476 Dynamics of the Ecosystem 



in abundance, they reduce the nutrient supply concomitantly for a 

 time. Following the subsequent complete breakdown of thermal 

 stratification, wind stirring and reduced illumination stop diatom 

 growth as the area returns to its winter condition once more. How- 

 ever, the same deep stirring during the winter brings the nutrient- 

 rich water to the surface. This annual restoration of deep water to 

 the upper layers gives rise to the statement that "the sea is plowed 

 once a year." 



Consumers. When we turn to a consideration of the consumer in 

 the typical food chain of the open sea, we are confronted with a 

 situation of special interest because of the nature of the producers. 

 The simple fact that the plants at the base of the food chain are micro- 

 scopic in size has far-reaching repercussions in the oceanic ecosystem. 



(a) Herbivores. Scarcely any of the larger marine animals are 

 able to feed directly on diatoms because of their small size. The 

 menhaden is one of the few fish of commercial importance that can 

 do so. The gill rakers of this species are usually long and provided 

 with interlocking hooks that form a fine, sieve-like structure by means 

 of which the plant cells can be filtered from the sea in sufficient 

 quantities. The production of the menhaden thus represents a two- 

 link food chain— the shortest possible and a rarity among the large 

 animals of the sea in complete contrast to the situation on land. In 

 the terrestrial environment the availability in respect to size and 

 abundance of trees, grass, and other plant producers is such that deer, 

 elk, sheep, and other large ruminants can feed directly on them. Thus 

 short food chains are common on land. 



Since most of the larger animals in the sea are unable to use phyto- 

 plankton as food, they must depend for their nutriment upon "middle 

 men" or "key industry animals" of intermediate size. On the sea 

 bottom many mollusks and other sessile invertebrates possess feeding 

 mechanisms that enable them to filter the smaller planktonic forms 

 from the water. In the open sea planktonic crustaceans, especially 

 copepods and euphausiids, fill this niche. Copepods can filter out the 

 smallest diatoms by forcing a current of water through a meshwork of 

 bristles attached to a special set of appendages. We might think of 

 these small crustaceans acting as miniature harvesting machines, 

 packaging the microscopic plant cells. The filter-feeding zooplankton 

 incorporate the substance of the diatoms and represent parcels of 

 food of sufficient size for larger animals (Fig. 13.5). 



When the importance of copepods as essential middlemen in the 

 economy of the sea was recognized, special attention was directed 

 toward the factors controlling the food supply of these animals. 



