For the last five weeks I’ve been on a research cruise aboard the ARSV Laurence M. Gould off the west Antarctic Peninsula (WAP). This region has received a lot of attention in recent years as one of the fastest warming places in the planet – annual temperatures have increased 7°C over the last 50 years. Because of past sealing and whaling and ongoing fishing the WAP is hardly a pristine ecosystem (remember, say no to Chilean sea bass, aka the Patagonian toothfish, one of the most pressured predators in the region). Nonetheless it’s under much less direct human pressure than most places on Earth which, combined with the rapid rate of warming, makes it a good site to observe the impact of changing climate on marine ecosystems. To carry out these observations the WAP hosts a long term ecological research site (the Palmer LTER), one of 25 NSF funded LTER sites. The Palmer LTER is unique in being one of only two marine-pelagic LTERs, and also one of only two LTERs located in Antarctica (the other is in the McMurdo Dry Valleys). The cruise I participated in takes place each year during the Antarctic summer. It is tasked with making a variety of measurements throughout the Palmer LTER and otherwise supporting science activities connected with long-term observations there.
The Laurence M. Gould has extremely limited access to the internet, so unfortunately it wasn’t possible to upload articles in real time. Instead I’ll try to summarize the cruise in a couple of articles after the fact.
I mentioned that the climate of the WAP is changing fast. Twenty years ago the whole peninsula would have been considered a polar environment, hosting a polar marine ecosystem. The outlying islands that extend in a sweeping arc out to South Georgia Island would have been considered subpolar. The subpolar ecosystem is now moving south in a hurry, changing the distribution of nutrients, primary production, and marine species. Because it’s hard to picture what’s going on below the ocean’s surface it’s a little difficult to visualize these changes in the marine ecosystem. A comparable change in the terrestrial environment might be the encroachment of desert on a region of rich grassland. Deserts aren’t all bad (I rather like them), but they can’t support the biomass or diversity of grasslands. In the parlance of economics they can’t support the same level of ecosystem services.
In the WAP a huge driver of ecological change is sea ice. The species that occupy the Antarctic Peninsula are divided into two groups: ice dependent and ice independent. The ice dependent species have an absolute requirement for sea ice and include Adélie penguins, leopard seals, crabeater seals, sea ice algae, and most importantly, krill. The krill relationship with sea ice isn’t totally straightforward. Adult krill are happy to feed on phytoplankton in the water column and have no real sea ice requirement. Juvenile krill on the other hand, are dependent on the dense layer of lipid rich algae that grow on the underside of sea ice to support their growth. No juvenile krill means no adult krill, a problem because everything else in the Antarctic marine ecosystem depends on adult krill. Without krill there is no way to transfer biomass from primary producers at the base of the foodweb to higher trophic levels. To return to the grassland analogy, removing the krill from the ecosystem would be like removing all the hoofed mammals from the Serengeti. All the predators and scavengers would disappear too. Ultimately even the grass would fail, because the presence of large grazers has benefits like soil aeration and nutrient and seed dispersal. In the WAP it would be worse than this because the biomass far exceeds that of a grassland of comparable size, and this biomass fits critically into the ecology of such globe-trotting species as the humpback whale, orca, and Wilson’s storm petrel (thought by some to be the world’s most abundant bird).
The general trend in the WAP is for reduced sea ice and primary production in the north of the study region, and increased primary production in the south. All the factors influencing this change aren’t yet clear, nor is it clear how these changes will manifest in the future. It will take decades of observation to clarify the trend and the mechanisms behind it. One likely driver of the southward shift in primary production is the reduced availability of micronutrients supplied by glacial meltwater. The Peninsula, like the rest of the continent, is covered with glaciers. Glaciers melt at their base where they contact rock, and this mineral-rich meltwater is a key source of iron and other micronutrients to the marine ecosystem. It’s counterintuitive that warming would reduce the availability of meltwater. The availability of these micronutrients have to do with the distribution of meltwater in the ocean however, not the rate of melt. Freshwater is much less dense than seawater, so glacial meltwater “floats” overtop seawater, forming a micronutrient rich lens. In the presence of sea ice this lens is protected from wind driven vertical mixing, and for a brief period each summer following sea ice retreat there is a strong spatial overlap between micronutrient rich water and sunlight. The result is a “bloom”, a sudden explosion of growth among the primary producers. In the absence of sea ice storms dilute this surface water with micronutrient poor water from deeper in the ocean. By summer the amount of photosynthesis that can be supported is much reduced.
Nothing is that simple however. In the WAP the upwelling of micronutrient poor deep water through marine canyons appears to strongly support primary production. Like upwelling water the world over this deep water, while deficient in micronutrients, is rich in macronutrients like nitrate. So the reality is probably that both water sources, and a reasonably stable water column, are required to support the level of primary production the rest of the ecosystem depends on. Returning to the grassland analogy, tweaking the delivery and mixing rate of surface and deep water is equivalent to the jetstream shifting its course over land, depriving a historically wet region of water and limiting the amount of plant growth that it can support.
So, from the standpoint of primary production, two important things are happening in the WAP. First, the reduction in sea ice means there are fewer ice algae to support the growth of juvenile krill. Second, the reduction in sea ice is increasing the rate of vertical mixing in the water column, reducing the growth of phytoplankton – the food source of adult krill.
This year was an exception to the current trend, with ice conditions rebounding a little after years of decline. It was nothing approaching the coverage that was normal 15 or 20 years ago, but it was sufficient to support an impressive phytoplankton bloom and will hopefully slow the loss of some of the ice dependent species like the Adélie penguin, whose numbers have been dwindling. Next post I’ll talk a bit more about some of our surprising findings regarding the fate of all that carbon produced during the bloom…
For more on the Palmer LTER check out this excellent, non-technical article in the magazine of the Oceanography Society: West Antarctic Peninsula: an ice-dependent coastal marine ecosystem in transition