An Introduction to krill and sea ice

The crustacean order Euphausiacea, more commonly known as krill, comprises 86 species of shrimp-like zooplanktonic organisms that are distributed throughout the worlds oceans. What sets this group apart from other groups of zooplankton however is the sheer biomass that represents it in the worlds oceans. In 2004, Lancraft et al found that the vast majority of the dominant zooplanktonic groups in an area of the Antarctic ocean were krill species. It is this majority that makes krill a significant source of food within various food chains, providing sustenance for a multitude of marine organisms ranging from fish to some species of seal (most notably the Crabeater) to baleen whales, that take advantage of krill’s swarming behaviour. The fact that some of these organisms are dependent on the abundance of krill means that any significant change in numbers of krill would cause severe consequences for these ecosystems, making krill an important example of a foundation of ‘bottom-up’ ecosystem control (Hariston et al, 1960).

Of course populations of such biomass require a sizeable food supply and it is for the most part algae and phytoplankton that krill rely on for sustenance, thus making the abundance and relative whereabouts of it’s populations reliant on where this food supply is abundant. Therefore, it is worthy of interest why a significant portion of the world’s krill populations reside within the polar regions, most notably the Southern Ocean, where krill species (especially the Antarctic krill, Euphausia superba) are immensely abundant and form the foundation of most Antarctic ecosystems. While krill are also present in the Arctic Ocean, they are not nearly as abundant or significant to the ecosystem, with amphipod and copepod crustaceans being the main source of food (Arndt and Swadling, 2006)

The reason that krill are found in such an extreme environment is that they are also home to immense quantities of algae and phytoplankton on par with those in lower latitudes. What is of particular note is that krill will not only be frequently distributed in the vicinity of sea ice (Smetacek and Nicol, 2005), but have been seen to show increased recruitment following periods of extended sea ice coverage, suggesting a positive relationship between the two (Hewitt et al, 2003). This is noteworthy due to the fact that sea ice would normally not permit significant planktonic abundance beneath it due to the severe limitations it puts on light entering the water column, thus limiting the presence of autotrophic organisms and organisms that feed on them.

Before I explore the relationship between krill and sea ice, it must first be noted that this sea ice, while being a regular feature of the polar environment, is constantly dynamic. The area of ocean it covers at both poles will increase during winter months and then thaw in the spring and summer months. This remains true to varying degrees for the Arctic and Antarctic, with Antarctic sea ice cover exhibiting a greater degree of seasonal change than that of the Arctic, due to the fact that the Arctic sea ice is largely land-locked. (Arndt and Swadling, 2006). This process of expansion and diminishment occurs on a massive scale, with Antartica’s sea ice expanding to roughly five times it’s normal coverage every year, making the presence of sea ice a significant factor affecting how polar ecosystems function. Also, the ephemeral nature of sea ice means that organisms must develop a certain plasticity in their morphologies and life history strategies to associate themselves with it.

The characteristic of sea ice that intrinsically links it to the surrounding ecosystem is it’s role of providing a habitat for autotrophic organisms such as algae to grow. Firstly, it is thoroughly permeated by a network of tubes filled with brine that play host to communities of biota, the majority of which are diatoms (Brierly and Thomas, 2002). The underside of the sea ice also acts as a substratum for algae, providing a more readily available food source for grazers. This, combined with the release of trapped nutrients and organisms upon the thaw of the ice provides a significant source of food, as well as a diet supplement, for krill and the ecosystem as a whole.

As well as utilizing the sea ice habitat for food, krill are also capable of using crevices in the ice above them as a shelter from predators, both as adults and during their larval stages, indicating a behavioral adaption to the sea ice as well as adaptions in life history.

It is the extent of krill’s reliance upon sea ice as a food source and a shelter for both larvae and adults, as well as it’s significance for the ecosystems krill plays a role in and how this intricate link with this ephemeral habitat will be affected by the ongoing threat of climate change that I will explore.

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