Secchi disk:  Simple technology for ocean science

We still have a lot to learn about the ocean.  Technology is helping us to uncover more and more about this blue planet, but not all technology needs to be complicated.  Some can be absurdly simple. The Secchi disk doesn’t look like much – a black and white/all white disk attached to some rope, a measuring tape, or a pole, with a weight suspended underneath.  It’s not even complicated to use – just lower it into the water until you can’t see it anymore (the point known as the Secchi depth).  Excited?  No me neither.  But this simple technology tells us something important – the turbidity of the water at that particular point, at that particular time.  

Turbidity is a measure of water clarity.  Particles in the water prevent light penetrating through the water column.  So the more particles there are, the less light can penetrate, and the more turbid it is.  Particles that restrict light can be almost anything – from silts and clays to plankton and other tiny critters.  Secchi disks in cruder forms (china plates!) have been around since the early 1800’s , but the actual disk itself was first developed and extensively used by Pietro Angelo Secchi – a priest and astronomer who undertook oceanographic research whilst aboard the papal yacht L’Immacolata Concezione.  There are of course more efficient and accurate ways to measure turbidity these days, but the Secchi disk is still in use.  Here are a couple of open access pieces for your enjoyment:

Building up a picture of the Baltic Sea

Because Secchi disks are ‘old school technology’ they have collected data on turbidity for a long time.  In some places – like the Baltic Sea – Secchi disks have been used repeatedly for a several years.  In this paper by Oer Sanden of Linkoping University and Bertil Jakansson of the Swedish Meteorological and Hydrological Institute compare the Secchi depths of two separate time periods – 1919 to 1939 and 1969 to 1991.  With this data, the researchers were able to get a grasp on the scale and magnitude of eutrophication (phytoplankton blooms as a result of increased nutrient) in the Baltic Sea.  Their analysis indicated that the Secchi depth was declining by an average of 0.05 meters per year, indicating that phytoplankton was indeed increasing, and thus turbidity decreasing.  The discussion is particularly interesting as the researchers explore the possible reasons why this decline would be seen.  You can access the paper here

Tying all the data together Long term data is great.  The more long-term data we have, the more we can start to understand trends and patterns and determine long-terms changes.  One of the problems we have is tying together data collected using different techniques as the measurements may not necessarily be consistent – especially as newer technology is often much more accurate than older technologies.  In this paper by Daniel Boyce and colleagues from Dalhousie University, Secchi disk data is tied to satellite remote sensing and newer in-situ measuring data.  They can do this because the Secchi disk data is surprisingly robust.  As long as the researchers can determine the methodology used to collect Secchi depths, then the information is comparable.  This is great news because this team of researchers were able to create a “globally integrated chlorophyll time series extending 120 years into the past”.   Chloro-what-now you ask?  Chlorophyll in the ocean is an indicator of phytoplankton biomass - tiny free-floating one-celled ‘plants’ that float freely in the ocean.  Very cool.  See their paper here

Get Involved!

Because the Secchi disk is so simple, it is a prime candidate for citizen science research.  Dr Richard Kirby of the Plymouth University Marine Institute is just one of many scientists around the world concerned with reports that in some areas oceanic plankton is declining, particularly as temperatures increase.  To get a better handle on what’s happening at a global scale, Richard is asking help from boat owners.  All you need to do is make your own Secchi disk, download a free app, and follow some very simple instructions.  Read more about this project – including how you can get involved and make your own disk - at Plymouth University'spage here

Image:  Dr Richard Kirby from Plymouth University demonstrates that the Secchi disk is so simple, even a scientist can use one.  Credit:  Image made available via the media pack.