I like to move it, move it: Krill boogie down all year

Kane, Mary K., et al. “Krill motion in the Southern Ocean: quantifying in situ krill movement behaviors and distributions during the late austral autumn and spring.” Limnology and Oceanography (2018). DOI: 10.1002/lno.11024

Figure 1 – Krill as imaged by the krill camera in the Antarctic (Adapted from Kane et al., 2018).

Krill, the shrimp-like plankton that whale like to snack on, are important little buggers in many of the world’s oceans (Fig. 1). In Antarctic ecosystems in particular, they are productive nutrient recyclers, critical to fluxing carbon to the sea floor, and are the major link between microscopic photosynthesizes and larger organisms. Scientists have long understood how important Antarctic krill are, but have a lot of trouble studying them. The Southern Ocean in general is incredibly hard to sample due to adverse weather conditions. Moreover, plankton tend move around quite a bit making it difficult to study their behavior without pulling them out of the water.

The challenges associated with studying krill have led scientists to make some assumptions about where they live and how they behave. For decades, the story went like this: krill live exclusively in the upper ocean during the summer months, feeding on phytoplankton and being eaten by bigger animals. In the winter, the krill move directly under sea ice in preparation for the algal blooms associated with spring melt. This accepted dogma, combined with limited technological capabilities, meant that most studies of krill focused on the upper water column in the spring and summer months.

Figure 2 – Deploying the krill camera. The cameras are in a row along the time of the frame (Adapted from Kane et al., 2018).

Scientists have begun to look past these assumptions and are coming up with new ideas about how krill live and influence their environment. Mary Kane, a PhD student at the University of Rhode Island, and her colleagues wanted to test some of these new theories. In particular, they wanted to look at how krill are distributed from the surface to the bottom, where they live at different times of year, and how they move. To do this, Kane need a new sort of tool, one that would be able to count the organisms at many different depths and record their behavior.

Kane and her team developed a camera-based approach to look at krill at high spatial resolution – each image provides a snap shot for counting individuals in the population at a given depth ­– and densely sampled in time – individuals can be tracked in a series of frames (Fig. 2). The cameras, and a suite of environmental sensors, were mounted onto a frame that could be raised and lowered from a crane on the deck of a ship.

The group selected three bays along the West Antarctic Peninsula to deploy their instrument. They sampled each site several times in both early spring and late fall, sending their camera all the way to the bottom to look at krill all the way through the water column. Images from 37 deployments were analyzed to count and track krill.

Based on that analysis, and observation of the coincident environmental data, Kane found that krill are more active in the winter and live deeper than previously thought. The image data revealed that they hang out near the bottom of the ocean all year. Krill tend to cluster close the surface in warmer months to feed on phytoplankton, but were still found near the bottom. In the winter, they almost exclusively lived close to

Figure 3 – A model of krill behavior. The panel on the left shows the krill hanging around the bottom in the winter. The right depicts them eating phytoplankton at the surface (Adapted from Kane et al., 2018).

the bottom, presumably feeding on organic matter in the sediment (Fig. 3). Moreover, the data suggested that the krill actively swim year around as opposed to lying dormant beneath the sea ice.

Kane’s observations indicate that scientists need to reconsider the role of krill in in Antarctic food webs and nutrient cycling throughout the year. For example, krill might be responsible for bring more iron, a critical element for photosynthesizers, to the surface. Understanding such cryptic connections is crucial to assessing how these high latitude regions might respond to continued climate shifts. In order to do so, many more studies, using creative techniques like Kane’s, will need to be done. This is no small order and one that will require cooperation on the part of many actors, scientific and otherwise.

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