Determining the bioavailability of contaminants and assessing the quality of sediments

Background, Aims and Scope Our study determined the role of light, aeration, grain size, amount of sediments and of water, density of animals and presence of additional food on the movement of Ilyanassa obsoleta. Spiked relative to reference sediments were also tested for avoidance or preference by the mud snails. Our goal was to develop exposure conditions to be used in future assessment studies of the environmental quality of sediments inhabited by mud snails. The behavioural response of the snails towards different concentrations of contaminated Halifax Harbour sediments placed in one half of a tank relative to reference sediments in the other half is examined in Part 2 and a chemical link investigated. Methods Animals were exposed in 7 L aquaria at 12°C to their native sediments placed in one or two halves of a tank and the effect due to the presence of a light source, air bubbles, sediment grain size, added algae examined relative to the snails' behaviour. Preference or avoidance of one half of a tank relative to the second half, the immersed or dry glass surface represented the various media to be chosen by snails. After choosing the optimum set up, exposures were performed in triplicate, under slightly differing initial placement of the snails to compare means and variability in the results. Three specific biogenic chemicals, cholesterol, coprostanol and a fatty acid methyl ester (FAME) were spiked at 0.002, 0.01, 0.05, 0.25 and 1.2 mg/g, dry weight of sediment. A hexane, dichloromethane and methanol extract of harbour sediment were also spiked in reference sediments. In all cases, the location of the snails was recorded over a period extending up to 96 hrs. Results Twenty snails exposed during a 72 hrs period in commercial 7 L tanks containing 50 g of sediments (wet weight) in the bottom half of a tank filled half way with seawater represented the chosen conditions to test the avoidance/preference behaviour of snails for part of a tank. The presence of additional food on sediments attracted snails within 24 hrs of exposure. Discussion A general lack of preference was indicated in exposures to three biologically derived compounds spiked in reference sediments, i.e. a C-18 FAME, cholesterol and coprostanol. The repelling properties of a harbour sediment extract pointed to a role for non polar chemicals. Conclusions It is proposed that behaviour reflects a balance between the repelling effects of contaminants and the attractive value of food. The present study provides a robust green assessment tool that is easy to set up to assess the quality of sediments. Recommendations and Perspectives The interpretation of field observations regarding the abundance of animals during environmental assessments must consider both nutritional and toxicological causes. Additional laboratory exposures are needed to determine which chemicals play a role in the field.