36 



UNDERWATER GUIDE TO MARINE LIFE 



While it may be true that httle is known of the deep sea, we do not beheve 

 that an unknown hnk in the food chain is necessary to explain the abundance 

 of life in the depths. The sun is still the source of energy, although indirectly, 

 but two food chains, which are linked together, are really involved, one at the 

 surface and one in the depths (fxg. 8). Life in the depths is connected to the 

 fundamental reaction of photosynthesis at the surface and is not self-sustaining, 

 but the basic form of life in the depths is not photosynthetic; it is bacteria 

 which cause decay and nitrification. Bacteria is a sort of "plankton of the depths" 

 with respect to food chains there, and since bacteria is most abundant at the 

 bottom, so are the animals that feed on bacteria. 



SUN 



X ^ 



PLANT PLANKTON ^HERBIVORES ^CARNIVORES 



TO PLANT PLANKTON 



DrcAY ^ ORGANIC 



BACTERIA MATTER 



\ 



AMMONIA ^-NITRIFYING BACTERIA- 



-NITRITES AND NITRATES 



Fig. 8. Deef sea food chains. Animals of the deef scattering layer (DSIS) and deep 

 sea are dependent wpon two sources for their hasic food: I) the constant rain of dead 

 animals from the surface and 2) the deep water plankton consisting of various 

 bacteria and simple plants. Both of these sources are dependent upon the sun, directly 

 or indirectly, for energy. Since surface life is most abundant in the first 250 feet of 

 water and bottom life is most abundant in the deep sea, a graphic representation of 

 the numbers of animals at various depths takes the form of an hourglass as shown on 

 the left. 



So far, we have dealt only with food chains involving a few animals. When 

 a whole community is considered, the food chains of all the species must be 

 added up to form a food cycle. Figure 9 shows a greatly simplified cycle of the 

 coral reef. 



Food chains have a great effect on animal numbers. Carnivores, for instance, 

 could never become more numerous than the herbivores they eat. It is estimated 

 that for every planktonic animal there are seven planktonic plants. The numbers 

 of planktonic animals per baleen whale is almost inconceivable. Figure 10 

 shows a pyramid of numbers in which the most numerous forms of life are 

 shown to be the photosynthetic plankton and the least numerous are the large 

 carnivores. This concept explains the relative rarity of the extremely large, 

 voracious predators like the killer whale and the white shark. 



