the DOC (HPOA), which typically comprises about half of the DOC, the dissolved Hg-HPOA relation is stronger than the dissolved Hg-DOC relation. The HPOA fraction includes aromatic DOC, which associates with and transports Hg. We also evaluated optical properties of DOC to determine their suitability as surrogate indicators of dissolved Hg. For three sites in the Northeast, a simple laboratory measurement of ultraviolet (UV) absorbance at 254 nm was a reliable proxy for HPOA (r²>0.9), and the dissolved Hg-UV relation was strongly linear (r²≈0.9). To build on these findings, we measured in situ dissolved organic matter fluorescence (FDOM), another optical property, at the same three sites for one year. FDOM proved to be a strong proxy for DOC and, hence, dissolved Hg. In situ measurements therefore provide high-temporal resolution that allows inference of in-stream dynamics and processes controlling DOC and Hg, and can greatly improve the accuracy of flux calculations. These easily measured proxies (UV absorbance and FDOM) foster improved understanding of when and how inorganic Hg moves to downstream methylation sites, and in some settings also serve as direct indicators for dissolved methylmercury.