70 - The Effect of Seagrass Recolonization on the Sediment Hg Vertical Profiles of a Recovering Coastal Lagoon

"The progressive implementation of protective environmental legislation, production, export, and trade bans are decisive steps towards a mercury-free economy. For historically impacted sites, however, restoration measures are necessary to prevent exposure and minimize possible effects of climate change. With that in mind, the possible use of seagrass (Zostera noltei) recolonization as a blue-green restoration infrastructure is currently being tested in the historically contaminated area of in Ria de Aveiro (Portugal). The main purpose of this work was to evaluate the effects of Zostera noltei recolonization on the sediment vertical Hg profiles.

For that purpose, a combination of solid phase mercury, pore water reactive mercury and labile Hg fractions (through DGT passive samplers) profiles were measured in the top 20 cm of sediment, with a 1 cm resolution. This was performed in three distinct areas (bare-bottom, natural seagrass meadow and transplanted seagrass meadow) and seasons (summer and winter) to take into account different plant activity periods.

Sediment Hg concentrations ranged from 3.3 to 8.2 mg kg-1, with the highest concentrations recorded in the deepest layers. Inversely, the highest recorded mercury concentrations in both the interstitial water and DGT samplers was found in the top sediment layers, ranging within 0.04 – 0.22 mg kg-1. Overall, strongly significant correlations were found between pore-water and DGT mercury profiles, highlighting the efficiency of this novel technology to reflect the labile Hg fraction of sediments.

No consistent differences were observed between seasons in any of the studied matrices, while significant differences were observed between the bare bottom and the vegetated sites, confirming the significant role of plant cover on the sediment biogeochemistry.

Ongoing studies are evaluating the mercury fluxes at the sediment-water interface to further clarify the effect of Zostera noltei on these processes.

This work was partially funded by project RemediGrass (PTDC/CTA-AMB/29647/2017)."

Authors: Vítor Oliveira