(J < 



Z Q 



< ? 



Q E 



NO SPOROPHYTE 



RECRUITMENT 



-.* 50 SPOROPHYTES/ cm 2 



. '50 SPOROPHYTES/ cm 2 



13 14 15 16 17 



MEAN TEMPERATURE CO 



Figure 29. Irradiance-temperature re- 

 sponse surface for Macrocystis pyrifera 

 sporophyte recruitment. Sporophytes 

 counted on sections of rope initially 

 inoculated with spores in the laboratory. 

 When spores developed into gametophytes, 

 the ropes were put in the field on the 

 apparatus shown in Figure 28. 



Since temperature and nutrients are 

 inversely correlated (Section 2.5), these 

 laboratory results suggest that it is the 

 light-nutrient interaction that is 

 important in the field (Deysher and Dean 

 pers. comm.). 



Different sediment loads could lessen 

 the production of sporophytes by reducing 

 irradiance levels below the compensation 

 irradiance point (Table 1). Gametophytes 

 were not killed by sediment, but failed to 

 grow. In another study, DeVinny and Volse 

 (1978) found that sediment cover of 

 107 mg/cm 2 (0.45 mm thick) could reduce 

 gametophyte survival by 90%. These 

 results are similar to those of Norton 

 (1978), who found that a covering of 

 sediment reduced light levels below the 

 growth compensation point for the kelp 

 Sacchariza polyschides . Sediment also 

 affects scour, nutrients, and micrograzer 

 activity as well as light. 



The availability of nutrients, 

 particularly nitrogen, can affect the 

 growth of macro-algae (see Section 2.5). 

 Some species of large algae can maintain 

 growth during periods of low ambient 

 nutrients by building internal nitrogen 

 reserves. In the case of Macrocystis 

 pyrifera , sporophyte tissue may have a 

 nitrogen level 10,000 times that of the 

 external supply (Gerard 1982b). Wheeler 



and North (1981) found that N0 3 " varied 

 seasonally in inshore waters, and that 

 growth rates of fronds were also seasonal. 

 However, there was no clear correlation 

 between growth rates and ambient N0 3 . 

 Gerard (1982b) tested the effect of 

 nitrogen depletion on growth rates of 

 fronds j_n situ by moving a plant from an 

 inshore kelp forest to an offshore area 

 with a lower nitrogen environment. The 

 transplant was moored at a similar depth 

 to the offshore site (^ 7 m). She found 

 that fronds maintained growth for two 

 weeks, presumably as a result of internal 

 nitrogen reserves, but that growth rate 

 decreased in the third week. Despite the 

 relatively small nitrogen-storage capacity 

 of Macrocystis , she concluded that 

 nitrogen saturation is uncommon in 

 southern California kelp forests. 



Dean et al. (1983) did two types of 

 experiments to test for the effects of 

 nitrogen levels on sporophyte production. 

 In the gametophyte outplant experiments 

 described earlier, nitrogen levels in the 

 water column were measured. There was no 

 correlation between the production of 

 sporophytes and nitrogen levels, 

 suggesting that nutrients play a 

 relatively minor role in recruitment when 

 other factors such as light, temperature, 

 and sedimentation are considered. 

 However, temperature and nutrients are 

 highly correlated, and the work of 

 Zimmerman and Kremer (1984) indicates that 

 nutrients must be measured at shorter time 

 intervals than in these experiments. Dean 

 et al. (1983) also did nitrogen addition 

 experiments to assess the effects on 

 sporophyte production. Racks of 

 gametophytes were situated above trays of 

 fertilizer, rendering the nitrogen 

 concentration in the vicinity of the 

 outplants greater than ambient. In these 

 experiments, fertilizing the substrata did 

 increase sporophyte recruitment. They 

 also found that the plates on the bottom 

 showed a greater increase in nitrogen than 

 those higher in the water column, probably 

 because of higher current velocities near 

 the surface which diffused the fertilizer. 

 This can also be the case for ambient 

 nitrogen, with vertical stratification and 

 high N0 3 " concentrations only near the 

 bottom (Gerard 1982a). In this case, the 

 interaction between nutrients and water 

 currents was important. 



96 



