FIG. 28-3 Characteristic holoplankton (Sverdrup e^ o/. 1942). 

 (A) Protozoa: a) foramlnlfera Globigerina, b) dinoflagellate 

 Gymnodinium, c) tintinnid Stenosomella, d) tintinnid Flavella, e) 

 radiolarian Proioeysfh, f) another radiolarian, g) dinoflagellate 

 Noc^Vuco. 



amounts for the shells of mollusks, the skeletons of 

 corals, some protozoans and worms, certain algae, 

 the other organisms and may be precipitated out of 

 the water by bacteria. Silicon is required by sponges, 

 some protozoans, and the phytoplankton diatoms. 



These salts keep cycling through the ecosystem, 

 but additions to the supply come continually from the 

 land, being washed into the oceans by the rivers. 

 Neritic waters are especially fertile and support a 

 great mass and variety of animal life because of this 

 land drainage and the pattern of water circulation 

 on the continental shelf. Biological productivity de- 

 creases progressively from shallow waters over the 

 continental shelf, to deeper waters, to the open ocean, 

 but is also high over offshore banks and in areas of 

 upwelling. Substantial amounts of nitrogen salts are 

 also swept out of the air by precipitation, and there is 

 nitrogen-fixation by bacteria. 



FIG. 28-3 (B) Coelenterates and ctenophores: a) connb-jeiiy 

 Pleurobrachia, b) siphonophore Velella, c] jellyfish Aglanfho, 

 d) siphonophore Diphyes. 



It is of interest that atoms of phosphorus, nitro- 

 gen, and carbon occur in sea water in ratios of 

 1:15:1000 and in plankton in ratios of 1:16:106. 

 This means that there is an overabundance of carbon 

 available in the sea for absorption by the phytoplank- 

 ton, but phosphorus and nitrogen may be limiting 

 for further increases in the population of organisms 

 (Redfield 1958). 



Oxygen 



The oxygen supply of sea water comes by diffu- 

 sion from the air at the surface and from photosyn- 

 thesis of green plants down to the compensation 

 point. It is continuously used at all depths in respira- 

 tion of animals and plants and in the decomposition 

 of organic matter. 



The oxygen content of sea water (Hedgpeth 

 1957) is seldom limiting for the occurrence of ani- 

 mals, except in the deeper waters of the brackish 

 Black and Caspian Seas where it is practically ab- 

 sent. Oxygen concentration is especially high on 

 shores where there is splashing of waves. Surface 

 waters of the Atlantic Ocean commonly have 4.5 to 

 7.5 cc/1 and abyssal regions may run over 5 cc/1. 

 Oxygen is somewhat less abundant in the Pacific and 

 Indian Oceans. Oxygen may be reduced to lower 

 concentrations between 100 and 1500 m, because of 

 its use in animal respiration and in decomposition, 

 than at lesser depths where there is photosynthesis 

 or at greater depths where the abundance of animals 

 is greatly diminished. 



Marine animals have a variety of mechanisms and 

 adaptations for respiration (Flattely and Walton 

 1922). Greatest difficulties occur in shore animals 

 at low tide when they are exposed to the air, but 

 the need for oxygen at this time is decreased in many 

 forms by curtailment of activity. Some crabs, barna- 

 cles, snails, and fish have become almost amphibious 

 in being capable of respiring in air, although at re- 

 duced rates, as well as in water. Pure mud bottoms 

 may present anaerobic conditions a short distance 

 below the surface, but mud bottoms mixed with sand 

 contain an abundant and diversified fauna. 



PLANKTON 



Composition 



The plankton of the sea includes a great variety 

 of forms, even more than in fresh water (Biglow 

 1926, Hardy 1956). Rotifers, however, are uncom- 

 mon in marine plankton and cladocerans are much 

 less important. 



The nannoplankton consists mostly of flagellates, 



356 Geographic distribution of communities 



