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Organic matter drives high interannual variability in methylmercury concentrations in a subarctic coastal sea
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Kemiska institutionen.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Kemiska institutionen.
2017 (Engelska)Ingår i: Environmental Pollution, ISSN 0269-7491, E-ISSN 1873-6424, Vol. 229, s. 531-538Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

Levels of neurotoxic methylmercury (MeHg) in phytoplankton are strongly associated with water MeHg concentrations. Because uptake by phytoplankton is the first and largest step of bioaccumulation in aquatic food webs many studies have investigated factors driving seasonal changes in water MeHg concentrations. Organic matter (OM) is widely accepted as an important driver of MeHg production and uptake by phytoplankton but is also known for strong interannual variability in concentration and composition within systems. In this study, we explore the role of OM on spatial and interannual variability of MeHg in a subarctic coastal sea, the northern Baltic Sea. Using MeHg (2014: 80 ± 25 fM; 2015: <LOD; 2016: 21 ± 9 fM) and OM measurements during late summer/early fall, we find that dissolved organic carbon (DOC) and humic matter content explain 60% of MeHg variability. We find that while labile DOC increases MeHg levels in the water, humic content reduces it. We propose that the positive association between MeHg and labile DOC shows that labile DOC is a proxy for OM remineralization rate in nearshore and offshore waters. This is consistent with other studies finding that in situ MeHg production in the water column occurs during OM remineralization. The negative association between water humic content and MeHg concentration is most likely due to humic matter decreasing inorganic mercury (HgII) bioavailability to methylating microbes. With these relationships, we develop a statistical model and use it to calculate MeHg concentrations in late summer nearshore and offshore waters between 2006 and 2016 using measured values for water DOC and humic matter content. We find that MeHg concentrations can vary by up to an order of magnitude between years, highlighting the importance of considering interannual variability in water column MeHg concentrations when interpreting both short and long term MeHg trends in biota.

Ort, förlag, år, upplaga, sidor
Elsevier, 2017. Vol. 229, s. 531-538
Nyckelord [en]
Methylmercury, Labile dissolved organic matter, Humic matter, Hg methylation, Interannual variability
Nationell ämneskategori
Kemi
Identifikatorer
URN: urn:nbn:se:umu:diva-139654DOI: 10.1016/j.envpol.2017.06.008ISI: 000410010200056PubMedID: 28646796OAI: oai:DiVA.org:umu-139654DiVA, id: diva2:1142691
Tillgänglig från: 2017-09-20 Skapad: 2017-09-20 Senast uppdaterad: 2019-12-16Bibliografiskt granskad
Ingår i avhandling
1. Formation, uptake and bioaccumulation of methylmercury in coastal seas: a baltic sea case study
Öppna denna publikation i ny flik eller fönster >>Formation, uptake and bioaccumulation of methylmercury in coastal seas: a baltic sea case study
2019 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
Abstract [en]

Methylmercury (MeHg) is a potent neurotoxin which can bioaccumulate to harmful levels in aquatic food webs. Methylmercury formation is a predominantly biotic process which involves phylogenically diverse microorganisms (e.g. iron- or sulfate-reducing bacteria). The formation of MeHg is related to the presence of organic matter (OM) which contains substrates essential for methylating microbes and reduced sulfur ligands (thiols, RSH) that form strong bonds with inorganic mercury (HgII) and affect its bioavailability. In aquatic systems, MeHg is bio-concentrated from the water column to the base of the food web and this step is crucial for MeHg levels found at higher trophic levels. Trophic transfer processes of MeHg in the food web are also of great importance. Discharge of OM in coastal areas affects light conditions needed for phytoplankton growth, and promotes heterotrophy, i.e. bacteria production. This may lead to a shift from the phytoplankton-based to the longer bacteria-based (microbial loop) food web and influence the amount of bioaccumulated MeHg in higher trophic levels. Methylmercury levels in predatory biota is thus affected by the bioavailability of HgII for methylation (studied in Paper I & II), MeHg speciation in the water column, crucial for MeHg incorporation at the base of the food web (Paper III), and the structure of the pelagic food web (Paper IV).In this thesis, it is shown that OM can act as a predictor of dissolved MeHg levels in estuarine and coastal systems. It impacts MeHg levels both by affecting HgII bioavailability (through Hg complexation with humic matter) and the activity of methylating microbes (providing metabolic electron donors) (Paper I). Moreover, elevated concentrations of particulate and dissolved HgII and MeHg, are associated with the presence of pelagic redoxclines in coastal seas. The redoxcline affects HgII speciation in the water column and its bioavailability for methylation (Paper II). It is further shown that the molecular structure of ligands in MeHg complexes affects the kinetics of MeHg uptake in phytoplankton. Rate constants for association of MeHg to the cell surface of a green algae were higher in treatments containing smaller thiol ligands of simpler structure than in treatments with larger thiols and more “branched” structure (Paper III). Finally, it is demonstrated that MeHg bioaccumulation in zooplankton can increase in systems with highly heterotrophic food webs and enhanced loadings of terrestrial OM (Paper IV). Such conditions are expected to occur in northern latitude coastal systems following climate changes.

Ort, förlag, år, upplaga, sidor
Umeå: Umeå University, 2019. s. 49
Nyckelord
Mercury, methylmercury, bioaccumulation, mesososm, isotope tracers, methylation, demethylation, stability constant, kinetic model, coastal sea, ICPMS, LC-MS/MS
Nationell ämneskategori
Kemi
Identifikatorer
urn:nbn:se:umu:diva-166499 (URN)978-91-7855-177-4 (ISBN)
Disputation
2020-01-24, KBE301, Lila hörsalen, Linnaeus väg 6, Umeå, 09:00 (Engelska)
Opponent
Handledare
Tillgänglig från: 2019-12-20 Skapad: 2019-12-16 Senast uppdaterad: 2019-12-18Bibliografiskt granskad

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Skrobonja, AleksandraBjörn, Erik

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