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The Robot Ocean Network

Automated underwater vehicles go where people cannot, filling in crucial details about weather, ecosystems, and Earth’s changing climate.

Oscar Schofield, Scott Glenn, Mark Moline

Heating Up Antarctica

As complex as hurricane forecasting may be, it pales in comparison to interpreting changes in ocean physics on a global scale, and then connecting those changes with local effects such as sea ice coverage or species decline.

Many questions oceanographers face are so complex that they require the combined data of several robotic platforms that span the range of spatial and temporal scales of marine ecosystems. Linking global changes to local effects has been difficult to impossible using conventional strategies.

One setting that illustrates the importance of bridging these scales is the western Antarctic Peninsula, which is undergoing one of the most dramatic climate-induced changes on Earth. This region has experienced a winter atmospheric warming trend during the past half-century that is about 5.4 times the global average (more than 6 degrees Celsius since 1951). The intensification of westerly winds and changing regional atmospheric circulation, some of which likely reflects the effect of human activity, has contributed to increasing transportation of warm offshore circumpolar deep water onto the continental shelf of the peninsula.

This water derives from the deep offshore waters of the Antarctic Circumpolar Current, the largest ocean current on Earth, and is the primary heat source in the peninsula. The altered positions of this current are implicated in amplifying atmospheric warming and accelerating glacier retreat in the region. Monitoring and tracking the dynamics of the warm offshore deep current require a sustained global presence in the sea, which is now being accomplished via the Argo network of autonomous floats. Data from Argo suggest that the Antarctic Circumpolar Current has exhibited warming trends for decades.

2013-11SchofieldF8.jpgClick to Enlarge Image The increased presence and changing nature of the deep-water circulation has implications for the local food web. The Western Antarctic Peninsula is home to large breeding colonies of the Antarctic Adélie penguin ( Pygoscelis adeliae ), which live in large, localized colonies along the peninsula even though food resources are abundantly available along the entire inner continental shelf. This concentration of the population has raised a persistent question: What turns specific locations into penguin “hot spots?”

The locations of the Adélie colonies appear to be associated with deep seafloor submarine canyons, which are found throughout the continental shelf of the peninsula. This colocation has led to a hypothesis that unique physical and biological processes induced by these canyons produce regions of generally enhanced prey availability. The canyons were also hypothesized to provide recurrent locations for polynyas (areas of open water surrounded by sea ice), giving penguins year-round access to open water for foraging. But linking the regional physical and ecological dynamics to test the canyon hypothesis had been impossible, because brutal environmental conditions limited spatial and temporal sampling by ships.

Robotic AUVs now offer expanding capabilities for observing those conditions. As part of our research, for the past five years we have been using a combination of gliders and propeller AUVs to link the transport of the warm offshore circumpolar deep water to the ecology of the penguins in the colonies. Gliders surveying the larger scale of the continental shelf have documented intrusions of deep, warm water upwelling within the canyons near the breeding penguin colonies. These intrusions of warm water appear to be ephemeral features with an average lifetime of seven days, which is why earlier, infrequent ship-based studies did not effectively document them. Associated with this uplift of circumpolar deep water along the slope of the coastal canyon, the gliders found enhanced concentrations of phytoplankton, providing evidence of a productive food web hot spot capable of supporting the penguin colonies.

Satellite radio tagging is being used to characterize the foraging dynamics of the Adélie penguin, and has shown the majority of their foraging activity was centered at the slope of the canyon. These more localized foraging patterns were used to guide sampling of the physical and biological properties with a propeller AUV, because strong coastal currents hindered buoyancy-driven gliders. The propeller AUV data were used to generate high-resolution maps, which revealed that penguin foraging was associated with schools of Antarctic krill. The krill in turn were presumably grazing on the phytoplankton at the shelf-slope front.

It took the integration of all three classes of robotic systems (profilers, gliders, and propeller AUVs) to link the dynamics of the outer shelf to the coastal ecology of the penguins. But in the end, that combination of techniques turned out to be just what was needed to settle a long-standing mystery of penguin biogeography. Better understanding of these processes is critical to determining why these penguin populations are exhibiting dramatic declines in number—for example, the colonies located near Palmer Station in Antarctica have declined from about 16,000 to about 2,000 individuals over the past 30 years. Ongoing and future robotic deployment will help address how climate-induced local changes in these deep-sea canyons might underlie the observed declines in the penguin populations, which themselves are serving as a barometer for climate change.

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