Sir Alister Hardy

Sir Alister Hardy:
An ocean colour pioneer
(1896 - 1985)

by Trevor Platt and Venetia Stuart

This article has been commissioned by the IOCCG and has appeared in the backscatter magazine, published by the Alliance of Marine Remote Sensing (AMRS)

British scientist Sir Alister Hardy was one of the pioneers of modern biological oceanography. He began his scientific career after demobilisation from the First World War (during which he served in the Northern Cyclist Battalion). As a newly-qualified young scientist, he was assigned in the 1920's to study the relation between herring and its (planktonic) food supply. At that time, the standard method for assessing the abundance of plankton was to tow a net behind a slowly-moving ship. The net was made from the fine silk gauze used in the milling industry for sieving flour. Hardy noticed that replicate catches made by these nets at closely-spaced stations often gave quite different values for the local abundance of plankton, even when the replicate tows were made with the utmost care. He concluded that this was evidence of an intrinsic variability in the natural abundance of plankton, decided that such variability must be important, and devoted a considerable fraction of his career to its study.

He designed and had built the Hardy Continuous Plankton Recorder, an instrument that could be towed for long distances from ships. Water flowed through the instrument and through a piece of the same fine silk used to make plankton nets, capturing a small sample of the plankton. The forward motion of the ship was used to supply energy to advance the silk (which was carried in a continuous roll) in discrete steps, such that a continuous record of plankton abundance could be obtained at the scale of the ocean basin. This instrument was simple enough to be deployed by sailors on merchant ships, and thus many line miles of data could be obtained, which have added enormously to our knowledge of the structure and function of the marine ecosystem.

A modified version of this instrument is still being used by the Continuous Plankton Recorder (CPR) survey, in Plymouth, United Kingdom. This survey is coordinated by the Sir Alister Hardy Foundation for Ocean Sciences (SAHFOS) which is supported by grants and awards from several large nations and organizations, including the Intergovernmental Oceanographic Commission (IOC). Since the inception of the CPR survey, in 1931, approximately 4 million miles have been sampled and about 200,000 samples analysed.

Sir Alister Hardy on board the Discovery with his origional Continuous Plankton Recorder. Photograph reproduced from Great Waters Hardy, 1967 with kind permission of the publishers.

Currently, CPR's are towed over 80,000 nautical miles per year, producing some 4,000 samples annually. Over several decades, this unique plankton monitoring programme has provided a comprehensive data set on the spatial and temporal abundance and distribution patterns of upper layer plankton, on a broad geographical scale. The results from the CPR survey are now assuming even greater significance since the database extends from a period when marine pollution, exploitation and climatic effects were much less pronounced than they are today.

Because the local abundance of phytoplankton is often the principal determinant of the colour of the ocean, Hardy became interested in the occurrence and significance of differences in the colour of water masses. In his classical book `` The Open Sea'' (Hardy, 1956), he writes about the sharpness of the colour boundaries between water masses containing different kinds and abundances of plankton. These colour differences are often so strong that they are easily visible to the unaided human eye, with the result that they seem to make a line in the sea.

Describing an overflight of the English Channel, he writes ``In flying from Plymouth to the western mackerel grounds, we passed over a sharp line separating the green water of the Channel from the deep blue of the Atlantic; it ran on a slightly irregular course from the Lizard to the south-west as far as we could see to the distant horizon. Then while circling over the mackerel area we saw another equally definite boundary running from Land's End towards the Scilly Isles separating the deep blue water from a brown-green area lying to the north.'' Hardy goes on to say ``If these marked colour-changes can be correctly interpreted we may in the future find aircraft being used to make rapid surveys of the surface conditions in relation to fisheries.''

This overflight was made in the early 1920s when manned flight was still a novelty. Hardy was clearly far ahead of his time in realizing the significance of spatial differences in the kinds and abundance of plankton and in devising the means to study them. Until the advent of satellite remote sensing, the CPR provided the only means of collecting plankton data at large spatial scales: it was the remote sensing of the day. Further, by recognizing that colour differences in the ocean contained important biological information that could be surveyed rapidly with aircraft (satellites were still a technology of the future), he became a true pioneer of remotely-sensed ocean-colour science. As well as being a most distinguished scientist, Hardy was a painter of outstanding ability. Perhaps it was this combination of talents that enabled him to see so far into the future of ocean colour.

Reference List

Hardy, A. "The Open Sea", Fisher, J., J. Gilmour, J. Huxley, M. Davies, and E. Hosking, Eds., Collins, London, 1956.

This article appeared in the February 1997 issue of the backscatter magazine, published by the Alliance of Marine Remote Sensing

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