Modeling the Influence of Eutrophication and Redox Conditions on Mercury Cycling at the Sediment-Water Interface in the Berre Lagoon

Abstract

This study presents a specifically designed Mercury module in a coupled benthic-pelagic reactive-transport model -Bottom RedOx Model (BROM) that allows to study mercury (Hg) biogeochemistry under different conditions. This module considers the transformation of elemental mercury (Hg(0)), divalent mercury (Hg(II)) and methylmercury (MeHg). The behavior of mercury species in the model is interconnected with changes of oxygen, hydrogen sulfide, iron oxides, organic matter, and biota. We simulated the transformation and transport of Hg species in the water column and upper sediment layer under five different scenarios, combining various levels of oxygenation and trophic state in the Berre lagoon, a shallow eutrophic lagoon of the French Mediterranean coast subjected to seasonal anoxia. The first scenario represents the conditions in the lagoon that are compared with experimental data. The four other scenarios were produced by varying the biological productivity, using low and high nutrient (N and P) concentrations, and by varying the redox conditions using different intensity of vertical mixing in the water column. The results of the simulation show that both oxidized and reduced sediments can accumulate Hg, but any shifts in redox conditions in bottom water and upper sediment layer lead to the release of Hg species into the water column. Eutrophication and/or restricted vertical mixing lead to reducing conditions and intensify MeHg formation in the sediment with periodic release to the water column. Oxygenation of an anoxic water body can lead to the appearance of Hg species in the water column and uptake by organisms, whereby Hg may enter into the food web. The comparison of studied Pakhomova et al.Modeling Mercury Cycling at the SWI scenarios shows that a well-oxygenated eutrophic system favors the conditions for Hg species bioaccumulation with a potential adverse effect on the ecosystem. The research is relevant to the UN Minimata convention, EU policies on water, environmental quality standards and Mercury in particular.

Distribution and bioavailability of mercury in the surface sediments of the Baltic Sea

“…Oxygenation of an anoxic sediment leads to the appearance of Hg species in the water column and the potential bioaccumulation by organisms. However, as shown by Pakhomova et al (2018), any shifts in redox conditions in bottom water and upper sediment layer lead to the release of Hg species into the water column. The negative dependence between Hg concentration and redox potential was partly related to the station depth, which is confirmed by a statistically significant negative correlation between redox potential and depth (R Spearman = − 0.48, p = 0.003) ( Table A3).…”

HR3DHG version 1: modeling the spatiotemporal dynamics of mercury in the Augusta Bay (southern Italy)

“…This integrated model also estimates the total amount of mercury present in biological species that occupy the lowest trophic level of the food chain, i.e., phytoplankton popu- lations. For this purpose, we incorporated the Phytoplankton MERLIN-Expo model (Pickhardt and Fischer, 2007;Radomyski and Ciffroy, 2015) to describe the mechanism of mercury uptake in phytoplankton cells. Moreover, we reproduced the spatiotemporal dynamics of phytoplankton communities in seawater using the Nutrient-Phytoplankton model (Dutkiewicz et al, 2009;Morozov et al, 2010;Valenti et al, 2012;Denaro et al, 2013a, b, c;Valenti et al, 2015Valenti et al, , 2016aValenti et al, , b, c, 2017Morozov et al, 2019).…”

“…Finally, the biogeochemical models introduced in previous works (Soerensen et al, 2016;Pakhomova et al, 2018) provided neither the NPP from the Nutrient-Phytoplankton model (Baines et al, 1994;Brunet et al, 2007;Zhang et al, 2014) nor the load of POM-released Hg D obtained using the Phytoplankton MERLIN-Expo model (Pickhardt and Fischer, 2007;Radomyski and Ciffroy, 2015) (see Sect. 3.1).…”

“…Moreover, the role played by the spatiotemporal behavior of phytoplankton (La Barbera and Spagnolo, 2002;Fiasconaro et al, 2004;Valenti et al, 2004Valenti et al, , 2008Dutkiewicz et al, 2009;Morozov et al, 2010;Valenti et al, 2012;Denaro et al, 2013a, b, c;Valenti et al, 2015Valenti et al, , 2016aValenti et al, , b, c, 2017Morozov et al, 2019) and the mechanisms responsible for the uptake of Hg within cells (Pickhardt and Fischer, 2007;Radomyski and Ciffroy, 2015;Lee and Fischer, 2017;Williams et al, 2010) are taken into account as specific contributions to the scavenging process and Hg release process by POM, respectively. Also, seasonal oscillations of key environmental variables (velocity of marine currents, amount of precipitation, elemental and inorganic mercury concentration in the atmosphere, etc.)…”

Susane, a device for sampling chemical gradients in the benthic water column

“…The results demonstrate that production and consumption processes of such substances in the benthic layer do support such concentration gradients, despite turbulent mixing which tends to eliminate them. While the gradients' presence may be inferred from modeling studies (e.g., Pakhomova et al ), tools like Susane are helpful for their direct observation because they can collect the appropriate discrete samples. In the future, coupling such observations with hydrodynamics of the benthic layer will contribute to a better understanding of their biogeochemical fluxes near the SWI.…”