1) Do you love the ocean? Citizen scientists are collecting phytoplankton data in a unique study using a piece of equipment developed by the Pope's astronomer in 1865, and they're getting results. Read this thread to find out the full story and how you can join in. See 2 
2) In the late 18th century sailors learned to lower a white object, often a plate, from the side of their boat into the water as a sounding method to determine the water clarity; different bodies of water had different clarity, for example waters close to shore were often less clear due to suspended sediment. See 3
3) in 1864, the Pope wanted to chart the currents in the Mediterranean and asked the commander of his navy, Alessandro Cialdi, to conduct a survey. Cialdi knew that you could use a white disk to measure water clarity and that could tell the location of different water bodies and position of currents but, See 4
4) Cialdi wasn’t a scientist and the white disk method was rudimentary. However, the Pope employed a scientist and that was his astronomer Pietro Angelo Secchi. So Cialdi invited Secchi on a cruise onboard the Papal yacht called the Immaculate Conception and on that cruise, See 5
5) Secchi formalised using a white disc to measure ocean clarity and the scientific instrument called the Secchi Disk was born. Secchi published a paper on the currents of the Mediterranean in 1865 (). But what has this to do with phytoplankton and marine citizen science? See 6link.springer.com/article/10.100…
6) Away from estuaries and coasts the main determinant of water clarity is the phytoplankton and so the Secchi Disk became used by marine biologists to determine the biomass of phytoplankton in the water column. The more phytoplankton the less clear the water. Scientists take a measurement called the Secchi Depth, See 7
7) The marine Disk, as developed by Secchi, is a 30 cm white disk. A weight is hung below and it is attached to a marked line or tape measure. The Secchi Disk is lowered over the side of the boat and the depth at which it just disappears from sight is called the Secchi Depth and this measures the water clarity, or phytoplankton biomass. See 8
8) The Secchi Disk is arguably, the simplest piece of biological oceanographic equipment. In 2013 I created the SecchiDisk.org Citizen Science study for sailors, or any seafarer, to measure the phytoplankton using a DIY Secchi Disk: bbc.co.uk/news/education… See 9
9) I created the Secchi Disk study in response to a 2010 research paper () that suggested the oceans' phytoplankton had declined by 40% over the preceding 50 years. See 10 for a popular article.nature.com/articles/natur…
10) Phytoplankton drops 40% See 11scientificamerican.com/article/phytop…
11) The explanation given by the scientists was that a warmer surface ocean had reduced the nutrients reaching the surface from deeper waters; simply put, there was less fertiliser. The phytoplankton underpin the marine food chain and a 50% decline in their abundance would affect all other life in the sea. See 12
12) Since 2010, new studies have examined phytoplankton abundance and found varying results in different regions. A recent study published in 2023 has provided evidence for a greening of low latitude surface waters over the last 20 years, or more phytoplankton . See 13nature.com/articles/s4158…
13) Today, most phytoplankton abundance studies rely upon remote sensing of ocean colour by satellites. Satellite data only provides an integrated value however, that simplifies the biological and environmental vertical & horizontal heterogeneity. . See 14oceancolor.gsfc.nasa.gov
14) Therefore, in situ measurements are still very important and one of the simplest tools to take in situ measurements is the Secchi Disk that is weighted from below with ~ 1 kg and attached to a Fibreglass tape measure. See 15
15) There are not that many marine scientists to cover the oceans but, there are a lot of sailors and this is where citizen science can be a powerful tool. . See 16bbc.co.uk/news/science-e…
16) The Secchi Disk study is collecting data throughout the oceans. In 2014 the study achieved the first measurements in the ice-free Northwest passage . Today, it is now the world's largest citizen science marine phytoplankton study. See 17bbc.co.uk/news/science-e…
17) It is not just sailors that are taking part. Fishermen are too and people on holiday, See 18bbc.co.uk/news/science-e…
18) Passengers on expedition cruises to places such as Antarctica and the Arctic are collecting phytoplankton data from these remote areas . See 19cruiseindustrynews.com/cruise-news/20…
19) The Secchi Disk study has published two papers showing how citizen scientists can contribute to our knowledge of the phytoplankton and both articles are free to read, download and share: A) journals.plos.org/plosone/articl… and B) nature.com/articles/s4159…. See 20
20. This week the paper was published in the journal Big Earth Data using some of the citizen science data, which the authors say statistical analysis shows no deviation of the measurements performed by citizens in comparison to the official measurements. See 21tandfonline.com/doi/full/10.10…
21) So why not watch this video, make your Secchi Disk and take part in the Secchi Disk study if you go to sea. You'll help make the current map of citizen science data even bigger and better. It is of fundamental importance to understand the marine phytoplankton. See 22
22) The Secchi Disk study is supported by the UK-registered charity .SecchiDiskFoundation.org
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