Future Unwritten: Antarctic Sea Pens’ Secrets to Success

A sponge, Haliclonissa verrucosa, filter feeds in water off of Spume Island (Source: Chuck Amsler).

On the seafloor, beneath the cold, dark waters surrounding Antarctica, life blooms. Sea stars make their glacially-slow journeys along rocks, sponges rhythmically pulse water through their internal cavities, and one particular coral, the delicate sea pen Malacobelemnon daytoni, flourishes.

Sea pens are colonial, meaning that many individuals work together as a whole, each conducting a specialized task necessary for survival. The resulting shape resembles a quill pen, earning them their creative common name.

M. daytoni is one of the most abundant species in Potter Cove, located off the southwest side of King George Island in the South Shetland Islands. The environment of Potter Cove is heavily influenced by local glacial retreat, which discharges increasing quantities of sediment into the ocean. Researchers know little about benthic (defined as the lowest layer in a body of water) ecology in this region, challenging as it is to conduct scientific research in the cove’s remote, frigid waters. A recent paper in Marine Environmental Research by an international team from the Universidad Nacional de Córdoba and Institute of Marine Science analyzed the biochemistry of M. daytoni to understand its ecological success. They found that the key is a flexible, omnivorous diet and strategic reproductive techniques.

A sea pen in Potter Cove (Source: Ricardo Sahade).

Natalia Servetto, lead author on the study, is part of a group that has been studying the Potter Cove benthos since 1994, thanks to logistic support of the Instituto Antártico Argentina, the Alfred Wegener Institute and the National Scientific and Technical Research Council (CONICET). These efforts have revealed what Servetto called an “unexpected and marked shift in the system… linked to ongoing climate change processes” and to glacial retreat, which has increased sedimentation rates, affecting the benthic fauna. In the last few years, Servetto says, the abundance and distribution of M. daytoni has increased significantly, while many other Potter Cove invertebrates are becoming less abundant.

Why should the sea pen thrive while its neighbors perish? To answer this question, a team based out of the Argentine Carlini Station scuba dove every month for one year, sometimes through holes in the sea ice, to depths of 15 meters to take tissue samples from sea pens in Potter Cove.

This in itself is a feat. Chuck Amsler, a biology professor at University of Alabama at Birmingham who studies macroalgae and invertebrates in the Western Antarctic Peninsula, told GlacierHub that diving to study Antarctic life is a challenge because, “It’s damn cold!” However, Amsler added, “The scientific reward is that you have the opportunity to observe your study system directly. There is no substitute for the kind of insights one can get from that.”

Carlini Station provides access to the remote Potter Cove (Source: Natalia Servetto).

Many such insights came to fruition back in the lab, where the researchers analyzed the carbohydrate content, stable isotope ratios, and fatty acids in the coral tissues, looking for chemical clues to what the pens eat through the year. Just as a savvy New Englander might buy groceries with the seasons, eating peaches in summer, apples in autumn, root vegetables in winter, and fresh maple syrup in spring, the researchers found that the sea pen’s diet changes seasonally. In summer, M. daytoni feasts on copepods (a type of small invertebrate), phytoplankton, and a bit of macroalgae detritus. In autumn, the menu features more phytoplankton and microalgae, and in winter, macroalgae detritus and sediment are the sea pens’ bread and butter. In spring, their diet becomes fresher again when phytoplankton and microplankton return to the table.

This omnivorous, opportunistic feeding strategy allows the sea pens to eat whatever is available in a given season, reducing pressure on the species during times of food depletion. Such depletion peaks in winter and autumn, forcing M. daytoni to scavenge for organic sediment and detritus that become re-suspended from the seafloor. Sea pens have another major advantage over their neighbors: inorganic sediment from melting glaciers can clog the respiration and feeding mechanisms of filter-feeding invertebrates, while M. daytoni continues to chow down, undisturbed.

Amsler and his team begin a dive to study benthic macroalgae and invertebrates (Source: Maggie Amsler/Antarctic Photo Library).

Not only do they feed resourcefully, but the sea pens also optimize the energy they obtain. Servetto’s team found that lipid content in their tissues, associated with reproduction, increased in rapid bursts that were seasonally linked with higher food availability. This pattern suggests that sea pens can take the energetic resources offered by the environment at a given time and shunt them into reproduction, the most important process for any organism.

Beyond any single year, and beyond the bounds of the Potter Cove ecosystem, the opportunistic feeding and reproductive strategies of M. daytoni will help this species thrive. Nearly 90 percent of glaciers are retreating along the Antarctic Peninsula, causing environmental shifts that threaten many species, but could create an opportunity for sea pens to actually expand their range, Servetto says. As glacial retreat creates new ice-free areas, colonization may occur, according to Servetto, “at a previously unimagined speed.”

However, it’s not yet clear how the Antarctic coastal system will evolve as glaciers melt. Amsler says that decreasing sea ice cover will likely favor macroalgae and their associated communities of small, mobile invertebrates like amphipods and gastropods, and threaten shallow-water sessile invertebrates like the sea pens, probably pushing them into deeper water. Decrease in sea ice cover may also have broader climate impacts: the Antarctic benthos is a net carbon sink (a natural process that stores carbon), and as ice cover decreases, Amsler expects that benthic primary production will increase, removing even more carbon from the atmosphere.

Divers in Antarctica face difficult conditions, including brash ice (Source: Chuck Amsler).

No matter the outcome, the impacts of climate change on benthic Antarctic invertebrates will be manifold. Other forces, like ocean warming and acidification, will also affect M. daytoni and reshape benthic invertebrate communities, though Amsler says more work is needed to understand how. “I don’t know what the community will look like in 100 years, but I’m confident that it will be different from what we see today,” he predicted. As climate changes and glaciers melt, the flexible diet and efficient reproductive strategy of M. daytoni will give it an advantage in changing Antarctic coastal ecosystems.

Iceberg Killing Fields Threaten Carbon Cycling

The vast, unpopulated landscape of Ryder Bay, West Antarctica gives the impression of complete isolation. However, despite its barren, cold exterior, Antarctica plays an important role in regulating the Earth’s climate system. Located along the southeast coast of Adelaide Island, Ryder Bay is helping mitigate impacts of climate change by removing greenhouse gases from the atmosphere to the ocean, where these gases can remain for centuries. This repurposing is being done by benthos, microorganisms like phytoplankton that bloom during summer months and provide critical food supplies that maintain the marine ecosystem in Ryder Bay. Quietly residing on the floor of the Southern Ocean, benthos are encountering increased risks due to a changing climate. While the potential carbon recycling capacity of local marine ecosystems remains significant, the collapsing glaciers and ice shelves in Ryder Bay may threaten this productivity, according to an article in the journal of Global Change Biology.

West Antarctica during winter. (Source: Ashley Cordingley)
West Antarctica during winter (Source: Ashley Cordingley).

The carbon recycling process in the marine ecosystems is one of the strongest mechanisms helping to reduce the impacts associated with historic carbon emissions. Located along the continental shelf, benthos absorb carbon through photosynthesis; when these organisms die and fall to the ocean floor, this carbon is then stored in sediments. Undisturbed, the ocean can help thwart warming due to an enhanced greenhouse effect by removing carbon from the atmosphere and storing it in the ocean. David Barnes, a Marine Benthic Ecologist with the British Antarctic Survey and an author of the article,  pointed out to GlacierHub, “Trends in carbon accumulation and immobilization, which occur on the seabed, could be considered most important as these involve long-term carbon storage. [These trends] are perhaps the largest negative feedback on climate change.” However, because of shifting land dynamics, the increased frequency of iceberg creation is having a direct impact on the ability of the marine ecosystems to recycle carbon.

Iceberg shape and size is hard to estimate solely from its above sea level figuration. (Source: Ashley Cordingley)
Iceberg shape and size is hard to estimate solely from its above sea level figuration (Source: Ashley Cordingley).

As the Earth continues to warm, ice sheets and glaciers in Antarctica advance and become thinner, causing cracks and crevasses to form. These fissures, in turn, lead to unpredictable, large-scale breaks which create icebergs that discharge into the ocean. At the time of detachment, ice formations hit the ocean floor, obliterating the marine ecosystems below. Icebergs can continue to impact the benthos as they travel on the ocean.

Barnes described this problem to GlacierHub:  “At places like Ryder Bay, it would be very difficult to provide forecasting, because it is very frequent and a bit chaotic. The direction an iceberg travels depends on its shape, how deep its keel is, wind, and current speed. A smaller iceberg with a vertically flat side above water will easily catch wind like a sail, so if the wind is strong it will mainly follow wind direction. Conversely, a bigger iceberg with a deep vertical flat side might more easily catch current.”

According to NOAA, these icebergstypically rising 5 meters above the sea surface and covering 500 square meters in areaare large enough to inflict significant destruction. Dubbed “iceberg killing fields,” these places of impact can cause extensive disruption to the beneficial marine ecosystems along the ocean floor.

Divers assess seabed for ice scour damage (Source: Ashley Cordingley)
A diver assesses the seabed for ice scour damage (Source: Ashley Cordingley).

David Barnes works with the British Antarctic Survey to study the iceberg killing fields and measure the impact of iceberg-seabed collisions on marine ecosystems. The British Antarctic Survey has been monitoring the local marine ecosystems in Ryder Bay due to their sensitivity to environmental change and the surprisingly large role benthos play in removing carbon from the atmosphere. According to the report, “The scour monitoring has probably become the longest continuously running direct measurement of disturbance on the seabed anywhere in the world.” With roughly 93 percent of carbon dioxide being stored in our oceans, it is necessary to monitor how these potential carbon sinks may fluctuate, according to the Worldwatch Institute.  

According to Barnes’ findings, the benthos in Ryder Bay are experiencing high mortality rates due to the frequent and powerful collisions between collapsing ice shelves and the sea floor, often referred to as ice scour. “Since 2003, when it was first measured in Ryder Bay, ice scour has been less predictable and more variable (than many other environmental variables),” according to Barnes and the British Antarctic Survey. The heightened unpredictability of ice scour makes predicting and preventative measures challenging.

Collisions between icebergs and the ocean floor are frequent and damaging, with the “potential to halve the value of benthic immobilized carbon in the Ryder Bay shallows,” says Barnes. These measurements show a very high frequency of scouring in the shallows because of its proximity to the ocean floors in Ryder Bay, according to the article. In fact, on average, ice scour affected 29 percent of the seabed study area yearly, from 5 to 25 meters deep. In the past decade, Barnes found that only seven percent of the shallows had not been hit by icebergs. This scouring accounts for nearly 60 percent of total benthic fatality at a 5m depth. The high frequency and fatality rates associated with iceberg scour make it one of the “most significant natural disturbance events,” according to Barnes.

Extensive research conducted on the sea floor in Ryder Bay helps measure ice scour. (Source: Ashley Cordingley)
Extensive research conducted on the sea floor in Ryder Bay helps measure ice scour (Source: Ashley Cordingley).

Weekly ocean measurements of temperature, salinity and size-fractionated (micro, nano and pico) phytoplankton have been collected since 1997, says Barnes. The field work conducted by the British Antarctic Survey set up 75 ice scour markers gridded at 5, 10 and 25m. These grids are surveyed and replaced by researchers using scuba gear, allowing for the different scour depths to be calculated. Frequency of collisions is then calculated through the recording of disturbances for each meter squared in order to establish a detailed history and provide insight into potential future trends. Annual collection of faunal remains and boulders are integrated into the disturbance data sets. These collections will help further quantify the damages inflicted upon marine ecosystems and their abilities to sequester carbon.

While glaciers in polar regions seem inconsequential to our everyday experiences with climate, they have the ability to significantly influence the biological systems which remove greenhouse gasses from the atmosphere. Continued support of scientific endeavors in the polar regions are critical in order to understand the places and processes that play such a vital role in the Earth’s climate system. As Barnes states, “We have a huge and powerful ally [in the polar regions] in the fight against climate change, so let’s make sure we look after it.”