Trace elements and δ¹⁵N values were analysed in micronekton (crustaceans, fishes and squids) sampled in the south-western Indian Ocean. Myctophids were associated with high concentrations of arsenic at La Pérouse and MAD-Ridge seamounts, and with lead and manganese at MAD-Ridge and in the Mozambique Channel. The difference in cadmium, copper and zinc concentrations between micronekton broad categories reflected differing metabolic and storage processes. When significant, negative relationships were found between micronekton body size and trace element concentrations, which can possibly be attributed to differing metabolic activity in young and old individuals, dietary shifts and/or dilution effect of growth. No relationships were found between trace element concentrations and δ¹⁵N values of micronekton (except cobalt which decreased with increasing δ¹⁵N values), since most trace elements are not biomagnified in food webs due to regulation and excretion processes within organisms. All trace element pairs were positively correlated in fishes suggesting regulation processes.

Deep-water column micronekton play a key role in oceanic food webs and represent an important trophic link between deep- and shallow-water ecosystems. Thus, the potential impacts of sub-surface hydrocarbon plumes on these organisms are critical to developing a more complete understanding of ocean-wide effects resulting from deep-sea oil spills. This work was designed to advance the understanding of hydrocarbon toxicity in several ecologically important deep-sea micronekton species using controlled laboratory exposures aimed at determining lethal threshold exposure levels. The current study confirmed the results previously determined for five deep-sea micronekton by measuring lethal threshold levels for phenanthrene between 81.2 and 277.5 μg/L. These results were used to calibrate the target lipid model and to calculate a critical target lipid body burden for each species. In addition, an oil solubility model was used to predict the acute toxicity of MC252 crude oil to vertically migrating crustaceans, Janicella spinacauda and Euphausiidae spp., and to compare the predictions with results of a 48-h constant exposure toxicity test with passive-dosing. Results confirmed that the tested deep-sea micronekton appear more sensitive than many other organisms when exposed to dissolved oil, but baseline stress complicated interpretation of results.