Metals geochemistry and ecological risk assessment in a tropical mangrove (Can Gio, Vietnam)

Highlights

• Metal geochemistry was studied in the sediment of the Can Gio Mangrove forest.

• Metal were mainly associated to the residual fraction.

• Metal partitioning was strongly linked to organic matter cycling.

• Higher metal stocks were measured in the Avicennia stand than in the Rhizophora one.

• Metal presented low ecological risks to the ecosystem, except Mn and As.

Abstract

Mangrove sediments act as natural biogeochemical reactors, modifying metals partitioning after their deposition. The objectives of the present study were: to determine distribution and partitioning of metals (Fe, Mn, Ni, Cr, Cu, Co and As) in sediments and pore-waters of Can Gio Mangrove; and to assess their ecological risks based on Risk Assessment Code. Three cores were collected within a mudflat, beneath Avicennia alba and Rhizophora apiculata stands. We suggest that most metals had a natural origin, being deposited in the mangrove mainly as oxyhydroxides derived from the upstream lateritic soils. This hypothesis could be supported by the high proportion of metals in the residual fraction (mean values (%): 71.9, 30.7, 80.7, 80.9, 67.9, 53.4 and 66.5 for Fe, Mn, Ni, Cr, Cu, Co, and As respectively, in the mudflat). The enrichment of mangrove-derived organic matter from the mudflat to the Rhizophora stand (i.e. up to 4.6% of TOC) played a key role in controlling metals partitioning. We suggest that dissolution of Fe and Mn oxyhydroxides in reducing condition during decomposition of organic matter may be a major source of dissolved metals in pore-waters. Only Mn exhibited a potential high risk to the ecosystem. Most metals stocks in the sediments were higher in the Avicennia stand than the Rhizophora stand, possibly because of enhanced dissolution of metal bearing phases beneath later one. In a context of enhanced mangrove forests destruction, this study provides insights on the effects of perturbation and oxidation of sediments on metal release to the environment.

Introduction

Mangrove ecosystems are closely tied to anthropogenic activities. Due to their specific characteristics, i.e. their richness in fine particles and organic matter, mangrove sediments can act as natural sinks for metals originating from natural and anthropized watersheds (Clark et al., 1998; Marchand et al., 2011a; Tam and Wong, 1996). In mangrove sediments, the partitioning of metals between different phases, i.e. organic matter, iron-manganese oxyhydroxides, sulphides, carbonates (Tam and Wong, 1996; Zhang et al., 2014) is strongly influenced by sediment characteristics, i.e. pH, redox potential, particle size (Otero et al., 2009). Accordingly, the mangrove zonation, the hydrological conditions and the presence of different tree species will influence the above characteristics (Marchand et al. 2003, 2004) which will in turn influence metal accumulation and speciation in the sediments. In fact metals can be released from sediments into pore-waters in suboxic/anoxic conditions by the dissolution of bearing phases (Chakraborty et al., 2016; Marchand et al., 2012) during diagenetic processes, which can subsequently re-precipitate with another bearing phase (Noël et al., 2014). Then, dissolved metals can be exported to adjacent surface waters (Holloway et al., 2016; Sanders et al., 2012) and also be transferred to mangrove's living organism such as fishes, crabs, snails and mangrove's leaves and roots (Chakraborty et al., 2014; De Wolf and Rashid, 2008; Parvaresh et al., 2011). In a context of enhanced mangrove forests destruction at a worldwide scale i.e. at a rate close to 1% per year (Duke et al., 2007), the perturbation and oxidation of sediments can induce the dissolution of sulfide minerals, which can result in sediment acidification (Dent, 1986) and can enhance the release of metals from the sediment into the aquatic environment. Thus, the understanding of metal behaviors in mangrove ecosystems is highly relevant for the environment itself but also for human, as mangroves provide important ecosystem services (Lee et al., 2014, M. Brander et al., 2012).

In Vietnam, large mangrove areas were devastated by herbicide mixture during the war in the 70's, and the Can Gio Mangrove was one of the most heavily sprayed areas (Hong, 2001; Ross, 1975). However, mangrove restoration efforts have been realized and almost 40 years after, the rehabilitated mangrove is now more diverse in community structure than prior to the war (Hong, 2001). Nowadays, this mangrove is largely dominated by Rhizophora apiculata and Avicennia alba species, the latter one developing at lower elevation along tidal creek. The Can Gio Mangrove is located downstream Ho Chi Minh City, the economic capital of Vietnam with almost 10 million inhabitants. Mangrove river network acts as a unique gate for water outlet from Ho Chi Minh City to the South China Sea. Industrial activities, economic developments and rapid population growth are inducing high pressure on water and sediment quality of the adjacent river, the Sai Gon River (Babut et al., 2019; Nguyen et al., 2018a,b; Strady et al., 2017). Strady et al. (2017) stated that the main rivers and canals in the city were moderately contaminated by major metal(oid)s. Recently, Thanh-Nho et al. (2018) highlighted that metals can be transferred over long distance from upstream to the mangrove forest, and that elevated inputs of metals in the estuary were the result of enhanced runoff and soil leaching during the monsoon season. Consequently, taking into account the mangrove specific geochemical characteristics, the lack of wastewater treatment plants in emerging countries as Vietnam and the important local ecosystem services provided by the Can Gio Mangrove (Cormier-Salem et al., 2017; Kuenzer and Tuan, 2013), more attention should be paid on metals distribution, speciation, bioaccumulation and transfer in mangrove ecosystems. We note that so far, in the Can Gio Mangrove, only total metal concentrations in surface water, suspended sediment and surface sediment were investigated (Costa-Boddeker et al., 2017; Thanh-Nho et al., 2018) and the evaluation of the metal speciation in the sediment has not yet been assessed. It can be evaluated by the technique of sequential extraction (Tessier et al., 1979) which provides a classification of metals bound to different geochemical fractions. This technique also allow to characterize the diagenetic processes taking place in the sediment over a depth profile and to characterize the geogenic components and so origin of the sediments.

Within this context, the objectives of this study were: i) to investigate the geochemistry of Fe, Mn, Ni, Cr, Cu, Co and As in sediment cores of the Can Gio Mangrove by focusing on the metal speciation in the sediments and their concentrations in pore-waters; and ii) to assess the potential ecological risks of these metals accumulated in the sediments on the mangrove ecosystem by using Risk Assessment Code (Benson et al., 2017). The sediment cores were sampled on three specific environments characterized by different elevation (i.e. tidal inundation), organic matter content, redox stratification and plant cover: in the mudflat, beneath Avicennia stand and beneath Rhizophora stand.