Darkness in the abyss

Glacial melt isn’t only about sea-level rise. It’s about mud. Sediment to be precise. As old ice melts into the sea it brings down all that stuff it’s been sitting on, the mud it has scraped up and absorbed over decades and centuries. Newly exposed, ice-free beaches deteriorate too, putting more stuff out there. Clean water becomes murky and nutrient rich. A different ecosystem.

Potter’s Cove, and particularly the Carlini base on Isla 25 de Mayo, which the British call King George Island, has been a central place for studying the impacts. (As all the South Shetland Islands are now covered by the Antarctic Treaty no territorial claims are recognised, but the archipelago had long been a source of squabbles between the UK, Argentina and Chile. The picture below, from Google Earth, shows it perched off the tip of the Antarctic Peninsula.) The base has a really interesting history and is now a major research centre with multi-national collaboration, not least involving the Netherlands, Germany and Italy, exploring the impact of climate change from the deep ocean floor to the atmosphere. It’s not just the science: in 2013 Metallica did a very cool gig there - which is also notorious for being (sort of) silent, with a lot of the music only being heard through headphones. 

The long history of research here (as the base was first established in 1953) has enabled some better studies, so that for example the sea-floor has been the surveyed regularly since 1994, with a recent study published this November. The abstract says, in a straightforward way, that in just fifteen years the researchers have seen a “marked shift in an Antarctic benthic community that can be linked to ongoing climate change. However, rather than temperature as the primary factor, we highlight the resulting increased sediment runoff, triggered by glacier retreat, as the potential causal factor.” (Source: Sahade R. et al. “Climate change and glacier retreat drive shifts in an Antarctic benthic ecosystem.” Science Advances).

The researchers have found that the ecology in Potter’s Cove has shifted dramatically in those areas affected by suspended matter as the surrounding glaciers retreat. They have three stations where they survey in detail, Inner, Middle and Outer, with a big depth variation in the Middle area.

In 1994 there was a complex array of seasquirts and macroalgae in the all those areas. Today the squirts are nearly gone in the sediment rich waters nearer the shore and the deeper waters of the Middle Station replaced by creatures such as sea pens, sponges and sealice, an impoverished ecological mix compared to the diversity and rarity of what was there. (The picture is taken by divers at the research institute and shows a colony of squirts in Potter's Cove.)

This type of shift, which appears closely connected to available light, matches the increased colonization by seaweed, which is very sensitive to sunlight available at depth as well as access to ice-free coastal fringes. An interesting report (found here, but I admit I’ve only read the abstract of the actual article) points to macroalgae  (types of seaweed) losing strength in deeper water, because the extra sediment is blocking light, but spreading in the newly ice-free areas of the cove.

Perhaps the most frightening question is whether these are reversible changes. Suppose we all agree to the dramatic changes which might contain global temperature increases to 2° and glacial retreat is slowed, if not halted. Can these communities revert back to their previous state? Sahade’s team are cautiously pessimistic in their hypothesis, suggesting that the changes in the benthic communities may “represent a transition to an alternative equilibrium state of the ecosystem. Implications of sudden shifts can be of major importance because gradual changes in environmental variables, such as those observed in sedimentation, can have no or negligible effects until a threshold is reached and then rapid ecosystem shifts take place. If thresholds for the collapse and recovery of the system states do not coincide (hysteresis) then these shifts are difficult to reverse.” They go on to say that highly diverse ecosystems in the glacial fjords of Antarctic is an important characteristic of the region which is shifting with higher sedimentation.

By comparison, Arctic glacial edges have significantly less diversity and biomass. A 2005 article studying northern glacial fjords found "[b]enthic biomass decreased with increased proximity to the glacier, and also points to fewer numbers of species due to the disturbed and darkened environment. (Arctic glaciers move a lot more than those in the Antarctic, both as a result of seasonal change and global warming.) As glaciers retreat in the Antarctic Peninsula and the islands along its fringes, we may be seeing the beginnings of rapid step-changes in these unique underwater environments, changes which cannot be reversed even if humanity finally gets its act together. The deeps are indeed getting darker.

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  1. I am getting quite an education from reading your posts. Your attention to detail, choice of topics and ability to explain provocative and complex subject matter is superb.

  2. Pingback: COP15 and Antarctica: how soon, how fast, how deep? – Sailing to Antarctica

  3. Pingback: COP15 and Antarctica: how soon, how fast, how deep? - Sailing to Antarctica

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