Some 50% of the earth’s vegetation consists of microscopic plant organisms that drift in the few hundred uppermost metres of the earth’s oceans. As these organisms also account for about 50% of all the photosynthesis, or the trapping of atmospheric carbon into carbohydrates, keeping a close watch on their abundance and health is as vital as monitoring the forest and green cover on the earth’s land masses.

Plankton are the animals, plants, bacteria and other single-celled creatures that inhabit the upper part of the open sea. The part of this community, which uses light, with the help of chlorophyll, to convert carbon dioxide into plant matter are phytoplankton.

Mostly, they are too small to be seen except through a microscope, but in large numbers, they appear as a green shade in the water. Being living things that seek food and energy, they show drifting behaviour, sinking to deep water in search of minerals and nutrients, and rise to the surface to make the best of sunlight. Hence the name - from phyton, or plant and planktos or wanderer.

Various measurements indicate that phytoplankton abundance has been declining over the last century. This has implications for marine ecosystems, carbon cycling and fishery yields the world over. This apart, the phytoplankton level can serve as a sensitive indicator of the ocean warming and other conditions that bring about the changes. Hence the interest and effort and urgency for careful observation and monitoring.   

Magic of the Secchi disc

Methods of assessment include analysis of samples of seawater by spectrometric methods or by irradiating the sample and then watching for fluorescence, of the suspended plant material. But a faster method, to cover large areas, is either by measuring the transparency of seawater, or the amount of light reflected by the sea.  

This last method, of measuring the light reflected by the sea, using satellite imaging, is now the most productive method. But the data through this route has been collected only after 1979, which is not enough for long-term analyses. Fortunately, there is a supplementary resource of reasonably accurate transparency data available right from the year 1899, through surveys of the transparency of the sea, that used a simple device called the Secchi disk.

Father Pietro Angelo Secchi SJ was versatile priest-astronomer-scientist of Italy, who lived from 1818 to 1878. He had a successful academic career and rose to be the Director of the observatory of the Pontifical Gregorian University, in Rome, for 28 years. His work in astronomy was extensive, he compiled data of over 10,000 binary stars, he discovered three comets, including one that is named after him as comet Secchi,  he observed and drew maps of the moon, Mars and the surface of the sun, he did important work with the spectra of stars and developed a first system of classifying stars.

Plumbing new depths

Apart from his work in astronomy, Secchi also contributed to physics, meteorology and oceanography, and made a mark in this third field with his simple device, the Secchi disc, for measuring transparency of seawater. In 1865, Father Secchi was asked to map the clarity of the Mediterranean Sea for the Papal navy.

This is when he invented the simple contraption, just a black and white coloured, circular disk, which is lowered into the water by a rod or a line till it is no longer visible. The depth at which this happens was taken as a measure of clarity or opacity. This simple measure, which came to be known as the Secchi depth, became the unit for innumerable oceanography investigations for nearly a century.

But more interestingly, the Secchi depth is readily converted into a measure of the abundance of phytoplankton, the chief reason for the opacity of sea water. The oceanographic records of the past century thus become a valuable history of the rise and fall of phytoplankton, world-wide and over a long, continuous period.

The modern method of satellite imaging uses the level of sunlight that is reflected from the earth. Here, there is first the reflection of light by the atmosphere and then the reflection from the surface of the sea, which need to be accounted for before the relatively feeble reflection because of seawater opacity can be assessed. But the satellite method has the benefit of fast and accurate spectroscopic analysis and also the capacity to assess levels of not just phytoplankton but other organisms as well.  

Mapping the correlation

Daniel G Boyce, Marlon R Lewis and Boris Worm, of Halifax, Canada have reported in the journal, Nature, a review of satellite data, in conjunction with the record of Secchi depth measurements, to plot the variations on plankton levels, at local, regional and global scales, over the last century.

The review reveals strong correspondence between the phytoplankton, and hence chlorophyll record, and changes both in the leading climate indices as well as conditions of ocean temperature. The study also shows statistically significant long-term decrease in chlorophyll concentrations for eight of the ten ocean basis, as well on overall basis.

This finding is also consistent with the satellite observation of ocean colour, which indicates that fall in indices of phytoplankton productivity corresponds to increases in ocean warming. This is a grave feature, in a world that faces warming, as phytoplankton is basic to the food chain and the productivity of the sea.

The current methods of satellite based assessment are still affected by many factors, such as limited life spans of satellite observation posts and the need to standardise the kind of instrumentation.

There is then the matter of cost and complexity and competing demands on science funding by states. The work of Boyce and others may help ensure that this area of investigation does get the attention it merits.