Role of formation and decay of seston organic matter in the fate of methylmercury within the water column of a eutrophic lake
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Published:2023-04-14
Issue:7
Volume:20
Page:1459-1472
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ISSN:1726-4189
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Container-title:Biogeosciences
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language:en
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Short-container-title:Biogeosciences
Author:
Balzer Laura,Baptista-Salazar Carluvy,Jonsson Sofi,Biester Harald
Abstract
Abstract. Anoxic microniches in sinking particles in lakes have been
identified as important water phase production zones of monomethylmercury
(MeHg). However, the production and decay of MeHg during organic matter (OM)
decomposition in the water column and its relation to the total Hg
concentration in seston are poorly understood. We investigated total Hg and
MeHg in relation to chemical changes in sinking seston and hydrochemical
settings in a small and shallow (12 m deep) eutrophic lake during
phytoplankton blooms from April to November 2019. The results show that MeHg
proportions reach up to 22 % in seston in oxygen super saturation at the
water surface and highest values (up to 26 %) at the oxic–suboxic redox
boundary. MeHg concentrations were highest in May and November when algal
biomass production was low and seston were dominated by zooplankton.
Biodilution of MeHg concentrations could not be observed in the months of the
highest algal biomass production; instead, MeHg and THg concentrations in
seston were comparatively high. During suboxic OM decomposition and with
decreasing redox potential (Mn and nitrate reduction), the concentration and
proportion of MeHg in seston strongly decreased (<0.5 %),
whereas total Hg concentrations show a 3.8- to 26-fold increase with water
depth. Here, it remains unclear to which extent biodilution on the one hand
and OM decomposition on the other alter the MeHg and THg concentration in
seston. Changes in OM quality were most intense within or slightly below the
redox transition zone (RTZ). The concentrations of MeHg and THg in seston
from the RTZ were comparable to those found in the sediment trap material
which integrated the changes in seston composition during the entire
sampling period, suggesting that changes in the MeHg and THg content in the
hypolimnion below the RTZ are comparatively small. Our study suggests that,
in shallow eutrophic lakes, the water phase formation and decomposition of
MeHg is intense and controlled by the decomposition of algal biomass and is,
assumedly, largely disconnected from Hg methylation in sediments, similar to
what has been observed in deep oligotrophic lakes.
Publisher
Copernicus GmbH
Subject
Earth-Surface Processes,Ecology, Evolution, Behavior and Systematics
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