organism may be considerably higher if it possesses 

 a skeleton or shell. 



Organisms have various devices to remain afloat, 

 aside from swimming : absorption of large amounts of 

 water to form jelly-like tissues or sap, storage of gas 

 or air bubbles within the body, formation of light- 

 weight fat deposits in the body or oil droplets within 

 the cells, increase of surface area in proportion to 

 body mass thereby increasing frictional resistance. 

 Increase in the relative amount of body surface is 

 achieved by decrease in size, flattening, attenuation 

 of body form, extensions of body parts as antennae, 

 spines, tentacles, or cerci, surface hairs or tubercles 

 of various sorts, surface sculpturing, or formation 

 of colonies (Marshall 1954, Davis 1955). These de- 

 vices result in many strange shapes among plankton 

 organisms. When the organisms die, the protoplasm 

 disintegrates, special flotation mechanisms are usu- 

 ally destroyed, there is a loss of swimming move- 

 ments, and what is left of the organism sinks to the 

 bottom. 



Abundance 



The actual abundance of plankton varies 

 greatly from place to place and from one season to 

 the next. Smaller species tend to be more numerous 

 than larger ones. The mean annual abundance of 

 diatoms is commonly in the tens of thousands per 

 liter and for shorter periods during the year algal 

 blooms may increase the population to hundreds of 

 thousands of cells per liter (Ricketts and Calvin 

 1948). Zooplankton is, however, much less abun- 

 dant. It has been repeatedly noted that large popula- 

 tions of phyto- and zooplankton do not occur in the 

 same place at the same time. Various explanations 

 have been offered : one, that it is due to the feeding 

 of the zooplankton on the phytoplankton (Harvey 

 1934), and another, that it is due to the phytoplank- 

 ton, when abundant, producing conditions that are 

 inimical or toxic to the zooplankton (Hardy and 

 Gunther 1935). 



The total net zooplankton per unit volume of 

 water is some 16 times more numerous in the neritic 

 coastal waters off the Atlantic coast of North Amer- 

 ica than in the Sargasso Sea (Clarke 1940). The 

 abundance of plankton is generally higher in cold than 

 in warm ocean waters, correlated with the greater 

 amount of phosphate present in colder waters 

 (Harvey 1955). Annual productivity is probably 

 less in cold waters, however, because there are fewer 

 generations per year. Cold-water plankton tend to 

 be of larger individual size. There is generally a 

 greater variety of species in most taxonomic groups 

 in warm waters than in cold. Abundance of plankton 

 responds to, and has an effect on, the chemical con- 

 tent of the water. 



Yearly cycle 



In general, winter is characterized by minimum 

 levels of plankton. Nitrogen and phosphorus salts in- 

 crease in surface waters because of the decomposition 

 of organisms that have died during the preceding 

 months, to the lesser absorption by phytoplankton, 

 and to the greater mi.xing of waters from various 

 depths accompanying the loss of thermal stratifica- 

 tion. During the spring, as the result of increasing 

 light, reduction of vertical turbulence of the water, 

 and rising temperatures, the phytoplankton increases 

 to a maximum for the year. In temperate and boreal 

 regions the phytoplankton consists largely of diatoms. 

 The zooplankton at this time abounds in immature 

 stages. 



By summer, nitrogen and phosphorus become 

 diminished in the surface waters because of their use 

 by phytoplankton and lack of replenishment from 

 greater depths with the re-establishment of thermal 

 stratification. Phytoplankton consequently declines 

 rapidly, lacking nutrients and being consumed by 

 the increasing zooplankton population. Zooplankton 

 reaches its maximum during the summer but as it 

 exhausts its phytoplankton food supply, it also de- 

 clines. Decomposition of dead plankton in shallow 

 waters and the destruction of the seasonal thermo- 

 cline with vertical mixing of waters in the open ocean 

 again returns nitrogen and phosphorus to surface in 

 the autumn and this usually allows a second smaller 

 ma.ximum of plankton to develop. 



High arctic seas usually have a single maximum 

 of short duration in the summer. In the tropics, on 

 the other hand, there is generally no conspicuous 

 peak, although plankton tends to be more abundant 

 during the winter months (Bogorov 1958). In the 

 Indian Ocean, physical oceanographic changes asso- 

 ciated with the monsoons create a seasonal plankton 

 cycle. 



Diel movements 



Many zooplankters vary in the depth at which 

 they are most highly concentrated at different times 

 during the 24-hour diel cycle. The daily movements 

 to the surface waters at night and to greater depths 

 during the day is especially marked in the copepods 

 (Clarke 1934), and occurs in euphausiaceans, my- 

 sidaceans, amphipods, ostracod and decapod larvae, 

 pteropods, chaetognaths, polychaetes, siphonophores, 

 and tintinnids. Even some nektonic animals, such as 

 herring and squid, show vertical diel migrations. 

 Most phytoplankton are confined to the upper lighted 

 zone, although dinoflagellates have been shown to 

 have short, vertical, daily movements in response to 

 light (Hasle 1950). The vertical movements of these 



358 Geographic distribution of communities 



