a matter of scale: goodbye SeaWiFS

Today it was announced that the Sea-viewing Wide Field-of-View Sensor (SeaWiFS) mission was unrecoverable, marking the end of 13 years of ocean colour data. Now I love the ocean but even I can’t get upset about the demise of a satellite so don’t worry, this isn’t an eulogy to global satellite maps of the past. More a chance to think about what an amazing thing it is that we can even create these maps at all.

A map showing the global biosphere using chlorophyll a measurements to indicate the amount of phytoplankton in the ocean. Notice the high productivity in cold high latitude waters and coastal shelves which in turn support our economic fisheries.

Lets think about what this map is actually showing us. Ocean primary producers are the phytoplankton; tiny floating plants that play a major role in the global carbon cycle, oxygen production and marine food webs. The sheer mass of these little organisms floating around out there means this whole invisible forest packs as much of a punch in global carbon terms as the vast forests on the land. But they’re not easy to measure. This is Thalassiosira pseudonana, a marine diatom commonly studied in marine labs.

It looks impressive here but it’s actually about 4-6μm across. Micrometers. I did a quick search on clingfilm and that weigh in about around 10μm thick. So we’re talking about something so small it’s slimmer than clingfilm in oceans so vast they cover 2/3 of the Earth.

Luckily for us these little plankton need light for photosynthesis and so are restricted to the top ocean layers or the photic zone. Here they can gather in huge numbers, up to a million per teaspoon of seawater according to NASA. And to utilise this light phytoplankton contain light-harvesting pigments, such as chlorophyll. The visible light spectrum is absorbed by these pigments and the more phytoplankton present the more light is absorbed and so the colour we perceive the ocean to be from space will change. Using the bio-optical properties of the ocean we can determine changes in both phytoplankton abundance and dominant species using just subtle changes in ocean colour. And SeaWiFS does all this from 705km above the Earth. The straight line distance from John o Groats to Lands End is 970km. Viewing the collective properties of organisms thinner than clingfilm from 700km up in the atmosphere! We’re visualizing the microscopic at distances further than many people would like to drive in a day. Amazing. Not only have satellites such as this one allowed us to get a better handle on biogenic carbon sinks, they also allow us to get a better picture of the word around us and see things on the bigger scale. We can monitor the effect of ocean upwellings, currents and spring blooms.

The end of SeaWiFS does not mean the end of maps such as these so next time you see one of these maps remember you’re looking at the results of light absorption by organisms smaller than the eye can see measured by something so far away it took a rocket to get it there.

SeaWiFS ocean colour image: NASA Earth Observatory (2011) The Carbon Cycle [accessed 15.02.11] Available here
Thalassiosira pseudonana image: Alfred-Wegener-Institut (2004) Decoded gene sequence of the diatom Thallassiosira pseudonana. [accessed 15.02.11] Available here
SeaWiFS info:

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