l6] VEGETATIONAL TYPES OF SEAS 515 



almost uniformly high, though the content of plant nutrients may 

 be relatively low in the upper layer, and these may be only slowly 

 replaced. 



In the neritic province the chemical constitution is more variable, 

 salinities being usually lower than in the open ocean, and sometimes 

 markedly so. Moreover they are apt to undergo such seasonal or 

 sporadic fluctuations that the inhabitants may have to be euryhaline 

 in nature. However, plant nutrients such as phosphates and nitrates 

 tend to be more readily available in these shallower inshore waters 

 than elsewhere — a fact which is of special importance in the produc- 

 tion of Diatoms, the foremost of ' primary sea-foods '. Conse- 

 quently a unit volume or even unit area of the neritic water is 

 commonly far more productive than a similar unit of oceanic water, 

 though the latter as a whole, because of its extent and depth, provides 

 the bulk of inhabitable space on earth. (The free atmosphere is 

 scarcely to be considered habitable space except very close to the 

 surface of the earth, for the organisms that are found free in it 

 appear to be making only temporary excursions therein, however 

 protracted these excursions may seem.) 



Plankton 



It seems best, in dealing with the sea, to consider all phytoplankton 

 together — without separating any macroscopic floating matter as 

 pleuston. Moreover, barring accidental detachment, for example 

 of marine vascular plants, when either death or resettling must soon 

 follow, the ' regular ' phytoplanktonic organisms of the sea are all 

 lowly Thallophytes or Schizophytes. Thus under the heading of 

 marine phytoplankton are commonly included all of the floating or 

 drifting forms of plant life of the oceanic and neritic provinces of 

 the sea — as befits the Greek derivation of the world plankton, which 

 means ' wanderer ' — whether they be microscopic, as in the vast 

 majority of cases, or quite large, as in the case of Sargasso-weed 

 {see p. 521). Although the number of different groups of plants 

 normally represented in plankton is limited, there is no dearth of 

 variety in form, as Fig. 171 indicates. 



The general characteristics of phytoplankton were discussed in 

 the last chapter, much of which was devoted to the conditions of 

 life and phytoplanktonic communities of fresh waters. Those of the 

 ocean are again mainly microscopic and so will not be described in 

 detail. They are chiefly Diatoms and Peridinians (Dinoflagellates), 



