THE POOD CHAIN 



systems and the environmental condition associated with them. It is also 

 due to the individual metabolic requirements of the species themselves. 

 As yet we know very little about their intimate chemistry, we partly know, 

 and partly suspect that, as each individual species grows, absorbing nutrients, 

 it secretes minute amounts of organic compounds into the water around 

 it, to be acted on by the bacteria. As these differ slightly the resultant bacterial 

 effect is also slightly different and the chemistry of the sea is then more suited 

 to another species. The first dies down having reached some limiting factor 

 in its requirements to be replaced by a second, and so on. This starts a 

 biological history of the water, to add complexity to its physical history, 

 and here the disintegration of the nanoplankton may be as important as its 

 growth and survival. The spring and early summer diatoms are, in general, 

 followed by the dinoflagellates as these can exist in a lower nutrient con- 

 centration, in part due to an ability to use dissolved or particulate organic 

 matter as well as inorganic nutrients and sunlight (pp. 38, 39 and Fig. 29). 

 Again the species of dinoflagcllate to thrive will be dependent on the bio- 

 logical history of the water. Sometimes there are mixtures but very often 

 one species outnumbers all the others put together and is dominant over 

 perhaps a hundred square miles or more, whilst an adjoining water mass 

 will be dominated by a different species. 



On these plants feed the herbivorous animals which form a large pro- 

 portion of the general plankton and an even greater proportion of the 

 planktonic larvae of the bottom-living species. But not all are capable of 

 feeding on any species of plant, so that the biological history of the water 

 which has helped to determine the plant species is responsible in turn for the 

 type of animals that thrive. Generally speaking only the very smallest of the 

 animals, especially the small invertebrate larvae, feed entirely on the nano- 

 plankton, the others are unable to filter off such minute organisms in sufficient 

 quantity, an exception being Oikopleura (Plate XIX). Others might take some 

 nanoplankton but it would be so rapidly digested that it would be difficult to 

 recognize in the stomach contents; the actual structure of the filtering mechan- 

 isms suggests that many do not do so to any extent. Animals feeding on diatoms 

 and dinoflagellates are limited in their choice by the size and shape of the 

 food and whether they ingest the food whole, or break it up first. Copepod 

 nauplii, for example (p. 68), and even the dinoflagcllate Noctihica (Plate IX), 

 ingest their food whole, and complete diatoms such as Skelctoiienia and 

 Thalassiosira, and dinoflagellates such as Pcridiiiiuin and Ditiophysis can be 

 seen inside them ; but the spiny species of Cliactoceros, the long Rhizosnlenia 

 and the horny Ccratiiiiii arc not often taken. This applies also to the very 

 smallest stages of the young fish larvae. 



On the other hand the adult herbivorous copepods can usually break up 



I 123 



