Protozoans as a food source for Antarctic krill, Euphausia superba: Complementary insights from stomach content, fatty acids, and stable isotopes

Abstract

We studied the diet of Antarctic krill, Euphausia superba, at five stations across the southwest Atlantic sector in summer 2003 by analyzing stomach content, fatty acids, and stable isotopes on the same individuals. Our aim was to examine what each method could contribute to our understanding of krill nutrition and whether differences seen in growth rates were linked to their food. All three methods indicated clear regional differences in diet, but small ontogenetic and sex-related differences. Overall, diatoms were the most abundant item in the stomach, but at three of the stations, tintinnids, large dinoflagellates, and other armored flagellates dominated the identifiable biomass. Copepod remains were rare. Fatty acids profiles gave additional information about feeding on weakly silicified diatoms and athecate heterotrophic dinoflagellates, with the latter being the main food source at one of the stations. Two independent indices of carnivory, d 15 N and the fatty acid ratio 18:1(n-9)/18:1(n-7), were correlated among krill from the same swarm, suggesting consistent differences in diet between individuals. An internal index of trophic position, (i.e., d 15 N glutamic acid -d 15 N phenylalanine ) underlined the importance of heterotrophic food for the nutrition of krill, even in summer. Highest growth rates of krill were found during a diatom bloom and coincided with a mixed diet, large digestive gland, and fast stomach passage. However, even in a nonbloom, flagellate-dominated system, krill were able to sustain medium growth rates when feeding on heterotrophic dinoflagellates. Each method supplied specific information on krill nutrition, and the true picture is only revealed when the various methods are used together.Studies of trophic relationships are central to our understanding of ecosystems. On one hand, they shed light onto the feeding ecology of individual species. On the other hand, they indicate general paths of nutrient and energy transfer through the food web. At the base of pelagic food webs are various algal groups and microbes whose importance for sustaining higher trophic levels changes, both regionally and during phytoplankton succession (Kiørboe 1993). Those qualitative and quantitative differences in diet are reflected in the biochemical composition and physiology of grazers and secondary consumers (St. John and Lund 1996).A range of methods has been used to study trophic relationships, with each having strengths and drawbacks. Stomach content analysis is the most direct approach, with food particles examined under the microscope. However, items differ in their digestibility and therefore recognition, and results only represent the recently ingested food. Feeding incubations give insights into food selectivity and ingestion rates, but might suffer from perturbation of complex natural food assemblages by confinement. The use of bioindicators, which get passed on from food sources to the consumer, is an alternative approach. Some fatty acids are specific to certain algal groups or to cop...

15N/14N ratios of amino acids as a tool for studying terrestrial food webs: a case study of terrestrial insects (bees, wasps, and hornets)

“…As a result, CSIA of amino acids overcomes several of the challenges associated with bulk isotopic methods (e.g., O'Reilly et al 2002;Post 2002). Consequently, the CSIA of amino acids has been used to elucidate the trophic position of organisms in recent ecological studies of shrimp in the sub-Antarctic archipelago (Pakhomov et al 2004), krill in the Antarctic (Schmidt et al 2004(Schmidt et al , 2006, plankton in the central Pacific (McCarthy et al 2007), yellow-fin tuna in the eastern tropical Pacific (Popp et al 2007), zooplankton near Hawaii (Hannides et al 2009), and penguin in the Southern Ocean (Lorrain et al 2009). Recent evidence suggests that CSIA of amino acids is also useful in the evaluation of terrestrial food webs (Chikaraishi et al 2010a;Naito et al 2010;Styring et al 2010).…”

Role of krill versus bottom-up factors in controlling phytoplankton biomass in the northern Antarctic waters of South Georgia

“…How dense krill swarms interact with their food sources in shallow shelf regions is poorly known and beyond the scope of the present dataset. However, a possible explanation for our weaker on-shelf GLM relationships is that the krill here had switched to alternative food sources such as copepods or heterotrophic flagellates (Atkinson & Snÿder 1997, Pakhomov et al 1997a, Schmidt et al 2006. Indeed, in any area with several hundred g of krill under 1 m 2 , our calculated removal of about onethird of phytoplankton stocks per day is clearly unsustainable (Atkinson et al 2001).…”

“…The actual mechanisms of the krill-food interaction would probably occur at a smaller spatial scale, for example, at the scales of mobile krill swarms that contain actively feeding individuals (Antezana & Ray 1984, Schmidt et al 2006). However, logic would suggest that the mechanism of the various bottom-up and top-down controls varies greatly in time and space.…”

Comparisons of Feeding Ecology of Euphausia pacifica from Korean Waters Using Lipid Composition

“…또한, 18:1(n-9)는 후생동물에서 흔하게 나타나 는 지방산이며, 18:1(n-7)은 규조류 지시자 역할을 하는 지방산 16:1(n-7)의 사슬 연장(chain-elongation)으로부터 유래하기에 이들의 구성비는 동물플랑크톤의 섭식성향(육 식, 초식 또는 잡식성)을 지시하는 지표로도 활용되고 있 다 (Hagen et al 2007;Schmidt et al 2006 Table 2). 단일불포화지방산 중, 주로 식물플랑크톤에서 기원하는 것으로 알려진 지방산 16:1(n-7)과 18:1(n-7) (Sargent et al 1987;Falk-Petersen et al 2000)은 동중국해의 개체에 서 가장 낮게 나타났으나(약 3%), 와편모조류로부터 주로 기원하는 것으로 알려진 22:6(n-3) (Graeve et al 1994b;Falk-Petersen et al 2000)은 높게 나타나(13.5 ± 0.6%) 와 편모조류를 포함하는 미세조류가 채집 당시 동중국해 크 릴의 주요 먹이원이었던 것으로 추정할 수 있다.…”