The Seasonal Variation in Biological Composition of Certain Plankton Samples from the North Sea in Relation to Their Content of Vitamin A, Carotenoids, Chlorophyll, and Total Fatty Matter

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Abstract
A combined chemical and biological study of the plankton of the southern North Sea has been made. Three vertical hauls, with a Hensen net (silk cone, 60 meshes to the inch) were taken at six positions between Flamborough Head and the South-West Patch of the Dogger Bank, monthly from January to November 1936. One haul was used for biological examination, the second for determinations of percentage of ether-soluble matter and the third to determine carotenoids, chlorophyll, vitamin A and total solids. The biological results showed that the total plankton depended on three diatom outbursts, the largest in May, another little inferior in numbers in August, and a small one in October. Peridinians and the zooplankton occurred successively in relatively greater numbers with each outburst, but reached their maximum in August when the mass of the plankton was at its greatest. Carotene, chlorophyll and vitamin A were definitely detected in the gross plankton extracts, whilst positive tests for fucoxanthin were only obtained on a few isolated occasions. An examination of a large phytoplankton sample (obtained by tow net) containing Rhizosolenia styliformis and Biddulphia sinensis only, showed that no vitamin A as such was present. Carotene and xanthophyll, however, were present, in the ratio of 1 : 1.82, which is comparable with the ratio typical of land plants. Total carotenoids equalled 0.1% calculated on dry weight. Of the chemical constituents the seasonal variation of chlorophyll most nearly coincided with the total mass of the plankton. The carotenoids reached their peak slightly before the maximal biological development, whilst the vitamin A content reached its maximum in the month after the spring diatom outburst and much preceded the maximum for the plankton crop as sampled by the Hensen net.