Data from: Dissolved organic carbon modulates mercury concentrations in insect subsidies from streams to terrestrial consumers

Mercury (Hg) concentrations in aquatic environments have increased
globally, exposing consumers of aquatic organisms to high Hg levels. For
both aquatic and terrestrial consumers, exposure to Hg depends on their
food sources as well as environmental factors influencing Hg
bioavailability. The majority of the research on the transfer of
methylmercury (MeHg), a toxic and bioaccumulating form of Hg, between
aquatic and terrestrial food webs has focused on terrestrial piscivores.
However, a gap exists in our understanding of the factors regulating MeHg
bioaccumulation by non-piscivorous terrestrial predators, specifically
consumers of adult aquatic insects. Because dissolved organic carbon (DOC)
binds tightly to MeHg, affecting its transport and availability in aquatic
food webs, we hypothesized that DOC affects MeHg transfer from stream food
webs to terrestrial predators feeding on emerging adult insects. We tested
this hypothesis by collecting data over two years from 10 low-order
streams spanning a broad DOC gradient in the Lake Sunapee watershed in New
Hampshire. We found that streamwater MeHg concentration increased linearly
with DOC concentration. However, streams with the highest DOC
concentrations had emerging stream prey and spiders with lower MeHg
concentrations than streams with intermediate DOC concentrations; a
pattern that is similar to fish and larval aquatic insects. Furthermore,
high MeHg concentrations found in spiders show that MeHg transfer in adult
aquatic insects is an overlooked but potentially significant pathway of
MeHg bioaccumulation in terrestrial food webs. Our results suggest that
although MeHg in water increases with DOC, MeHg concentrations in stream
and terrestrial consumers did not consistently increase with increases in
streamwater MeHg concentrations. In fact, there was a change from a
positive to a negative relationship between aqueous exposure and
bioaccumulation at streamwater MeHg concentrations associated with DOC
above around 5 mg/L. Thus, our study highlights the importance of stream
DOC for MeHg dynamics beyond stream boundaries, and shows that factors
modulating MeHg bioavailability in aquatic systems can affect the transfer
of MeHg to terrestrial predators via aquatic subsidies.