Heavy metals, trace elements and sediment geochemistry at four Mediterranean fish farms
Highlights
► Trace elements levels in sediments were studied in 4 Mediterranean fish farms. ► Sediment grain size, organic content and redox affect element behavior. ► For all elements, impacted anoxic sediments had higher levels than reference sites. ► Some elements in oxic sediments had lower levels at impacted than reference sites. ► Cu, Zn, Fe can affect aquatic life in anoxic-fine sediments, As in all sediment types.
Introduction
The distribution and fate of elements in marine sediments received considerable attention in the 1970s following the Minamata incident and, as a result, comprehensive protection of public health from the most toxic heavy metals and elements in seafood was achieved (McIntyre, 1995). It is known that changes in the redox regime, free sulphide and organic matter affect the behavior of various metals and element species (Belias et al., 2003, Sutherland et al., 2007, Brooks and Mahnken, 2003, Russell et al., 2011, Jaysankar et al., 2009, Sarkar et al., 2004, Smith et al., 2005, Chou et al., 2002, Peltola et al., 2011). Such changes are known to occur in the proliferating dead zones where the seabed is anoxic (Diaz and Rosenberg, 2008).
Sediments consist of inorganic and organic particles with complex physical, chemical and biological characteristics (Sarkar et al., 2004). They can scavenge some elements, thus acting as an adsorptive sink with metal concentrations many times greater than in the water column. Sediments are therefore an appropriate matrix in which to monitor contamination (Sarkar et al., 2004, Giarratano and Amin, 2010, Cukrov et al., 2011, Carbonell et al., 1998). However, sediments are not only a sink but also a possible delayed source of these contaminants into the water column due to desorption, remobilization processes, redox reactions and degradation of sorptive substances (Cukrov et al., 2011, Sarkar et al., 2004, Christophoridis et al., 2009). Therefore metal contamination of surficial sediments could directly affect seawater quality (Mendiguchía et al., 2006) and constitute a long-term source of contamination to the food web (Burton, 2002, Cukrov et al., 2011).
Three-quarters of wild fish stocks are fully fished, overfished or depleted according to the Food and Agriculture Organization (FAO) (Giarratano and Amin, 2010). Aquaculture currently provides a considerable proportion of edible fish which is expected to increase in future decades in order to meet the needs of the growing human population (Duarte et al., 2009). Fish farm wastes (uneaten feed, faeces, metabolic excretion products and effluent chemical species such as medicines) can accumulate on sediments below or near the fish cages. This organic material represents a potential risk of contamination to the wider environment, exhibiting a variety of biological, chemical and ecological effects (Schendel et al., 2004, Salazar and Saldana, 2007). Much of the environmental impact of fish farms is local (Kalantzi and Karakassis, 2006), but fish farms can generate long-lived, persistent contaminants that may have (far-field) long range environmental effects. In this case metals and trace elements can be used as tracers to estimate the (far-field) long range influence of aquaculture (Smith et al., 2005). Even though the accumulation of heavy metals and trace elements in sediments below aquaculture cages has been identified to have a significant impact on benthic communities (Salazar and Saldana, 2007) it has received relatively little attention (Dean et al., 2007, Jaysankar et al., 2009).
The Mediterranean is one of the most oligotrophic marine water bodies in the world. This is reflected in the low carbon/high redox conditions prevailing in marine sediments (Karakassis and Eleftheriou, 1997). Fish farming sites occasionally represent “hypoxic or anoxic islands” in this highly oligotrophic continuum which are likely to induce changes in metal and element behavior and their interactions with the local marine organisms. In the Mediterranean, there has been limited study on the effects of heavy metals and trace element enrichment of sediments under fish cages, with research thus far focused mainly on Cu, Zn, Pb, Cd and Fe (Belias et al., 2003, Basaran et al., 2010, Pergent et al., 1999).
The objective of this study was to use state-of-the-art analytical techniques to determine the concentration of a wide range of metals and trace elements in a large number of samples originating from several fish farms with different background conditions. Simultaneously the determination of relevant geochemical variables was carried out in order to investigate the factors that control elemental distributions in sediments.
Section snippets
Study areas
Sediment samples were collected at four seabass (Dicentrarchus labrax) and gilthead seabream (Sparus auratus) farms in Greece — two located in the Aegean Sea (AEG1 and AEG2) and two in the Ionian Sea (ION1 and ION2). The sampling areas were selected from a larger group of fish farms in an effort to maximize variance in terms of physical (water temperature, water content, depth, sediment particle size) and geochemical characteristics (redox, sulphide, chl-a, organic matter) in the data set (
Results
The mean values of the surface sediment physical (depth, water temperature, sediment water content, grain size) and chemical (surface and subsurface redox potential, sulphides, labile and refractory organic matter, chlorophyll-a, phaeopigments, TON, TOC) characteristics are shown in Table 1. The mean values of sediment element concentrations at the four fish farms are included in Table 2.
MDS ordination analysis of metal concentration data (Fig. 1) showed that the samples clustered visibly
Elements distribution in sediment
In the present study, the concentrations of most elements in the sediments at the sampling stations directly beneath and close to the fish cages were significantly higher than in the respective reference stations despite the fact that some of these elements were not found in elevated concentrations in the fish feed, which is the main source of material precipitating on these sediments. Thus these differences can not only be explained by feed but also by other factors such as the environmental
Conclusions
Uneaten fish feed and faeces are the main sources of elements in the sedimentary environment surrounding fish farms. Sediments around fish cages are an important scavenger of elements due to the changes of environmental conditions induced by aquaculture industry. Fine-grain size particles, high organic content, sulphide enrichment and low redox potential are the main factors that control element accumulation and determine how available the elements are to aquatic life. The effects of elements
Acknowledgments
This study was partly supported by the European 6th Framework Programme (ECASA project, project no. 006540). Thanks are due to I. Glampedakis, V.N. Kouroubalis, S. Kiparissis, N. Gotsis, K. Sevastou, I. Magiopoulos and V. Kalogeropoulou for assistance with sampling. Owners of the four fish farms have kindly provided access to their site. Helpful comments and suggestions by three anonymous reviewers are gratefully acknowledged.
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