‘Marine Snow’ Trilogy: Episode 1

Dr Nathan Briggs, National Oceanography Centre, UK

16/12/2019

EPISODE 1: What is “Marine Snow” and how does it help keep the earth cooler?

On land, plants use sunlight to take carbon dioxide from the air and convert it into organic matter as they go through photosynthesis. In the ocean, tiny, plant-like cells called “phytoplankton” do the same thing, taking carbon dioxide out of the water as they drift in the sunlit upper ocean. As long as this organic matter is “stored” in the phytoplankton, this means there is less carbon dioxide in the ocean. The upper ocean is closely connected with our atmosphere, thus carbon storage in phytoplankton leads to less carbon dioxide in the atmosphere as well.

So, does this mean that phytoplankton are like the trees of the ocean, locking up large amounts of carbon dioxide, keeping our planet cooler and our oceans less acidic?

Well, not exactly…

On land, trees can grow for centuries, storing more carbon each year, but in the ocean, tiny, single-celled phytoplankton do not. The organic matter in phytoplankton is usually consumed within days to weeks, either by the phytoplankton themselves or by the various, and tiny, “zooplankton” that eat them. When organic matter is consumed, its carbon is converted back to carbon dioxide, which makes its way back into the ocean (and atmosphere). Nevertheless, phytoplankton play an important role in carbon storage.

How? One important answer to this question has to do with the deep ocean… and “marine snow”.

Take a look at the photo below. Does it remind you of falling snow? This “marine snow” is in fact organic matter, originally produced by phytoplankton, which has aggregated into larger particles that sink slowly from the surface to the deep ocean. This marine snow is one of the reasons we have travelled 1000 km west of the southern tip of Chile. Specifically, we want to know how much marine snow is produced here and how deep it sinks before it is consumed. Here in the Southern Ocean, below the sunlit surface waters, currents drive some water back to the surface and other water deeper still, where it will remain out of contact with the surface for 100s to 1000s of years.

The image on the left is an example of the large clump of marine snow we are finding in the Southern Ocean during the CUSTARD research expedition. It will most likely be consumed within a few days. In that time, which current will it sink to? The upwards current? In this case, the carbon dioxide locked within it will be released back into the atmosphere. But if it is consumed within the downward current, the carbon dioxide it releases will be “stored” in the deep ocean, keeping it out of the atmosphere for centuries, and keeping the planet a little bit cooler (or reducing its warming) during that time.

The various research teams on board are trying to learn more about the formation, sinking, and consumption of marine snow in different ways.

My team’s role is to study marine snow in its “natural habitat” by lowering cameras into the water and counting the number, shapes and sizes marine snow particles that we see at different locations and different depths. We also look for the zooplankton that may eat the marine snow, or may eat phytoplankton and produce “fecal pellets” (plankton poo) that sink, adding to the marine snow. On the right is our “Red Camera Frame” being lowered into the water.

What have we found so far?

Two different research sites, with very different amounts and types of marine snow. At our southern site (60°S), where we took the marine snow picture above, we found lots of aggregates, they were big (some over centimeters across!), and we found them deep (well past 1000 m). This means they were either sinking fast or they were consumed less quickly than usual.

We also found some odd shapes. Many of the marine snow aggregates photographed with our “Underwater Vision Profiler” system, like the picture on the left, appeared to be “ring” or “donut” shaped! We are not sure whether these shapes are natural or caused by water flow around our camera, but either way these shapes might tell us something about how marine snow forms and interacts with ocean turbulence.

In our northern site (54°S) we have seen very few aggregates so far, but we have seen one smaller (less than 1 mm) aggregate with a half-millimeter zooplankton attached, presumably eating. We will continue to monitor marine snow at these sites over the next four weeks. Any changes in conditions, combined with the many other measurements on board, from turbulence to photosynthetic rates, may help us understand the cause of our southern “snowstorm”. And, who knows, we may also solve the mystery of the marine “snonuts”!

All photographs courtesy of Dr Nathan Briggs, NOC