Solids in the sea
Solids in the sea
When we think of the ocean, the first thing that comes to our mind is probably just water – billions of litres of water. But if we filter seawater through a filter disc with a mesh of smaller than 0.45 µm (or 0.00045 mm), we find some amount of particles. In some areas in the ocean we find lots of particles, such as near river mouths where several grams of sediment can be found in 1 litre of seawater Other areas however have almost no particles in seawater, for example in the deep Southern Ocean around Antarctica. Here the waters are so clear that visibilities of 50 m are very common.
There are many different types of particles in the ocean in all sorts of shapes and sizes. We can group these particle types into three broad categories. The first group are particles from sediments introduced by rivers and estuaries or by aerosols, such as dust or volcanic ash. These rock-derived particles are inorganic compounds (even though rivers and estuaries carry a significant load of organic material too). Although they are very important for the ocean chemistry, we haven’t quite understood yet how particles and seawater interact. For example much of the iron, which is an important nutrient, is vastly introduced to seawater by dust. However, these dust particles do not only introduce elements into the ocean, they also remove them by scavenging. The balance between elemental input and output by particles is one of the big challenges in modern chemical oceanography.
Saharan dust plume over the East Atlantic. Image: www.nasa.gov
Another type of particles are living and dead organisms – or particulate organic matter (POM). This type of particles can range in size between a couple of nanometres to up to tens of metres, — if we count the large animals as particles too! The deep ocean typically has lower POM than in the upper few hundred metres of the surface ocean (on average 19.2 and 192 micrograms per litre, respectively (1)). Organic particles are a very important component of the carbon cycle in the surface ocean, because POM consists of roughly 50% organic carbon. Many of the tiniest organisms, such as algae, take up atmospheric inorganic carbon, i.e. CO2, and reduce it to organic carbon by photosynthesis. These microorganisms can cover huge areas in the ocean during their bloom in spring or autumn.
Satellite image of algae activity in the Southern Ocean. Image from NASA’s Earth Observatory (www.nasa.gov).
The third type of particles are the ones that humans have introduced into the ocean, such as plastic or chemicals forming solid compounds. A recent estimate on how much plastic currently floats in the large ocean gyres of the global ocean accounts to about 5.25 trillion pieces of plastic weighing almost 270,000 tons (2). With 4.8 to 12.7 million tons estimates for 2010 (3) the mass of plastic waste we dump into the ocean is even greater. Plastic and microplastic in the ocean is a very problematic type of particles for marine life – and for us. Larger plastic pieces, such as shopping bags, are easily confused with jelly fish. Jelly fish are the main food source for sea turtles, but also sharks and other large fish. In 2013 a sperm whale shored in Spain – cause of death: 17kg of swallowed plastic.
The Great Pacific Garbage Patch. Image: Courtesy of the NOAA Marine Debris Program.
References:
Millero, F.J., 2005. Chemical Oceanography, 3rd edition. 520 pages, CRC Press.
Eriksen, M., Lebreton, L.C.M., Carson, H.S., Thiel, M., Moore, C.J., Borerro, J.C., Galgani, F., Ryan, P.G., Reisser, J., 2014. Plastic Pollution in the World’s Oceans: More than 5 Trillion Plastic Pieces Weighing over 250,000 Tons Afloat at Sea. PLoS One 9, e111913. doi:10.1371/journal.pone.0111913
Jambeck, J.R., Geyer, R., Wilcox, C., Siegler, T.R., Perryman, M., Andrady, A., Narayan, R., Law, K.L., 2015. Plastic waste inputs from land into the ocean. Science (80-. ). 347, 768–771. doi:10.1126/science.1260352