Role of formation and decay of seston organic matter in the fate of methylmercury within the water column of a eutrophic lake

. 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;


Determination of methyl Hg in sediment samples
In order to extract MeHg from the sediment samples, between ~0.5 -1 g of material was measured into new 50 mL Falcon tubes and between 20 -100 μL of an internal standard, an isotopically enriched Me 200 Hg standard with concentration 1.1 ng -1 added and left to equilibrate for an hour.After equilibration, 10 mL KBr (1.4M), 2 mL CuSO4 (2M) and 10 mL dichloromethane, DCM (CH2Cl2) were added to each tube, which was capped and left for 45 min.In order to extract MeHg, the samples were rotated at 85 RPM on a sample rotor for 45 min, then centrifuged for 5 min at 3000 RPM.Glass Pasteur pipettes were used to manually transfer the lower (clear) layer containing DCM and the extracted MeHg to a new 50 mL Falcon tube.After adding 10 mL of Milli-Q (MQ) water to the pipetted liquid, the DCM was purged in a warm water bath at 45 °C, and the extraction is complete.MeHg was analyzed using a Tekran® Model 2700 Automated Methylmercury Analysis System connected to an Inductively Coupled Plasma Mass Spectrometer, Thermo-Fisher X-series 2 (ICPMS).Prior to analysis, half the extracted sample was ethylated using sodium tetraethyl-borate (NaTEB) at pH 4.9 (using 225 μl 2M acetate buffer).The resulting data was manually adjusted in Excel (Microsoft) to determine the MeHg peak area.The concentration of MeHg for each sample was subsequently calculated using mass-bias (MB) corrected signals derived through signal deconvolution.Three massbias vials, each containing 0.5 ppt ambient Hg ethylated in sodium tetraethyl-borate (NaTEB) at a pH of 4.9 (using S11 correction factor.The mean (in ng MeHg/g sediment) and %RSD of the replicates was: 4.30 ± 9.12%, 2.74 ± 10.40 %, and 1.20 ± 10.74 %.Five blanks, containing only reagents, were tested concurrently with the sampled material to ensure no contamination of MeHg occurred during the extraction process.Certified reference material (ERM-CC580, estuarine sediment) analyzed were on average 110 % of the certified value (75 ± 4 ng g -1 ).

Figs
Figs. S1 to S8Determination of methyl Hg in sediment samples

Fig. S1 .
Fig. S1.Depth profiles of O2 saturation (%), pH and concentrations of dissolved Mn [µg L -1 ] and Fe [µg L -1 ] in the lake Ölper water column from April to November 2019.The depth of the sharp decrease in O2 concentration and start of Mn reduction (RTZ) are shown in each panel (shaded light blue).

Fig. S2 .
Fig. S2.Depth profiles of temperature (T) [C°], pH, conductivity (EC) [µS cm -1 ], chlorophyll (Chla) [µg L -1 ] concentrations in the lake Ölper water column from April to November 2019.The depth of the sharp decrease in O2 concentration and start of Mn reduction (RTZ) are shown in each panel (shaded light blue).

Fig. S4 .
Fig. S4.Depth profiles of C [%], N [%], S [%] concentrations and calculated C/N ratio in the seston in lake Ölper from April to November 2019.The depth of the sharp decrease in O2 concentration and start of Mn reduction (RTZ) are shown in each panel (shaded light blue).Concentrations of C [%], N [%], S [%] and the C/N ratio of the sediment trap material collected during the 141 days between May 6 th and September 24 th are given in the grey box below.

Fig. S5 .
Fig. S5.Depth profiles of MeHg [ng g -1 ] concentrations, percentage of MeHg [%] of THg (MeHg-%), THg concentrations in seston [µg g -1 ] and dissolved THg [ng L -1 ] in the water phase (THg-w) of lake Ölper from April to November 2019.The depth of the sharp decrease in O2 concentration and start of Mn reduction (RTZ) are shown in each panel (shaded light blue).Concentrations of THg [µg g -1 ], MeHg [ng g -1 ], MeHg-% [%] of the sediment trap material collected over the 141 days between May 6 th and September 24 th are given in the grey box below.

Fig. S6 .
Fig. S6.Depth profile of DOC -[mg L -1 ] concentration in the lake Ölper water column from April to November 2019.The depth of the sharp decrease in O2 concentration and start of Mn reduction (RTZ) are shown in each panel (shaded light blue).Note that each column has an individual scale to better illustrate changes with depth.Depth profiles with the same scale in all columns are shown in Fig. S7.

Fig. S7 .
Fig. S7.Depth profile of DOC -[mg L -1 ] concentration in the lake Ölper water column from April to November 2019.The depth of the sharp decrease in O2 concentration and start of Mn reduction (RTZ) are shown in each panel (shaded light blue).

Fig. S8 .
Fig. S8.Dry and homogenized seston from lake Ölper from April to November, 2019; shown with one row per sampling day.The average color of each sample (corresponding to one sampled depth in m) is illustrated in boxes below each sample.Indicating decomposition during sampling or dominance of zooplankton in May and November.