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Abstract Title: Examining Spatial and Vertical Variations of Total and Methyl Mercury in Sediments at a Subtropical Mangrove Ecosystem
Presenter Name: Shaoyi Wang
Company/Institution: The Chinese University of Hong Kong
Session: Mercury in Marine Ecosystems
Co-Authors: Shaoyi Wang,Martin Tsui

Abstract Information :

Mangrove swamp, covering approximately 137,000 km2 of the tropical and subtropical coastal area, sediments in mangroves with strong reducing conditions may play an important, yet not well understood, role in producing highly toxic methylmercury (MeHg). Specifically, we know litter how THg and MeHg distribute in these coastal ecosystems and how they would be impacted by the presence of mangrove trees. In this study, we examine the spatial and vertical distributions of THg and MeHg in a traditional shrimp pond (locally called Gei Wei) in Mai Po Nature Reserve in Hong Kong, China. There are channels surrounding a large area of mangrove trees and reed beds. We have collected sediment cores along a transect from the channel to the shore of the mangrove stands (Kandelia obovata), with sediment cores collected below and above water surface. We also collected a sediment core from the open water area. In our initial measurements, we used hot digestion of sediments with a mixture of nitric acid and hydrogen peroxide, we regard the THg in a more reactive form (THgreactive). Initial data of THgreactive showed relatively low levels across the sediment cores (from 0-10 cm), being lower in sediments above the water (7.9 to 12.4 ng/g wet wt.) and little higher within the channel (9.8 to 17.9 ng/g wet wt.), and being similar to the core within the open water area (12.5 to 15.6 ng/g wet wt.). Overall, THgreactive increased from the top (0-2 cm) to the bottom layers (2-4, 4-6, 6-8, and 8-10 cm). We are continuing to collect more sediment cores along with other physiochemical measurements (e.g., total sulfide, organic matter), and will analyze THg in reactive and recalcitrant forms (through the digestion with aqua regia). We will analyze MeHg in sediments to identify the key zones of Hg methylation in these organic-rich sediments.

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