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Eutrophication increases phytoplankton methylmercury concentrations in a coastal sea - a Baltic Sea case study Anne L. Soerensen, Amina Traore Schartup, Erik Gustafsson, Bo G. Gustafsson, Emma Maria Undeman, and Erik Björn Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02717 • Publication Date (Web): 05 Oct 2016 Downloaded from http://pubs.acs.org on October 12, 2016
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Environmental Science & Technology
Eutrophication increases phytoplankton methylmercury concentrations in a coastal sea - a Baltic Sea case study
Anne. L. Soerensena*, Amina T. Schartupb, Erik Gustafssonc, Bo G. Gustafssonc, Emma Undemana,c, Erik Björnd a
Stockholm University, Department of Environmental Science and Analytical Chemistry, Stockholm, Sweden b Harvard University, John A. Paulson School of Engineering and Applied Sciences, Cambridge MA, USA c Stockholm University, Baltic Nest Institute, Baltic Sea Centre, Stockholm, Sweden d Umeå University, Department of Chemistry, Umeå, Sweden * corresponding author (e-mail:
[email protected]; phone: +46 86747278)
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Environmental Science & Technology
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Abstract
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Eutrophication is expanding worldwide, but its implication for production and
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bioaccumulation of neurotoxic monomethylmercury (MeHg) is unknown. We developed a
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mercury (Hg) biogeochemical model for the Baltic Sea and used it to investigate the impact of
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eutrophication on phytoplankton MeHg concentrations. For model evaluation we measured
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total methylated Hg (MeHgT) in the Baltic Sea and found low concentrations (39±16 fM)
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above the halocline and high concentrations in anoxic waters (1249±369 fM). To close the
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Baltic Sea MeHgT budget we inferred an average normoxic water column MeHg production
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rate of 2×10-4 d-1. We used the model to compare Baltic Sea’s present-day (2005-2014)
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eutrophic state to an oligo/mesotrophic scenario. Eutrophication increases primary production
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and export of organic matter and associated Hg to the sediment effectively removing Hg from
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the active biogeochemical cycle; this results in a 27% lower present-day water column Hg
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reservoir. However, increase in organic matter production and remineralization stimulates
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microbial Hg methylation resulting in a seasonal increase in both water and phytoplankton
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MeHg reservoirs above the halocline. Previous studies of systems dominated by external
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MeHg sources or benthic production found eutrophication to decrease MeHg levels in
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plankton. This Baltic Sea study shows that in systems with MeHg production in the normoxic
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water column eutrophication can increase phytoplankton MeHg content.
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Environmental Science & Technology
1. Introduction
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Monomethylmercury (MeHg) is a neurotoxin that bioaccumulates in aquatic food webs
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affecting MeHg production and its levels in food webs (e.g. 3-5). While many studies have
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investigated the role of eutrophication on isolated aspects of the mercury (Hg) cycle, its
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impact at the ecosystem level is unknown.3 Here we adapt an existing nutrient cycling model
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to include cycling of Hg species and phytoplankton accumulation of MeHg in the Baltic Sea,
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and use it to quantify the impact of eutrophication.
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. Eutrophication in coastal areas and continental seas is expanding worldwide2 potentially
Eutrophication leads to greater net primary production, therefore increasing organic
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matter (OM) settling and decreasing light penetration. These in turn change redox conditions
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in water column and sediments.6, 7 The most comprehensive study on the impact of
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eutrophication on coastal marine Hg cycling3 used a conceptual model to show that by
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increasing primary production eutrophication leads to a decrease in MeHg concentrations in
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biota. One important driver of this effect was proposed to be that the increase in
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phytoplankton biomass decreases the amount of MeHg in individual cells. This phenomenon,
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known as growth biodilution, has been observed in laboratory experiments4 and in both
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marine and freshwater systems.8-10 The conceptual model for coastal systems was in line with
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early work that concluded that divalent inorganic mercury (HgII) methylation did not take
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place in normoxic (oxygen (O2) >2 mL L-1) marine water columns.11 However, several recent
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studies reported HgII methylation in normoxic waters,12-14 possibly linked to the
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remineralization of organic matter.15, 16 Moreover, genes (hgcAB) coding for HgII methylation
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ability in microorganisms have been found in some normoxic and anoxic marine waters.17, 18
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Therefore the impact of eutrophication needs to be revisited using a model that includes in
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situ HgII methylation in the water column.
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An increase in OM settling increases the biological oxygen demand. Severe
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eutrophication can result in bottom water hypoxia (
