„ 4 DISCOVERY REPORTS 



in the English Channel the number of faecal pellets found in a plankton sample bears 

 a distinct relation to the amount of plant food available. These workers draw the con- 

 clusion that the amount of plant food eaten depends rather on the amount of food 

 available than on the dietary requirements of the animal population. If therefore a 

 rapidly increasing zooplankton concentration consumes phytoplankton to such a large 

 degree it is obvious that any checking in the growth rate of phytoplankton by a limitation 

 of an essential nutrient salt will exert a large influence on the amount of phytoplankton 

 available at that time. During summer in the Antarctic both phyto- and zooplankton 

 are present in enormous concentrations but it is significant that the highest concentra- 

 tion of zooplankton occurs after the maximum concentration of phytoplankton. 



As further evidence of the effect of a limitation in the amount of silicate available 

 Dr Hart tells me that the robust, spiny phase of Corethron criophilum (which is a 

 dominant species in the early part of the phytoplankton season in the Antarctic zone) 

 tends to change over in late summer to the spineless chain form with very thin, fragile 

 walls. Also, Dinoflagellates tend to become important in the extreme northern part of 

 the northern region of the Antarctic only in February and these organisms probably 

 contain less silicate than equivalent quantities of diatoms. 



Although silicate is considered to be quickly returned to the sea water, phosphate on 

 the other hand is held for some considerable time as organically combined phosphorus 

 compounds and as such is unavailable for the needs of phytoplankton growth. Thus 

 when phytoplankton is consumed and a very rapid regeneration of silicate takes place, 

 the growth rate of phytoplankton is not visibly increased as far as our experience holds. 

 In this sense then phosphate may possibly be limiting and when the required interval 

 for the breakdown of organically combined phosphorus has taken place, then some other 

 factor such as illumination may prevent a large subsequent outburst of phytoplankton 

 in the autumn. In this connection it may be added that some slight evidence does exist 

 for a small secondary autumnal outburst of phytoplankton in the south. 



Because, at the majority of Antarctic stations the amounts of phosphate and silicate 

 found in the surface layer never fall below the amounts present in winter in the English 

 Channel, it is possible that factors other than limitation of phosphate or silicate must 

 be invoked to explain the sudden falling off of the huge concentrations of phyto- 

 plankton in the Antarctic. Dr Hart (1934, pp. 183-193) has reviewed many such factors 

 and the reader is referred to this work for a full account. Dr Hart considers both 

 chemical and physical factors but thinks physical factors exert the strongest influences 

 upon phytoplankton production in the far south — such factors being the influences 

 due to weather, currents (including turbulence), light, ice conditions and temperature. 



