Euphausia superba – A key organism in the Antarctic marine ecosystem
The Antarctic krill Euphausia superba is a circumantarctic species that in the summer months sometimes forms, in the top 100 metres of the water column, huge swarms that can also be observed directly on the surface and make the water turn red. The animals display periodic daily vertical migrations connected with their feeding activity. During the day krill are found in water layers between 40 and 100 m while at night they rise to layers near the surface and primarily feed on microalgae [1].
Antarctic krill, Euphausia superba (photo: Carsten Pape)
The Antarctic krill Euphausia superba (Fig. 1) lives in the Southern Ocean. The adult animals feed on microalgae and zooplankton in the upper 200 metres of the water column and attain a maximum size of 68 millimetres. Females reach sexual maturity after two years of age, whereas males after three years. The species has an unusually long life span for a plankton organism of 5 to 7 years.
Euphausia superba reproduces during the Antarctic summer between December and March. After spawning, eggs sink to a depth of 800 to 1000 metres where the first larval stage (nauplius I) hatches and makes its way to the surface. While doing so, the young krill go through several larval stages. While the nauplius and metanauplius stage lives on its yolk reserves, the calyptopsis I larvae are the first stage that reaches the top water layer, where the larvae feed on the ample numbers of microalgae living there. This is followed by eight additional larval stages (calyptopis II, III, furcilia I-VI), which develop from the Antarctic autumn and winter to a juvenile animal in the following spring. Juveniles reach a size of 15 mm and look like adult animals. Development into a juvenile takes approx. 6 months.
Role in food chain: the key role of Antarctic krill in marine food chain of the Southern Ocean. (graphic: Alfred Wegener Institut)
Krill play a key role in the marine Antarctic ecosystem since they represent the main food source for numerous penguins, seals and baleen whales as well as sea birds and fish (Fig. 2). The abundance of krill in the Southern Ocean is estimated to be between 100 and 500 million tons [2]. To put this amount into perspective, annually, less than 100 million tonnes of all species of fish and shelfish are currently harvested from the world’s oceans. In the beginning of the 1970s a significant commercial krill fishery started and ever since this has been with 150,000 tons caught the largest in the Southern Ocean [3].
Long-term regional studies have shown that the krill stock is subject to substantial fluctuations from year to year that are connected to global climate change. I In the southwestern Atlantic sector of the Southern Ocean, which contains more than 50% of the krill stock, in the last 30 years the population has declined from over 100 per m2 by up to 80% [4]. Because of the key role of krill in the food web, this decline has a considerable influence on the stock of its consumers. In the area of the western Antarctic peninsula, which represents one of the most rapidly warming regions on the globe, the example of the Adelie penguin has shown that the decline in the krill stock is accompanied by a decrease in breeding success and consequently in the penguin stock as well [5].
The long-term studies have also demonstrated that the population density of krill in summer evidently depends on the preceding extent and duration of the winter sea ice cover [4]. It is assumed that a prolonged winter sea ice cover favours survival of the following generation during the winter as well as reproduction of the adult krill in the following spring [6, 7].
The exact dependence of krill on sea ice has not been clarified as yet. Studies in recent years have shown that the various development stages of krill (larvae, juveniles and adults) differ in their dependence on sea ice and the associated biotic community. The dependence decreases with increasing age of the krill. While the winter sea ice cover and the related biocoenosis play a large role for the larvae, this is not essential for the adult animals. On the other hand, the extent of algal bloom in spring appears to have a significant influence on the reproduction success of the adults [3, 6-8].
In general, there is little understanding of the krill’s adaptation mechanisms to its environment, characterised by extreme seasonal changes (e.g. food supply, ice cover and length of daylight), as well as to the long-term changes caused by global warming (e.g. decline in sea ice cover, change in quantity and quality of food, rise in seawater temperature). The working group “Antarctic krill”, established at the Alfred Wegener Institute in 1999, focuses on these mechanisms. This knowledge is essential in order to gain an understanding of the influence of various scenarios of global warming on the lifecycle of krill and, in the end, on the marine Antarctic food web.
[1] V. Siegel: Krill und Salpen im antarktischen Ökosystem. In: G. Hempel, I. Hempel, S. Schiel (ed.): Faszination Meersforschung. Ein ökologisches Lesebuch. Hauschild. Bremen 2006. – [2] V. Siegel, Polar Biology 29, 1 (2005). – [3] B. Meyer: Ecophysiological studies on the overwintering of Antarctic krill, Euphausia superba – A Synopsis. Postdoctoral thesis for acquiring venia legendi for the subject of Marine Biology in Department 2 (Biology/Chemistry). University of Bremen 2009. – [4] A. Atkinson et al. Nature 432, 100 (2004). – [5] H. W. Ducklow et al., Philosophical Transactions of the Royal Society of London Series B 362, 67 (2007). – [6] B. Meyer et al., Limnology and Oceanography 54, 1595 (2009). – [7] B. Meyer et al, Marine Ecology Progress Series 398, 1 (2010). – [8] B. Meyer: Ecophysiological studies of overwintering krill. In: G. Hempel, I. Hempel (ed.): Biological studies in Polar Oceans - Exploration of life in icy waters. Wirtschaftsverlag NW. Bremerhaven 2009.
Dr. Bettina Meyer, Alfred Wegener Institute, Bremerhaven