Trophic relationships and health risk assessments of trace metals in the aquaculture pond ecosystem of Pearl River Delta, China

Abstract

Cadmium, lead, zinc, Chromium, copper, nickel and manganese in sediments and in aquatic organisms were collected from the aquaculture pond ecosystem of the Pearl River Delta (PRD), China and analyzed to evaluate bioaccumulation and trophic transfer in food chains, as well as the potential health risk of exposure to the Hong Kong residents via dietary intake of these aquatic products. The results revealed that based on the biota–sediment accumulation factor, omnivorous fish and zooplankton accumulated more trace metals from sediment than carnivorous fish. Concentrations of seven trace metals in aquaculture pond of PRD significantly decreased with increasing trophic levels, showing that these trace metals were trophically diluted in predatory and omnivorous food chains. The hazard index values of all fish species were smaller than 1 for adults and children, indicating there was no health risk from the multiple metals via ingestion of the freshwater fish for the inhabitants.

Introduction

The Pearl River Delta (PRD) is an important area for agricultural, commercial, and industrial development of China. The Pearl River Delta has undergone rapid economic development with significant population increase during the last three decades. Environmental contamination by trace metals and petroleum hydrocarbons, caused by anthropogenic activities, has been reported in recent studies (Cheung et al., 2008). In PRD region, most of the fish farmers are still using the traditional way of filling up fish ponds using river water (Ruddle and Zhung, 1988). Through atmospheric deposition, sewage outfalls, urban storm water and agricultural and industrial runoff, trace metals may enter fish ponds. In addition, the fish feeds have been blamed as the major sources of some trace metals to aquaculture (Lacerda et al., 2006, Lacerda et al., 2011). Sediments accumulate higher levels of trace metals than water, causing serious problems due to their toxicity and propensity to bioaccumulate (Chen et al., 2000). If toxic metals are accumulated in fish tissues exceeding maximum permitted concentrations, it will pose a human health risk.

The anthropogenic metal contribution in sediment can be estimated from enrichment relative to uncontaminated reference materials or widely accepted background levels. Enrichment factor (EF) and geoaccumulation index (I_geo) are two pollution indicators wildly used to assess sediment quality (Loska et al., 1997, Diaz-de Alba et al., 2011). EFs are used as an indicator to reflect the degree of environmental contamination by comparing with background values representative for uncontaminated sample materials (Presley et al., 1992). I_geo determines the status of contamination by comparing current metal contents with preindustrial levels (Müller, 1981). The concentration accepted as background value is multiplied each time by the constant 1.5 in order to take into account natural fluctuations of a given substance in the environment as well as very small anthropogenic influences (Loska et al., 1997). EF reflects the degree of environmental contamination by comparison with background values representative for uncontaminated sample materials. Iron or aluminum is commonly used as a normalization element to reduce the variations produced by heterogeneous sediment (Loska et al., 1997). The concentration ratio of ¹⁵N/¹⁴N, expressed relative to a standard (i.e., δ¹⁵N), has been shown to increase with increasing trophic level because of the preferential excretion of lighter nitrogen isotopes (Deniro and Epstein, 1981, Minagawa and Wada, 1984). This technique has been used as a tool to determine the trophic relationships and to estimate the biomagnifications of organic and inorganic contaminants in aquatic ecosystems (Campbell et al., 2005, Cui et al., 2011).

Most studies on trace metal contamination in aquaculture environments in the PRD are focused on offshore aquaculture (Qiu et al., 2011) comparatively, there is a lack of information concerning fresh water fish pond environments (Cheung et al., 2008). In addition, most studies investigated the quality and risk assessment of sediment or fish tissue samples, and trace metal transport from sediment to organisms (bioaccumulation) (Qiu et al., 2011). In actually, However, aquatic organisms can also accumulate trace metals from their feeds and with trophic transfer and the potential for biomagnification. This information is very useful for fisheries management in freshwater fish pond environment. This is the first study to use stable isotopes of nitrogen to investigate the biomagnification rates of trace metals in various species of fish at different trophic levels in aquaculture pond ecosystems in the Pearl River Delta, China. With increasing public awareness concerning the safety of foods originating from mainland China, the major objectives of the present study were to investigate the trace metal contents contained in the freshwater aquatic products, the trace metals bioaccumulation and biomagnification in aquatic food chains of freshwater fish ponds in the PRD (the main source for consumption in Hong Kong), and evaluate the potential health risks of exposure to the Hong Kong residents via dietary intake of these products.