408 THE ALGAE 



(a) between mean and extreme low water-marks of spring tide; 



(b) around mean low water-mark of neap tides ; 



(c) around extreme high water-mark of neap tides. 



This uniformity suggests that the physiographic or tidal factor 

 is paramount. However, if the zones occupied by major algal 

 species in different locaUties are compared, after allowance is made 

 for splash and wave action, it is found that they do not always co- 

 incide (Fig. 206), so that even if the tidal factor is the major causal 

 factor its effect is commonly modified locally by other factors. 



One very important factor is the inter-relationship between 

 respiration and photosynthesis. Normally the latter is in excess of 

 the former, but at different temperatures and at different depths 

 (depending on light intensity (Fig. 212B)) in the water the two 

 rates coincide. This is known as the compensation point and below 

 that depth, if conditions remained uniform, an alga could not sur- 

 vive because breakdown would exceed build up. During short 

 periods an alga may be able to exist under such conditions. There 

 will be, for each species, however, some level at which the periods 

 below the compensation point exceed the periods above. This will 

 effectively form a lower limit to the algal belt. It is likely that the 

 lower levels of many of the species occupying the lower Uttoral 

 and the infra-httoral are set by this phenomenon, but at present all 

 too Uttle work has been done upon it. Apart from the work by 

 Ehrke (193 1) and Clendenning and North (1958)^ on Macrocystts 

 pyrifera an investigation of the Uttoral Hormosira hanksiiy has led to 

 resiilts which are summarized in Fig. 214, where it will be seen 

 that under a low hght intensity the compensation point is reached 

 at about i m. or less, at optimum light intensity (in relation to 

 emergent conditions) the compensation point is reached at about 

 4 m., whilst at still higher light intensities it is not attained until 

 nearly 6 m. In Fig. 220 the relationship of the compensation point 

 to sea temperature is shown. Another means of approaching the 

 problem was used by Klugh and Martin (1927), who studied the 

 growth rates of various algae in relation to submergence by measur- 

 ing plants and then tying them to floats which were suspended in 

 the water at different depths. After some months the floats were 

 pulled up and the plants remeasured. The curves (Fig. 221) show 

 that maximtun growth occurred between i and 2 m. where 



^ Kelp investigation programme at Scripps Institute, California and published 

 as special reports. 



