title: Dissolved organic carbon modulates mercury concentrations in insect subsidies from streams to terrestrial consumers.
abstract: 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 2 years from 10 low-order streams spanning a broad DOC gradient in the Lake Sunapee watershed in New Hampshire, USA. 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 ~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.
authors.affiliation: Department of Biological Sciences, Dartmouth College, 78 College Street, Hanover, New Hampshire, 03755, USA. ramsa.chavesulloa@wgu.edu.
authors.forename: Ramsa
authors.initials: R
authors.lastname: Chaves-Ulloa
authors.affiliation: Department of Biological Sciences, Dartmouth College, 78 College Street, Hanover, New Hampshire, 03755, USA.
authors.forename: Brad W
authors.initials: BW
authors.lastname: Taylor
authors.affiliation: Department of Biological Sciences, Dartmouth College, 78 College Street, Hanover, New Hampshire, 03755, USA.|Environmental Studies Program, Bates College, 7 Andrews Road, Lewiston, Maine, 04240, USA.
authors.forename: Hannah J
authors.initials: HJ
authors.lastname: Broadley
authors.affiliation: Department of Biological Sciences, Dartmouth College, 78 College Street, Hanover, New Hampshire, 03755, USA.
authors.forename: Kathryn L
authors.initials: KL
authors.lastname: Cottingham
authors.affiliation: Department of Natural Sciences, Colby-Sawyer College, 541 Main Street, New London, New Hampshire, 03257, USA.
authors.forename: Nicholas A
authors.initials: NA
authors.lastname: Baer
authors.affiliation: Cary Institute of Ecosystem Studies, PO Box AB, Millbrook, New York, 12545, USA.
authors.forename: Kathleen C
authors.initials: KC
authors.lastname: Weathers
authors.affiliation: Environmental Studies Program, Bates College, 7 Andrews Road, Lewiston, Maine, 04240, USA.
authors.forename: Holly A
authors.initials: HA
authors.lastname: Ewing
authors.affiliation: Department of Biological Sciences, Dartmouth College, 78 College Street, Hanover, New Hampshire, 03755, USA.
authors.forename: Celia Y
authors.initials: CY
authors.lastname: Chen
mesh_terms: D000818:Animals; D002244:Carbon / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007313:Insecta / Q000737:chemistry / Q000502:physiology*; D008628:Mercury / Q000737:chemistry*; D045483:Rivers / Q000737:chemistry*; D013112:Spiders / Q000737:chemistry / Q000502:physiology; D013696:Temperature
chemical_list: D002244:Carbon; D008628:Mercury
keywords: aquatic insects; aquatic-terrestrial linkages; bioaccumulation; dissolved organic carbon; emergence; food web; mercury; nonlinearity; resource subsidy; spiders; streams; threshold