sparse or unavailable. Wanner surface water from the south is very low in nutrients, and 

 when there is an intrusion of this water along the southern California coast, it is possible, if 

 not probable that the plants suffer from nutrient deficiency rather than high temperature 

 degradation, or perhaps there is a synergistic effect. An experiment to investigate the com- 

 bined nutrient and temperature effects on the growth rate of M. pyrifera must be completed 

 to better define temperature and nutrient limitations and thus more accurately characterize 

 potential ocean farm geographic ranges. 



The initial sustained temperature limitation is therefore tentatively put at 20°C. 

 Temperatures above this value appear to have a degrading effect on the kelp off southern 

 California and this area will probably be the first source of plants for early experimental 

 farms. The 20°C assumed hmiting temperature may eventually be increased if southern 

 (Bahia Tortugas) plants are used or if genetic manipulation in the future creates strains of 

 greater temperature tolerance. (Note: Figure 1 1 shows the approximate ranges of Macro- 

 cystis if limited to 20°C and with varying amounts of surface cooling from artificial 

 upwelling.) 



Currents 



The magnitude of the current field to which the proposed marine farm is exposed is 

 of great importance to its overall success. Currents of high velocities relative to the substrate 

 will entail high cost structures and will tend either to cause the kelp to trail horizontally at 

 or near the depth of the substrate, thus reducing light level and productivity. In extreme 

 conditions such currents might cause the plants to break away. In combination with high 

 waves they could cause damage to the substrate structure itself. Conversely, a low velocity 

 water movement is required to replenish nutrients in the vicinity of the plants and carry off 

 waste products. 



Besides major current systems, other forms of water movement are important. These 

 include local wind-derived currents, swell and chop, internal waves, and natural and artificial 

 upwelling. Artificial upwelling may become very important as a water movement factor in a 

 large marine farm where the other forms may be severely diminished by the frictional char- 

 acteristics of the plants (Ref. 16). 



Preliminary experiments at the University of California, Santa Barbara, indicate that 

 for tissue excised from adult Macrocystis , a maximum photosynthetic rate (ml O9 evolved/ 



cm'^ blade/hr) is observed with a water velocity of 10 cm/sec or approximately 0.2 knot (W. 

 Wheeler, personal communication). In several as yet unpublished experiments, W. Wheeler 

 utilized a variable flow-through system to measure the O9 evolution from Macrocystis tissue 



while keeping nutrients and light levels constant. The rate of photosynthesis with no water 

 movement was measured to be approximately 1/5 that at 10 cm/sec. At a velocity of 4 

 cm/sec (0.08 knot) the photosynthetic rate was 1/2 maximum. It is apparent from these 

 experiments that some degree of water movement is required to increase nutrient availability, 

 and thus the rate of photosynthesis, above that supported by purely diffusive processes. 



10 



