Bioaccumulation of Hg, Cu, and Zn in the Azores triple junction hydrothermal vent fields food web

Abstract

In this work, mercury (Hg), copper (Cu) and zinc (Zn) concentrations and tissue distribution are determined in seven benthic invertebrates species (the key species) from the Mid Atlantic Ridge (MAR) hydrothermal vent fields. The samples were collected from three hydrothermal vent fields – Menez Gwen, 840 m; Lucky Strike, 1700 m and Rainbow, 2300 m – near the Azores Triple Junction. These fields are characterized by different depths, geological context and chemical composition of the hydrothermal fluid, particularly the metal content, which is reflected by the metal concentrations in the organisms. Indeed, our results show that organisms from Menez Gwen presented the highest Hg concentrations, while those from Lucky Strike and Rainbow were richer in Cu and Zn. The potential transfer of these metals through two trophic links are also evaluated and include (1) the mussel Bathymodiolus azoricus and the commensal worm Branchipolynoe seepensis, and (2) three different species of shrimps and the crab Segonzacia mesatlantica. No evidence of Hg biomagnification in either of the vent food chains is clearly observed but an increase in Hg accumulation from prey to predator in the crustacean food chain. The same pattern was observed for Cu and Zn, even though these metals are not known to be generally biomagnified in food chains.

Introduction

Deep-sea hydrothermal vent communities develop in the interfacial zone where hydrothermal vent fluids and seawater mix. This zone is characterized by its instability, leading to continually fluctuating environments. Thus, vent organisms are constantly switching from an environment enriched in the reduced chemical compounds from vent fluids to cold oxygenated seawater. The fluids are especially rich in sulphides, methane and heavy metals, which are potentially toxic for the exposed organisms (Fisher, 1990, Childress and Fisher, 1992).

The three hydrothermal vent fields, i.e. Rainbow, Menez Gwen and Lucky Strike, located on the Mid Atlantic Ridge (MAR), near the Azores Triple Junction (ATJ), are characterized by their different end-member fluid chemical compositions, depths, geological contexts and associated biological communities (Desbruyères et al., 2000, Desbruyères et al., 2001).

The hydrothermal fluid emitted at Rainbow displays by far the highest metal concentrations, while the fluids collected in the Menez Gwen site display the lowest. The variation in fluid composition is related to phase separation processes (boiling/distillation of sub-surface vent fluids) and to the nature of the basement rock (basaltic and/or ultramafic) (Charlou et al., 2000, Charlou et al., 2002, Desbruyères et al., 2000, Douville et al., 2002). In these regions, the presence of two food chains in the prevailing food web have been highlighted using stables isotopes (Colaço et al., 2002) and include (1) the mussel Bathymodiolus azoricus, the commensal worm Branchipolynoe seepensis and the whelk Phymorhynchus sp., and (2) different species of shrimps and the crab Segonzacia mesatlantica.

Mercury (Hg) is a non-essential metal that can be found in all surface media. It is the only metal that consistently biomagnifies through the food chain, i.e., predators accumulate higher tissue amounts than their food contains (Monteiro et al., 1996). High temperatures found in the Earth’s mantle zones induce high Hg mobility, leading to its continuous diffusion into the lithosphere surface. Diffusion into the lithosphere may be huge as in the case of the New-Zealand hydrothermal vents, where sediments contain pure Hg and Hg sulphides as a consequence of release from vent fluids (Stoffers et al., 1999). The only Hg data on the vent fluids from the MAR available indicates that the concentrations are very low; most of the times below the detection limit of commonly used analytical technics (Kadar et al., 2005).

Copper (Cu) and zinc (Zn) are essential metals vital to enzymes and respiratory pigments (White and Rainbow, 1985). Hydrothermal fluids are rich in Cu and Zn in comparison with the background seawater. Transfer of metals through food chains has been considered in studies dating back to the 1960s (Bryan, 1964, Hoss, 1964). It is now considered that the dietary exposure is the major route for metal bioaccumulation for many marine animals (Wang, 2002).

The objective of this study is to determine the behaviour of Hg, Cu and Zn along the hydrothermal vent food chains of the MAR. Tissue concentrations and distribution of the three metals were determined for the key invertebrate species from the three Azores hydrothermal vents fields (Menez Gwen, Lucky Strike and Rainbow) in order to investigate the transfer of metals between prey and predator.