FACTORS INFLUENCING PHYTOPLANKTON PRODUCTION 333 



tically (Hart, 1934, pp. 185-6). Frequent observation of immense numbers of faecal 

 pellets accompanying a comparatively poor phytoplankton during the post-maximal 

 decrease have been made, mainly in the Northern and Intermediate Regions, in the 

 course of the work at sea. As described in the itinerary these observations suggested 

 that heavy grazing by zooplankton herbivores was in part responsible for the decrease, 

 and is thus probably the most important biological factor influencing production. 



With the data available for earlier work it was impossible to use calculations of 

 minimum crop from observed decrease in nutrients because of the lack of repeated 

 observations in one area over short intervals of time. The speed of horizontal move- 

 ments of the surface layers made it seem improbable that such calculations could ever 

 be usefully attempted (Hart, 1934, pp. 184-5). Since that paper was written numerous 

 repeated series of observations at short time intervals have been obtained which render 

 such calculations possible. They must always remain somewhat speculative, but as 

 the following considerations should show, they support the view that temporary shortage 

 of silica combined with the grazing-down factor, are largely responsible for the post- 

 maximal decrease. This view is also largely supported, on general grounds, by the work 

 of Clowes (1938). 



Minimal crop calculations based on observed reduction of nutrient substances in the 

 sea were first made by Moore et <?/. (1914) and Atkins (1926). They are made by simple 

 proportion from the observed reduction and the minimal amounts of the particular 

 substances present in phytoplankton, or, as with CO2 assimilation, equivalent quantities 

 of carbohydrate. The figures for amounts of the various substances present in the 

 plankton are derived from divers separate investigations quoted by Cooper (1933, 

 pp. 741 et seq.). It has become usual to express the results of such calculations in 

 metric tons wet weight of phytoplankton per km. sq. of sea surface, the depth covered 

 by the investigation being duly taken into account. An example of the method of 

 working is as follows: 



At station L 4 in the English Channel, Cooper (1933, p. 743) records a drop of 

 ii6mg./m.3 in nitrogen content, over the whole water column (72-0 m.) between 

 4 December 1930 and 10 July 1931. Nitrogen has been found to form 0-5% of the 

 wet weight of algae. It follows that at least 23,200 mg. or 23-2 gm. per m.^ of phyto- 

 plankton was produced during this period, for the initial figure refers to nitrate + nitrite 

 nitrogen only and takes no account of other less important sources of nitrogen known 

 to be available to the plants. The sum may be continued : 



23-2 gm. per m.* = 23-2 x 72 x 1,000,000 gm. per km. sq. on 72 m. depth 

 = 23-2x72,000 kgm. 

 = 1,670-4 metric tons. 

 Cooper (1933, p. 744) has compared the theoretical minimum production m the 

 English Channel on the basis of the observed reduction of carbon dioxide, phosphate, 

 nitrate and silica; obtaining good agreement by the first three methods, rendered even 

 closer by correction of the phosphate result for salt error (Cooper, 1938, p. 190). The 



