Citizen Science and Phytoplankton: The Secchi Disk Project

Dr Richard Kirby of Plymouth University is working with sailors around the world on his Secchi Disk program to study marine phytoplankton. All you need is a smart phone and a Secchi disk.

The importance of plankton

Phytoplankton live at the sunlit sea surface and they are the ocean’s primary producers – the ‘plants’ of the sea. Despite their small size they are so numerous that they account for approximately 50% of all photosynthesis on Earth. Since one of the by-products of photosynthesis is oxygen, they also produce 50% of the oxygen in the air we breathe – every second breath you take.

From their position at the bottom of the marine food web the phytoplankton determine the abundance of all creatures at every step above in the marine food web above. From the numbers of fish in the sea, to the number of whales, turtles, sharks, and seals, to the number of polar bears on the Arctic ice to the number of penguins in the Antarctic sea, and the seabirds in the sky.

The need for good data

Scientists are now debating the merits of a study that reports that phytoplankton had declined globally by 50% since the 1940s.

The Secchi Disk

Part of the problem in these studies, is that the oceans are vast and there are not that many scientists to study them and so the data is sparse. There are a lot of sailors however, and there is a very simple way to estimate phytoplankton in the oceans using a device ‘invented’ by the Pope’s Astronomer Pietro Angelo Secchi in 1865 and called the Secchi Disk.

It is this device and the opportunities offered by modern mobile phone technology that inspired a group of Plymouth Scientists. Led by Dr Richard Kirby, they have created a project that enables sailors to become seafaring Citizen Scientists to help create the largest global study of the marine phytoplankton – starting now and carrying on indefinitely into the future.

The aim is to establish a spatially extensive and long-term dataset to help address the debate surrounding changes in the ocean’s phytoplankton.

Secchi Disk FAQS

  • Why is this project important?
    The phytoplankton underpin the marine food chain and so we need to know as much about them as possible.
  • What are phytoplankton?
    Phytoplankton – also called microalgae – are microscopic plant-like cells and they are the sea’s main primary producers.
  • Who can use the Secchi App?
    The Secchi App is for seafarers and scientists.
  • Is the Secchi App free?
    The Secchi App is free and it is free to take part, however, you have to make a Secchi Disk.
  • What is a Secchi Disk?
    A Secchi Disk is a 30 cm, flat, white disk attached to a tape measure or a rope, and weighted from below by a small 200g weight.
  • Do all Secchi Disks have to be 30 cm in diameter?
    Yes. It is very important that everyone uses a disk that is the same size to standardise the survey.
  • Can a Secchi Disk be made from anything?
    Yes. A Secchi Disk can be made from any material so long as it is white, or it is painted white.
  • What is the Secchi Disk used for?
    A Secchi Disk is used to measure the Secchi Depth. The Secchi Depth measures the turbidity of the water, which is influenced by the amount of phytoplankton in the water column.
  • How do you use Secchi Disk to measure the Secchi Depth?
    With the sun behind you, you lower the Secchi Disk vertically into the water from the side of your boat. The point at which the disk just disappears from sight is the Secchi Depth.
  • How often should I take a Secchi Depth reading?
    You can take a Secchi Disk reading as often as you wish, every day, once a week, twice a month, or just occasionally.
  • Where should I take a Secchi Disk reading?
    You can take a Secchi Disk reading wherever you wish at sea so long as you cannot see the seabed and you avoid estuaries.
  • Should I measure the Secchi Depth at a particular time of day?
    Yes. The Secchi Depth should be measured between 10.00 hrs and 14.00 hrs.
  • Should I remove my sunglasses?
    Yes. Please do not wear sunglasses when you measure the Secchi Depth.
  • Should I clean the Secchi Disk?
    Yes. It is important to keep your Secchi Disk clean so that its visibility does not change.
  • Who will use the data I collect?
    The data you collect will help scientists around the world to understand the phytoplankton.
  • Do I need a network connection to use the Secchi App?
    You do not need a network connection to use the Secchi app. Secchi stores the data you collect while at sea until you get a network connection when you will be prompted to upload your data to the database. If you wish, you can defer the upload until another time in case you have joined a roaming network.
  • What happens if I submit a Secchi Depth when I have no network connection?
    In this case, the data will be stored on your phone as a pending reading. When you next get a network connection you will be prompted to either submit or delay until later.
  • How much data am I uploading to the database?
    Secchi data occupies about 4 kb per reading. Photographs occupy about 50 kb.
  • How long will the project run?
    This project begins now and continues indefinitely into the future.

Watch this

Ocean Drifters: a secret world beneath the waves, from Plymouth University
(film narrated by David Attenborough, on Vimeo)

Useful links

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