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Biochemistry

Iceberg Buffet: How giant icebergs bring food to plankton

Paper: Duprat, L.P.A.M., et al. 2016. Enhanced Southern Ocean marine productivity due to fertilization by giant icebergs. Nature Geoscience, v. 9: 219–221.

Credit: Gérald Tapp.

Credit: Gérald Tapp.

While the whole world is suffering from the effects of climate change, the poles are being hit especially hard. The average temperature is rising more quickly than in other regions of Earth, the ice is melting faster, and icebergs are breaking off from glaciers at unprecedented rates. But for some lucky little critters, the increase in these massive icebergs is a good thing.

When icebergs break off from glaciers, they carry with them sediments that the glaciers have scraped off the land on their way to the ocean. These terrigenous sediments – sediments that are originally from land but have been transported into the ocean by rivers or other means –can carry nutrients and trace elements like iron that are fundamentally important for the growth of phytoplankton. This growth is also known as primary productivity, and is a major component of the ocean’s carbon cycle, taking excess carbon and CO2 from the ocean – atmosphere boundary and transporting it to the depths of the ocean.

Previous studies have shown that chlorophyll levels – which are used to measure the amount of primary productivity on the ocean surface – tend to be higher near icebergs. This occurs because the meltwater plumes from the icebergs contain significant concentrations of iron and other nutrients. Iron is rare and can limit phytoplankton growth in the Antarctic, so adding it to the surface – referred to as iron fertilization – can stimulate growth.

Giant iceberg in the South Atlantic. In June 2002, it was measured 50 km by 23.5 km. Credit: NASA Earth Observatory.

Giant iceberg in the South Atlantic. In June 2002, it was measured 50 km by 23.5 km. Credit: NASA Earth Observatory.

With icebergs less than 1 kilometer in length, chlorophyll levels up to three times higher than the average level have been detected, for up to six days after passage of the iceberg, up to a radius of a few kilometers. However, the influence of giant icebergs over 18 km in length on marine primary production in the Southern Ocean is less well studied, and fertilization from these icebergs may be much larger than previously suspected.

Smaller Numbers but Bigger Payload

Although rarer, giant icebergs exceeding 18 km in length make up about half of the total iceberg discharge volume from Antarctica, because of their immense size. This makes these slightly icebergs just as important to primary productivity as the “small” ones. Several dozen such icebergs exist in the Southern Ocean at any given time, and may linger for years. They also come from a range of geologic environments around Antarctica, and are thus likely to have different iron and nutrient characteristics.

Dr. Luis Duprat and colleagues at the University of Sheffield in the UK set out to study how these massive icebergs might feed plankton growth. Using remote sensing and the MODIS Aqua satellite, they analyzed 175 satellite images of open ocean color in the Antarctic Weddell Sea and Southern Ocean, before and after the passage of 17 giant icebergs between 2003 and 2013. The color was used to detect enhanced chlorophyll levels and primary productivity.

Figure 1: mean chlorophyll concentrations in the Southern Ocean in 2011 (a) and in 2014 (b). You can see the path of one sample giant iceberg by the black line in (b). Chlorophyll concentrations are higher in 2014, and also higher in the down-current path of the iceberg. Credit: Duprat et al.

Figure 1: mean chlorophyll concentrations in the Southern Ocean in 2011 (a) and in 2014 (b). You can see the path of one sample giant iceberg by the black line in (b). Chlorophyll concentrations are higher in 2014, and also higher in the down-current path of the iceberg. Credit: Duprat et al.

They found that these giant icebergs can create a radius of enhanced primary productivity levels up to 10 times the iceberg’s length. These patches can linger for more than a month after the iceberg has passed through the area, and are over an order of magnitude larger than the areas created by smaller icebergs. Additionally, there were no statistically significant differences between the geographical origins of giant icebergs in their fertilization effect a month after passage. However, there were differences that correlate well with the large-scale geology of Antarctica. Almost all of coastal East Antarctica is composed of rock that will be less easily weathered than the rocks of the Antarctica Peninsula, and thus icebergs from East Antarctica will tend to have more sediment and iron.

What does this all mean for climate change and the carbon cycle? The Southern Ocean is a significant sink in the ocean component of the global carbon cycle, contributing ∼10% of the ocean’s total carbon sequestration through a mixture of chemical and biologically driven processes. However, due to its low concentration of dissolved iron, its contribution is lower than that of the smaller South Pacific and Indian Oceans. Adding iron to the system from these massive icebergs boost the process. Duprat estimates that up to a fifth of the Southern Ocean’s downward carbon flux originates with giant iceberg fertilization, and suggests that if giant iceberg calving increases this century, as expected, this negative feedback on the carbon cycle may become more important.

Read more recent oceanbites posts on the relationship between plankton and sea ice:

Arctic could become more biologically productive as ice melts

Plankton fill up ice-free summer homes

Zoe Gentes
I’m a Knauss Marine Policy Fellow working in the US House of Representatives. I have an M.S. in Oceanography and a B.S. in Geologic Oceanography from URI, with a minor in Writing and Rhetoric. When I’m not writing and editing, I enjoy rowing, rock climbing, skiing, and reading.

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