540 CYCLES OF ORGANIC AND INORGANIC SUBSTANCES 



been carried out at coastal stations throughout the entire year. 

 Lohmann (1908) was perhaps the first, who made not only a 

 quantitative census of the phytoplankton species but also calcu- 

 lated the total biomass, expressed as cubic millimeters per 100 

 liters. He likewise compared the food requirements of the standing 

 zooplankton crop with the a\'ailable abundance of the phyto- 

 plankton and recognized that during the winter the zooplankton 

 must feed on detritus to a very large extent. 



Despite this early work, many problems centering on the 

 phytoplankton population in the oceans are still unsolved. Our 

 attention must first turn then to the activities of phytoplankton, 

 the first component in the system considered here. The dark-light 

 bottle method for oxygen production and that of C^^ fixation were 

 milestones in the development of our understanding of its role. 

 Yet there is much more to be done. Productivity, for example, 

 must be correlated with the standing crop. In short, we must know 

 the rate of assimilation more exactly. This \'aries between three 

 and thirty in relation to the life histories of individual phyto- 

 plankton species. It may be possible to correlate productivity not 

 only with chlorophyll concentration but also with the truly active 

 substance within the cells, the proteins. 



In general, phytoplankton assimilates material until no more 

 nutrients are present in the surrounding water provided that 

 there is sufficient light available and that there are no inhibiting 

 substances present to prevent this activity (Steemann Nielsen, 

 1955; j0rgensen, 1956). This activity of the phytoplankton 

 may utilize all the specific nutrients which then become limiting 

 factors within a few days or perhaps a few weeks. In the sea, this 

 process is much more rapid than on land. Unfortunately, our 

 knowledge of the assimilation potential and more especially of the 

 reverse, the decomposition under natural conditions, is relatively 

 limited. This has been discussed during the Congress by Steemann 

 Nielsen (1959). We must try to relate these two processes with the 

 truly active substance in the phytoplankton, the proteins. Special 

 attention should also be given to the respiration of the phyto- 

 plankton when there is insufficient light. 



