231 - Assessing the Spatial-temporal Variability of MeHg Biogeochemistry and Bioaccumulation in the Mediterranean Sea with a Coupled 3D Model

"Understanding the dynamics controlling MeHg production and bioaccumulation at the base of marine food webs is crucial for understanding the pathways of human exposure and predicting the potential impacts of environmental changes. Previous research pointed out that the Mediterranean Sea has a high methylation potential and high levels of bioaccumulation and biomagnification driven by its peculiar physical-biogeochemical, and ecological features.

We developed a coupled transport-biogeochemical-bioaccumulation model (OGSTM-BFM-Hg) and used it to investigate the spatial and temporal variability of MeHg distribution and bioaccumulation in the plankton food web of the Mediterranean Sea, within a 3D domain with a horizontal resolution of about 6 km. Modeled MeHg concentrations are characterized by strong zonal gradients related to primary production, in agreement with observations from previous field studies. The analysis of monthly dynamics in different Mediterranean subbasins highlighted that summer stratification of the water column is an important process for the buildup of a sub-surficial maximum of MeHg concentrations, which is disrupted when winter convection occurs.

The modeled bioaccumulation dynamics in the plankton food web are characterized by high spatial and temporal variability driven by plankton phenology. Plankton MeHg enrichment relative to water concentrations, expressed as BAF (bioaccumulation factor) is maximum for the smallest phytoplankton group (picophytoplankton) and for the group representative of carnivorous mesozooplankton. The overall content of MeHg in plankton is highest in areas of the Mediterranean Sea where picophytoplankton is abundant and MeHg water concentrations are relatively high, such as the Tyrrhenian Sea and Southern Adriatic Sea. Biomagnification is maximum in areas of higher primary production where the trophic web can sustain longer food webs up to carnivorous zooplankton, such as the Alboran Sea and the Southern Western Mediterranean Sea.

The results of the hindcast simulation suggest cascading effects of increasing water temperature through the decline of deep convection events in the Northern Western Mediterranean Sea that results in higher MeHg concentrations in the intermediate waters, and in turn in enhanced bioaccumulation. The model will be used to carry out long-term simulations under the climate change scenarios RCP4.5 and RCP8.5."

Authors: Ginevra Rosati, Donata Canu, Paolo Lazzari, Cosimo Solidoro