l6] VEGETATIONAL TYPES OF SEAS 523 



benthic Green, Brown, and Red Algae are greatly reduced by the 

 disappearance of stenohaline types, as is the general luxuriance of 

 growth ; moreover some of the persisting Brown Algae, particularly, 

 may take on characteristic dwarf forms. On the other hand the 

 remarkably euryhaline Green Alga Etiteromorpha intestinalis, which 

 is widely familiar in fresh and brackish waters as well as in normal 

 sea-water and brine, is reported to undergo optimum development in 

 diluted sea-water, though it has been suggested that this may be 

 only where there is an influx of nutrient materials such as those 

 contained in sewage. Also liable to be subjected to marked varia- 

 tions in salinity — as well as in temperature, degree of desiccation, 

 and so forth — are the littoral Algae that are exposed to the air for 

 varying periods between tide-marks or that grow in small tidal 

 pools. As might be expected in view of the upward or downward 

 salinity-trends which these pools have to undergo as a result of 

 evaporation, or of rains and the influx of freshwater streams, respec- 

 tively, these littoral Algae have been demonstrated to display greater 

 tolerance to variations in osmotic pressure than do Algae growing 

 below low-tide mark. In addition to the Etiteromorpha, Fucus 

 ceranoides and species of Uha are known to be widely euryhaline. 



In reaction, sea-water is somewhat alkaline, owing to most of its 

 contained carbon dioxide being combined in the form of carbonates 

 and bicarbonates and to the removal, by plants during photosynthe- 

 sis, of the (acidic) carbon dioxide formed on dissociation of these 

 carbonates and bicarbonates. Especially in tidal pools is the alka- 

 linity apt to be marked, a pH of as much as 10 being sometimes 

 reached after several hours' exposure by the tide. Although most 

 Algae will tolerate a pH of up to at least 9, pH 10 appears to be 

 too high a degree of alkalinity for many ' stenoionic ' species {i.e. 

 those which are narrow in their tolerance of changes in the relative 

 abundance of free acidic or basic ions), and it has been contended 

 that this is why such Green Algae as Ulva, having high photo- 

 synthetic rates, are prone to oust the frailer types, such as many 

 Red Algae. However, it should be remembered that evaporation 

 raises the salinity as well as the alkalinity, and so it seems possible 

 that the reason may be rather the failure of the frailer types to 

 respond to increases in osmotic pressure of the medium. 



It has also been suggested that the low concentrations at which 

 saturation of oxygen is attained in warm waters may have some 

 correlation with the less profuse development of Algae in many 

 tropical as compared with cold waters. However, according to 



