Krill Contribute to Ocean Carbon Storage in Patagonia

Waters in the sub-Antarctic region of Chilean Patagonia are fed by glaciers in one of the largest freshwater systems on Earth, the North and South Patagonian Icefields. A recent study published in Marine Ecology Progress Series found that Euphasia vallentini, the most abundant species of krill in Chilean Patagonian waters, play a key role in food webs. The study also discovered that this species of krill helps to sequester carbon in the oceansthey consume plankton, which take in carbon during photosynthesis, and discharge some of the carbon into deeper ocean waters through the production of fast-sinking fecal pellets. This is increasingly important as atmospheric carbon concentrations rise, as it contributes to the role of the oceans as a carbon sink.

The North Patagonian Icefield (Source: McKay Savage / Creative Commons).
The North Patagonian Icefield (Source: McKay Savage/Creative Commons).

Krill are small, shrimp-like crustaceans that are found in all of the world’s oceans. In an interview with GlacierHub, Humberto E. González, the lead author of the study from the Austral University of Chile, explained that krill form “a trophic [related to food and nutrition] bridge between the microbial community [bacteria, nanoplankton, microzooplankton] and the upper trophic layers [seals, whales, penguins, etc.]. Thus, they play a pivotal role in trophic flows.”

The study by González et al. focused on the region between the Magellan Strait and Cape Horn because of the unique biological, chemical and physical conditions created by the hydrological input from three different sources: nutrient-rich Pacific and Atlantic Sub-Antarctic Waters (waters that lie between 46°– 60° south of the Equator), and cold and nutrient depleted freshwater from Patagonian rivers and glaciers.

Waters that are more saline or that are colder have higher densities. However, as explained in the study, the effect of salinity exceeds the effect of temperature on density within this region, giving rise to strong saline stratification in the mixture of oceanic and freshwater terrestrial environments. This reduces the movement of important species between the benthic (the lowest level) and pelagic (open water) ecosystems in southern Patagonia.

The stratification also reduces upward and downward mixing of ocean water. This reduces carbon fluxes in the region, as the transport of carbon dioxide to deeper parts of the ocean through diffusion across layers occurs more slowly than the circulation of ocean waters with different carbon dioxide concentrations.

A map of the Strait of Magellan and the region where the study took place (Source: / Creative Commons).
A map of the region where the study took place. The icefields are located further north (Source: Creative Commons).

The team of scientists embarked on a research cruise in the region in October and November 2010, collecting chemical and biological samples at about forty different stations. Using a variety of techniques, they studied features such as the types and distribution of organic carbon in the waters, and the abundance and diet of E. vallentini. All this was done to better understand the role of E. vallentini in the region’s food web structures and in the transport of carbon to deeper layers of the ocean despite strong stratification.

In conversation with GlacierHub, González stated that “the species of the genus Euphausia (a functional group of zooplankton) play a paramount role in many disparate environments from high to low latitude ecosystems. Euphausia superba in the Southern Ocean and Euphausia mucronata in the Humboldt Current System are some examples.In this study, González et al. found that E. vallentini play a similarly important role in Southern Chilean Patagonia, consuming a range of plankton from nano- to phytoplankton and forming the dominant prey of several fish, penguin and whale species.

Krill, such as E. vallentini, form an important link in food chains between phytoplankton and larger animals (Source: Uwe Kils / Creative Commons)
Krill, such as E. vallentini, form an important link in food chains between phytoplankton and larger animals (Source: Uwe Kils/Creative Commons)

The study also found that E. vallentini play an important role in passive fluxes of carbon through the sequestration of carbon in fast-sinking fecal pellets, or poop. The plankton ingested by E. vallentini takes in carbon dioxide during photosynthesis, and about a quarter of the plankton ingested by E. vallentini is then passed out in fecal matter. These fecal pellets form the dominant component of particulate organic carbon (organic carbon particles that are larger than a certain size) fluxes in the region’s waters, helping to sequester carbon as they sink to the ocean floor.

This process is accelerated by E. vallentini’s vertical diurnal migrations, which occur despite the strong saline stratification of waters in southern Patagonia. Their vertical movements, from deeper parts of the ocean during the day to the surface of the ocean in search of food at night, occurs more quickly than the rate at which their fecal pellets sink, speeding up the transport of carbon to deeper ocean layers. As González explained, “the Patagonian krill [and] the squat lobster (Munida gregaria) are the main species responsible for the carbon export towards deeper layer of the fjords and channels (in southern Patagonia).”

Although scientists from the Commission for the Conservation of Antarctic Marine Living Resources estimate that the total weight of Antarctic krill exceeds that of humans on Earth, they may not be immune from the effects of anthropogenic climate change. Indeed, González stated that a greater input of freshwater to the ocean could reduce nutrient levels in upper layers of the ocean. This will reduce the productivity of fjords and channels, reducing the availability of food for krill, and creating serious implications for the marine ecosystems that they are part of. This research serves as a reminder that biological organisms play an important role in the effects of marine ecosystems on the world’s climate, as they do in terrestrial ecosystems. 

Glacier Melting Sets Free Organic Carbon

Research has shown that glaciers have a greater role than was previously known in the movement of organic carbon into and through aquatic ecosystems, including the oceans. Organic Carbon (OC) refers to carbon contained in organic compounds that is originally derived from decaying vegetation, bacterial growth, and metabolic activities of living organisms. It serves as a primary food source for marine organisms, particularly microbes. In addition, it contributes to the acidification of water. Particularly in freshwater ecosystems, excessive OC can result in a brownish coloration. In fact, the amount of OC is often used as an indicator of overall water quality.

Figure 1. Location of glacier DOC samples classified by type. a–d, Samples were collected from a wide variety of glacial environments including: Alaska (a), Tibet (b), Dry Valley glaciers in Antarctica (c), and the Greenland Ice Sheet (d). (Source: Hood et al.)
Figure 1. Location of glacier DOC samples classified by type. a–d, Samples were collected from a wide variety of glacial environments including: Alaska (a), Tibet (b), Dry Valley glaciers in Antarctica (c), and the Greenland Ice Sheet (d). (Source: Hood et al.)

A recent research shows that the increase in glacier runoff through melting and iceberg calving has led to a rise of OC flux entering marine and lacustrine ecosystems, and this flux is expected to grow in the coming decades. According to the article, glacier ecosystems accumulate OC from primary production on the glacier surface, particularly in cryoconite deposits, and also from the deposition of carbonaceous material derived from terrestrial and anthropogenic sources.

To quantify the total storage of OC in terrestrial ice reservoirs, the study integrates measurements of organic carbon from mountain glaciers, ice sheets in Greenland, and Antarctica Ice Sheet, with data from locations that span five continents (see Figure 1). It turns out that that largest amount of OC is located in Antarctica, followed by Greenland and mountain glaciers. However, it is found in the study that a large portion of the OC released from melting glaciers is from mountain glaciers and peripheral glaciers which exit from the Greenland ice sheets (see Figure 2). The surprisingly disproportionately high DOC export from mountain glaciers and Greenland is associated with their glacier mass turnover rate, which is higher than in Antarctica. Even as glaciers are losing ice through melting and caving at their lower ends, they continue to receive new snow at the top, which converts to ice—a process of flow, which contributes to the movement of OC through the glaciers.

Figure 2. Storage and flux of glacier DOC. Total glacier storage of DOC (a) and annual DOC export in glacier runoff (b) for MGL, GIS, and AIS.
Figure 2. Storage and flux of glacier DOC. Total glacier storage of DOC (a) and annual DOC export in glacier runoff (b) for AIS (Antarctic Icesheet), GIS (Greenland Icesheet) and MGL (mountain glaciers). (Source: Hood et al.)

Dissolved organic carbon (DOC) and particulate organic carbon (POC), two major components of the OC, are both significant components in the carbon cycle, because they are primary food sources in aquatic food webs. In particular, DOC forms complexes with trace metals, which can be transported and consumed by organisms. This may have drastic affects on marine life, “because this material is readily consumed by microbes at the bottom of the food chain,” said U.S. Geological Survey research glaciologist and co-author of the research Shad O’Neel. The microbes are an important source of food for plankton and for larger organisms in the seas, including crustaceans and fish.

 

Iceberg Calving (Source: Flickr/Indistinct)
Iceberg Calving (Source: indistinct/Flickr)

The study raises questions of the implications of OC input for carbon dioxide concentration in atmosphere. The authors suggest that glacier-derived OC shows a high degree of biological availability, when compared to other terrestrial sources. Hence, it is more likely to result in more rapid decomposition of dead marine organisms, which otherwise would fall from upper zones of the oceans to deeper sections, where they would remain for long periods. This decomposition, in turn, contributes to carbon dioxide outgassing from the oceans to the atmosphere.

For another story about the effects of glaciers on ocean chemistry and ecology, look here.