Glacial Dropstones Home to Diverse Communities

Glacial calving can transport ice-rafted debris, or dropstones, to the ocean floor (Photo: Amanda Ziegler).

Beneath the glaciers of the West Antarctic Peninsula lie island-like habitats teeming with species from spiny icefish and sea squirts (like Cnemidocarpa verrucosa) to various species of starfish. According to a recent paper in Marine Ecology Progress Series, these unique habitats are formed around glacial dropstones, rocks once incorporated into glaciers. Dropstones move across the surface of Antarctica and are carried out to sea by icebergs that have calved off glaciers. As the icebergs melt, they drop these stones, which settle on the ocean floor. The fallen rocks provide a hard surface for sessile, or non-moving, species to securely attach, creating small communities surrounded by mud.

Many of the animals living on the dropstones reproduce by spawning into the water. The resulting larvae then drift with the ocean currents until they encounter suitable surfaces, like a dropstone, to settle and grow into adults. The dropstones provide a habitat, distinctly different from the surrounding seafloor, for those sessile creatures. They also attract mobile species who come to the dropstones for food and nesting sites. The researchers often saw spiny icefish, or Chaenodraco wilsoni, guarding eggs on a dropstone.

While dropstones take up very little of the surveyed seafloor (less than one percent), they contributed 20% of the overall species richness at depths of 437−724 m near the West Antarctic Peninsula (Source: Amanda Ziegler).

Amanda Ziegler, the lead author of the paper and a graduate student at the University of Hawai’i at Mānoa, told GlacierHub that this is the first known study of dropstone communities near the West Antarctic Peninsula. “Glacial dropstones have been studied in other habitats such as in the Arctic and in the Weddell Sea [to the east of the Antarctica Peninsula] where the heterogeneity produced by the stones was also found to increase the functional and taxonomic diversity of the megafaunal communities where dropstones were abundant,” she said. So these results are not unexpected but do provide new insight into the area, confirming that such communities are found in a wider range of areas than previously known.

The waters around Antarctica are too deep for divers, so Ziegler and her team relied on photographs taken by underwater cameras towed by United States Antarctic Program research vessels from 2008 to 2010 during the LARISSA Project. The photos came from three fjords along the West Antarctic Peninsula, all greater than 400 meters deep as well as similarly deep areas along the open continental shelf. In all, the team measured 2,972 dropstones and found that 467 were colonized by at least one megafaunal species. The researchers also found that while glacial dropstones make up less than one percent of the seafloor habitat, they are home to 20 percent of the species in the area. Ziegler noted that the team examined the relative importance of different environmental parameters in structuring the communities. “We asked, for example, does water temperature, sediment cover, or the size and abundance of dropstones affect the community composition and does this differ by site?” she said.

Dropstone communities of brightly-colored megafauna. The two left-most photos are from Flandres Bay and the rest are from Andvord Bay (Source: Amanda Ziegler).

Interestingly, not all dropstones examined were homes to communities of animals. To the researchers’ surprise, many were simply bare. Near Antarctica, shallow hard-substrate is often scarce, and therefore the communities that arise can be extremely dense and diverse. Ziegler expected the organisms to be space-limited, meaning the number of individuals and species is capped by the available habitat. Instead, they found many uninhabited, seemingly useable dropstones. The researchers possibly attribute this to a limited supply of larvae; there might not have been as high a density of sessile species that prefer hard substrate as expected. Ziegler also hypothesized that many of the bare dropstones could have been only recently deposited from icebergs.

These maps show the study areas along the West Antarctica Peninsula. The top left map shows the locations of the following three sites: (1) Andvord Bay, (2) Flandres Bay, and (3) Barilari Bay (Source: Grange & Smith 2013).

In addition to being objects of ecological research, dropstones can also indicate glacial and iceberg activity. Dropstones only occur where icebergs, calved from glaciers, have passed, so the distribution of dropstones can help paint a picture of their distribution and movement. During this study, the researchers were expecting a more predictable distribution of the dropstones originating from the glaciers in the area, but Ziegler said she was surprised by the abundance and size of the dropstones deposited out on the open shelf further than 100 km from the nearest active glacier. “This means that icebergs in this region move around a lot more than we expected, and we know now from our current studies in Andvord Bay that there is less melting of the icebergs and glaciers inside the fjords than expected,” she added.

The West Antarctic Peninsula has experienced rapid warming; how this affects drop stone communities is an area of future study (Photo: Amanda Ziegler)

It is unclear how climate change and the related change in glacial processes will affect the future of these island-like communities. Each new iceberg has the potential to carry more rocks out to sea, where they can be deposited as dropstones, but as the researchers found, there are already many unoccupied dropstones in the area. Along with glacial melt under increased warming, sediment released into the fjords might increase, quickly covering the dropstones and making them unsuitable for many sessile invertebrates.

According to Ziegler, sessile organisms, especially those that filter food from the water column, are particularly sensitive to fine-grain sediments released by melting glaciers. A new study by co-author Craig Smith hopes to better understand biological, chemical and physical oceanography of Andvord Bay, one of the fjords in this study, to better assess future changes in this ecosystem. Through this and similar research, we will learn if and how climate change will threaten these underwater communities.

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