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Antarctic krill, stuck with rocks in a hard place

Fuentes, V., G. Alurralde, B. Meyer, G. E. Aguirre, A. Canepa, A.-C. Wölfl, H. C. Hass, G. N. Williams, and I. R. Schloss (2016), Glacial melting: an overlooked threat to Antarctic krill, Scientific Reports, 6, 27234, doi:10.1038/srep27234.

Over the last several years, krill have been washing up on Antarctic beaches by the hundreds of thousands. Krill are small crustaceans that live in the open ocean and feed mostly on phytoplankton (tiny ocean plants). In the Southern Ocean, krill are the main food source for penguins, whales, and seals. They exist in huge numbers, enough to support all of these top predators, and they are found in the greatest densities close to shore. A group of researchers led by Veronica Fuentes from the Institute of Marine Sciences in Barcelona, Spain, documented fifteen mass strandings of krill in Potter Cove on King George Island, just north of the Antarctic Peninsula between 2003 and 2012.

Mass strandings of marine animals are common, but poorly understood occurrences. On coasts all over the world, dolphins, whales, sea turtles, and jellyfish are often found washed up on shore in great numbers. The largest recorded whale stranding occurred just last summer in Patagonia, Chile, when over 300 sei whales were found dead on beaches because of a toxic red tide, something that has led to mass strandings of marine mammals for millions of years. Pilot whales tend to swim towards shore when they are ill and, since whales are social creatures, many others often follow the sick whale. They become stranded when the tide goes out. The same phenomenon has also been observed in Cape Cod Bay, where the waters are shallow and the ebbing tide left over a hundred common dolphins stranded in 2012. Ship strikes, shark attacks, ingestion of marine debris, disorientation, harmful algal blooms, and sound levels from ship traffic, sonar, and oil drilling have all been suggested as possible reasons behind mass strandings.

The krill that washed up on King George Island were stranded for a different reason. In each of these mass stranding events, the krill were found with sediment clogging their digestive systems, sediment that came from melting glaciers.

Figure 1 - Map of area where krill were stranded. King George Island is just north of the Antarctic Peninsula and is almost entirely covered by a glacier. The shoreline where krill were found stranded is across Potter Cove to the south of Fourcade Glacier. When strong winds blow from the north, more sediment particles from glacier meltwater streams enter the cove. (Figure 1a in the paper.)

Figure 1 – Map of area where krill were stranded. King George Island is just north of the Antarctic Peninsula and is almost entirely covered by a glacier. The shoreline where krill were found stranded is across Potter Cove to the south of Fourcade Glacier. When strong winds blow from the north, more sediment particles from glacier meltwater streams enter the cove. (Figure 1a in the paper.)

Figure 2 – Photos of krill strandings. Krill washed up on the southern shore of Potter Cover with densities of ~1000 individuals per square meter. They made an easy meal for Gentoo penguins. (Figure 2 in the paper.)

Figure 2 – Photos of krill strandings. Krill washed up on the southern shore of Potter Cover with densities of ~1000 individuals per square meter. They made an easy meal for Gentoo penguins. (Figure 2 in the paper.)

The islands around Antarctica are rocky and covered by glaciers. When glaciers melt, they expose the boulders, rocks, and sediments beneath. Meltwater streams carry particles out to sea and deposit them into the waters where krill feed. Krill are used to feeding on things with hard exteriors – diatoms are a favorite phytoplankton food choice, and they have a hard glass shell. Krill can crush the shells open and press the enclosed material through a fine filter into their stomachs. Large rock particles present a problem, however, because they can’t be crushed open or squeezed through the filter. Instead, they get stuck, and block the filter from letting in actual food particles.

Fuentes and her colleagues conducted some experiments with krill in the lab to see how sediment particles of different sizes affected their ability to feed. First, they checked their ability to feed on phytoplankton – the more phytoplankton they added, the faster the krill fed. Good news! This meant the krill were healthy and feeding normally. Next they added sediments at a range of particle densities – some lower than what was seen during the krill strandings and some higher. They found that even low densities of particles in the water significantly decreased the krill feeding rates. When they increased sediment densities to very high levels, 80% of the krill stopped swimming and sunk to the bottom of their tanks. Bad news. The krill were no longer healthy and feeding normally.

The particle sizes used in the experiment were small compared to particles found in the krill that died in the strandings. The big particles had a catastrophic effect on krill at relatively low densities, but it took a much higher concentration of small particles to have the same effect (Fig. 4). This means that both particle size and density are important factors influencing the effect of sediment ingestion on krill. The lab experiments showed that krill continued to do worse as particle density increased, and the observations in Potter Cove showed that significant damage was done when there were large particles, even at relative low concentrations.

Figure 3 – Photo of a glacier meltwater stream. Sediments and pebbles are carried along with meltwater streams every summer. As temperatures have warmed, the streams have gotten stronger and deposited more particles into the ocean. (Figure 1b in the paper.)

Figure 3 – Photo of a glacier meltwater stream. Sediments and pebbles are carried along with meltwater streams every summer. As temperatures have warmed, the streams have gotten stronger and deposited more particles into the ocean. (Figure 1b in the paper.)

Figure 4 – Feeding efficiency vs concentration of sediment particles in the water. As higher concentrations of particles were added to the water, krill were less able to feel effectively (blue arrows). The yellow bars show an experiment conducted with small particles added to tanks; the purple bars show an experiment with large particles added. It required a much lower concentration of large particles (17 mg/L) to achieve the same effect that a higher concentration of small particles (100 mg/L) had on krill feeding efficiency (red circles). (Annotated version of Figure 3a and 3c in the paper.)

Figure 4 – Feeding efficiency vs concentration of sediment particles in the water. As higher concentrations of particles were added to the water, krill were less able to feel effectively (blue arrows). The yellow bars show an experiment conducted with small particles added to tanks; the purple bars show an experiment with large particles added. It required a much lower concentration of large particles (17 mg/L) to achieve the same effect that a higher concentration of small particles (100 mg/L) had on krill feeding efficiency (red circles). (Annotated version of Figure 3a and 3c in the paper.)

During most of the mass strandings of krill in Potter Cove, there were strong winds from the north or northwest 24 hours before the krill washed up. The Fourclade Glacier and its many glacial meltwater streams lie to the north of Potter Cove (Fig. 1), so winds from the north would spread the meltwater plumes and everything they carried across the little bay. Krill are generally pretty good at avoiding environments that aren’t good for them, but the strong winds probably trapped the krill against the southern shore along with the high concentration of sediments that had just been blown over. Particles found inside stranded krill still had sharp edges, a sign that they hadn’t been in the water for too long (think about a piece of sea glass – its edges are smooth because it’s been washed around in the ocean for a long time). The right combination of factors must be at work in order to create a mass krill stranding – glacier streams must be flowing strongly, probably as a result of recent warm temperatures, and the winds must be strong and coming from the right direction long enough to trap krill. Unfortunately, this combination occurs somewhat frequently in Potter Cove.

Krill is an incredibly important species in the Antarctic, and is the most abundant species in the coastal waters, but the majority of krill still live in the open ocean. This means that stranding events related to meltwater-deposited particles shouldn’t have a major impact on the overall health of the species. However, the coastal regions are where penguins, whales, and seals tend to feed. So, a significant decrease in the coastal stock could have a negative impact on these predatory species.

The strandings in Potter Cove occurred because krill ingested big particles that prevented them from properly feeding. While humans did not directly dump these sediments into the water, we have caused temperatures along the Antarctic Peninsula to rise as a result of our fossil fuel emissions, and these warmer temperatures have led to more glacial melting. Winds in the Southern Ocean are also getting stronger from the west and, in the case of Potter Cove, this could mean a lot more sediment deposition in the cove. Krill are an incredibly important part of the Antarctic food web. If sediments continue to fill the coastal waters at accelerated rates, there may be big problems in the future for krill and the species that rely on them.

Nicole Couto
I’m interested in how physical processes occurring in different parts of the ocean affect local ecosystems and climate. For my PhD research at Rutgers University (New Brunswick, NJ), I am studying the circulation and pathways of heat transport in the waters of the West Antarctic Peninsula continental shelf, one of the fastest warming regions of the planet. When I’m not thinking about the ocean, I do a lot of swim-bike-running and compete very uncompetitively on the Rutgers Triathlon team.

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