Can Acid Volatile Sulfides (AVS) Influence Metal Concentrations in the Macrophyte Myriophyllum aquaticum? Johannes Teuchies, †, * Maarten De Jonge, ‡ Patrick Meire, † Ronny Blust, ‡ and Lieven Bervoets ‡ † Department of Biology, Ecosystem Management Research Group, University of Antwerp, Universiteitsplein 1, 2610 Wilrijk, Belgium ‡ Department of Biology, Ecophysiology, Biochemistry and Toxicology Group, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerpen, Belgium * S Supporting Information ABSTRACT: The difference between the molar concentrations of simultaneously extracted metals (SEM) and acid volatile sulfides (AVS) is widely used to predict metal availability toward invertebrates in hypoxic sediments. However, this model is poorly investigated for macrophytes. The present study evaluates metal accumulation in roots and stems of the macrophyte Myriophyllum aquaticum during a 54 day lab experiment. The macrophytes, rooting in metal contaminated, hypoxic, and sulfide rich field sediments were exposed to surface water with 40% or 90% oxygen. High oxygen concentrations in the 90% treatment resulted in dissolution of the metal-sulfide complexes and a gradual increase in labile metal concentrations during the experiment. However, the general trend of increasing availability in the sediment with time was not translated in rising M. aquaticum metal concentrations. Processes at the root-sediment interface, e.g., radial oxygen loss (ROL) or the release of organic compounds by plant roots and their effect on metal availability in the rhizosphere may be of larger importance for metal accumulation than the bulk metal mobility predicted by the SEM-AVS model. 1. INTRODUCTION Metal toxicity is a potential risk for many aquatic ecosystems worldwide. 1 When released into surface water, the major part of these elements will accumulate in bottom sediments. 2 In organic rich, anoxic sediments the formation of insoluble metal sulfides is found to play a major role in reducing sediment metal availability and transfer to the surface water. 3,4 The amount of acid extracted sulfides (acid volatile sulfides or AVS) and simultaneously extracted metals (SEM) is proposed as a predictor for toxicity of certain trace metals in sediments (cadmium (Cd), copper (Cu), lead (Pb), nickel (Ni), zinc (Zn), chromium (Cr), and silver (Ag)). 5-7 In this model, the sediment is predicted to be non toxic when, on a molar basis, AVS concentrations exceed SEM concentrations ([SEM-AVS] < 0) and is mainly used to evaluate metal toxicity toward aquatic macro-invertebrates 8-11 but, has only poorly been evaluated with respect to metal bioavailability and toxicity to macrophytes. Recent studies indicate that certain aquatic invertebrates can accumulate sediment-bound metals when an excess of AVS is observed up to levels which largely exceed tissue concentrations from organisms of uncontaminated sediments. 12,13 This has been shown for benthic invertebrates which can be exposed to high metal concentrations due to the increased availability of ingested sediment-bound metals caused by the specific conditions within the animals’ guts (lower pH and presence of digestive enzymes and surfactants). 14 Also for aquatic plants, it can be expected that metal exposure is not necessarily related to metal availability in bulk sediments. 15 Wetland plants and macrophytes rooting in anoxic sediments often maintain aerobic root respiration via internal oxygen transport through aerenchyma. 16 An excess of oxygen in the roots can leak out (known as radial oxygen loss, ROL) resulting in an oxidized rhizosphere. 17 This can result in oxidation and dissolution of sulfides and consequently in the mobilization of metals near the roots while low metal mobility can still predominate in the reducing bulk sediment. 18 Metal-sulfide precipitation in sediments is a reversible process. Natural or anthropogenic governed sediment disturbance events as well as changes in the sediment redox potential of undisturbed sediments can result in dissociation of metal-sulfide complexes. 2 A subsequent increase in metal mobility and release into the surface water can be expected. 19,20 In this study, an experiment was performed where natural anoxic, AVS-rich sediments were exposed to both aerated and nitrogen-purged surface water under natural flow conditions. Macrophytes from a genus known to have the ability to create an oxidized rhizosphere (Myriophyllum aquaticum) 21,22 were included. The following objectives were investigated: • Is metal uptake by the macrophytes influenced by the sulfide concentrations in the bulk sediment? • Are changes in metal mobility, caused by surface water aeration, reflected in macrophyte metal concentrations? Received: February 29, 2012 Revised: July 31, 2012 Accepted: August 1, 2012 Published: August 1, 2012 Article pubs.acs.org/est © 2012 American Chemical Society 9129 dx.doi.org/10.1021/es300816y | Environ. Sci. Technol. 2012, 46, 9129-9137