Toxicity of physically and chemically dispersed fuels to the common Antarctic amphipod, Paramoera walkeri

Brief description

This metadata record contains the results from bioassays conducted to show the response of the common Antarctic amphipod, Paramoera walkeri to contamination from combinations of Special Antarctic Blend (SAB) diesel, Marine Gas Oil (MGO) and Intermediate Fuel Oil (IFO 180), chemically dispersed with fuel dispersants Ardrox 6120 and Slickgone NS.

Fuel only water accommodated fractions (WAF), chemically enhanced water accommodated fractions (CEWAF) and dispersant only treatments were prepared following the methods in Singer et al. (2000) with adaptations from Barron and Ka’aihue (2003). WAF was made using the ratio of 1: 25 (v/v), fuel to filtered seawater (FSW) following the methods of Brown et al. (in prep). Ratios for chemically dispersed treatments were 1: 100 (v/v), fuel to FSW and 1: 20 (v/v) dispersant to fuel. Dispersant only treatments were made using ratios for CEWAF, substituting the fuel component with FSW. Mixes were made in 5 L or 10 L glass aspirator bottles using a magnetic stirrer to achieve a vortex of 20-25% in the FSW before the addition of test media. The same mixing energy was used to prepare all WAFs for enhanced reproducibility and comparability of results (Barron and Ka’aihue, 2003). Mixes were stirred in darkness to prevent bacterial growth for 42 h with an additional settling time of 6 h at 0 plus or minus 1 oC. Extended stirring times were used following the recommendations determined as part of the hydrocarbon chemistry component of this project (Kotzakoulakis, unpublished data).

A dilution series of four concentrations were made from the full strength aqueous phase of each mix using serial dilution. WAF test concentrations were 100%, 50%, 20% and 10% while CEWAF concentrations were 10%, 5%, 1% and 0.1%. These concentrations were chosen in order to quantify the mortality curve and allow statistical calculation of LC50 values. To facilitate comparisons of dispersant toxicity in the presence and absence of fuel, dispersant only test concentrations reflected those of CEWAF treatments. WAF was sealed in airtight glass bottles stored at 0 plus or minus 1 oC for a maximum of 3 h before use. Fresh test solutions were prepared every four days to ensure consistent water quality and replace hydrocarbons that adsorbed or evaporated into the atmosphere.

Each test concentration was represented by five replicates with five FSW control beakers, with 10 P. walkeri individuals per replicate. Only healthy and active individuals were chosen with a size range of 7.9 plus or minus 0.7 mm for adults and 2.5 plus or minus 0.2 for juveniles measured from the base of the antennae to the widest part of the dorsal curve. Larger individuals and brooding females were not used to avoid unrelated deaths related to age or reproductive state (Sagar, 1980). Beakers were filled to 200 ml and were left open to allow the natural evaporation of lighter monoaromatic hydrocarbon components that would occur during a real spill. A small square of plankton mesh was placed in each jar to provide a substratum to reduce the stress of laboratory conditions and to help to stem cannibalism. Animals were not fed during experiments to avoid hydrocarbons adsorbed onto food pellets being ingested by the amphipods, thereby introducing an additional exposure pathway.

Experiments ran for a total of 12 d exposure duration. Experiments were run in cold temperature-controlled cabinets maintained at a temperature of 0 plus or minus 1 oC, fluorescent lights in the cabinets were set to a light regime of 18 h light, 6 h darkness, following the methods in Brown et al. (2017) to reflect Antarctic summer environmental conditions. Lethal and sublethal observations were made at standard ecotoxicology test times of 24 h, 48 h, 96 h, 7 d, 10 d and 12 d, with an additional observation at 8 d coinciding with one of the 4-day water changes. The health status of each individual was classified on a scale of one to four; one showing no effect up to four being mortality. Mortality was determined by a lack of movement and response to stimuli, particularly in the gills. Dead animals were removed and preserved in 80% ethanol at each observation period. Missing amphipods that may have been cannibalised were included in mortality counts as they were likely to have been moribund or already dead when eaten.

In order to simulate a repeated pulse pollutant, 90 to 100% of the test solution volume of each beaker was renewed with freshly made test concentrations every four days to replenish hydrocarbons lost through evaporation and adsorption and ensure consistent water quality. Beakers were topped up to 200 ml between water changes with deionised water to maintain water quality parameters.

Duplicate 25 ml aliquots of test concentrations were taken at the beginning and end of each experiment in addition to pre and post water change samples. Samples were immediately extracted with 0.7 μm of dichloromethane spiked with an internal standard of BrC20 (1-bromoeicosane) and cyclooctane. Samples were analysed using Gas Chromatography with Flame Ionisation Detection (GC-FID) and mass spectrometry (GC-MS). To determine actual exposure concentrations, four day measured TPH values were used to create a continuous exposure and evaporation profile over the 12 d test period following the methods outlined in Payne et al. (2014) and Brown et al. (2017).