Fatty acid composition in the endemic Humboldt Current krill, Euphausia mucronata (Crustacea, Euphausiacea) in relation to the phytoplankton community and oceanographic variability off Dichato coast in central Chile

Abstract

Fatty acids (FA) have been used as diet biomarkers. They have proven to be a valuable method of defining food web relationships, trophic positioning, and the dietary behaviors of marine species. The endemic krill species of the Humboldt Current System (HCS), Euphausia mucronata, is the most abundant krill species over the continental shelf, usually associated with coastal upwelling zones. This study aimed to quantify the changes in the FA composition of E. mucronata in the coastal upwelling zone off Dichato in central Chile concerning the phytoplankton community and environmental variability. We hypothesized that E. mucronata changes its FA composition according to the functional species groups that structure the phytoplankton community as well as environmental variability. Krill, phytoplankton, and environmental datasets were analyzed using multivariate analyses, considering samplings from January 2013 to August 2014. The total concentration of FAs was 550.5 ng/g (n = 42) of which the 77% corresponded to six FAs: palmitoleic acid (16:1n7), palmitic acid (16:0), vaccenic acid (18:1), oleic acid (18:1n7), eicosapentaenoic acid (EPA, 20:5n3) and docosahexaenoic acid (DHA, 22:6n3). FA composition did not change according to neither the season nor sampling date. Seasonal changes were observed in both community structure of the phytoplankton (i.e. species composition and functional groups) and sea temperature. EPA/DHA ratio, PUFA/SFA ratio and carnivory variability were seasonally different suggesting that E. mucronata had mostly a diatom-based herbivorous habit during austral spring and summer and, it could have omnivore or carnivore habits during austral autumn and winter. Diatom and ciliate abundances and, sea temperature were the best predictor variables explaining 64% of the total variation in the FA composition. This study suggests that, despite seasonal environmental variation (biotic and abiotic), this krill species fed mainly diatoms and could shift its trophic habit to carnivory according to season of the year.