Macrocystis sporophytes was observed on 

 only four occasions between 1977 and 1981. 

 These episodes coincided with four of the 

 eight periods when temperatures were at or 

 below 16. 3°C , and irradiance levels were 

 at or above 0.4 E/m 2 /day. This study 

 indicates that temperature and light are 

 interactive, which may have relevance for 

 geographic distributions. This is similar 

 to conclusions of laboratory studies of 

 Luning (1980) for species of Laminaria . 

 Appropriate measurements of TTght and 

 temperature have not been done in tandem 

 with recruitment, growth, and survivorship 

 studies in other localities. Such 

 studies, along with a thorough evaluation 

 of the biological differences among sites, 

 would provide important evidence for the 

 relative influence of these factors on 

 both the local and broad-scale 

 distribution of large brown algae. 



5.3 EFFECTS OF WATER MOTION 



General Hypothesis: Severe water 

 motion (^ 1 m/sec) limits the extent of 

 canopy and the distribution of 

 Macrocystis . 



This hypothesis is not easily 

 testable and has not been experimentally 

 examined. As discussed in Section 2.6, 

 stands of Macrocystis do not occur north 

 of central California where surge 

 conditions are strong and winter storms 

 are severe. Surface canopies in the more 

 protected areas of southern California 

 tend to fluctuate in 3- to 4-year cycles, 

 with older plants being prone to removal 

 by water motion. In central California, 

 however, winter storms tend to remove most 

 surface canopies, and there is a marked 

 seasonal change in their extent. On a 

 local scale, North (1971b) suggested that 

 the shoreward depth limit of Macrocystis 

 is determined by surge conditions. 



Severe winter storms were associated 

 with the recent El Nino conditions 

 (1982-83) along the entire coast of 

 California. These storms removed most of 

 the canopy plants at Point Loma in 

 southern California, with the greatest 

 mortality of plants in the shallowest (12 

 m) water (Dayton and Tegner 1984b). In 

 contrast, at Stillwater Cove in central 

 California, virtually all Macrocystis 

 plants were removed by these storms 



(Schiel and Foster, unpublished data). In 

 both cases, many Macrocystis and 

 understory kelps eventually recruited. 



Except for abrupt mortality of plants 

 due to storms, the factors associated with 

 large-scale phenomena such as El Ninos are 

 generally impossible to separate, however. 

 Water motion is seasonally severe while 

 temperatures are higher and nutrients 

 lower. 



The observations of Ebeling et al. 

 (MS.) on a reef near Santa Barbara 

 indicate that severe storms can have 

 opposing effects on kelp communities, 

 depending on algal abundances and sea 

 urchin behavior prior to and after the 

 disturbance (see discussion in Section 

 3.5). As with many studies, a possible 

 complicating factor is the lack of 

 knowledge about the intensity of algal 

 spore settlement. Sea urchin foraging 

 behavior may not have changed after the 

 first storm had there been a large 

 settlement of spores and subsequent large 

 recruitment of algae. Nevertheless, this 

 observational study indicates that water 

 motion, sea urchin behavior, and kelp 

 recruitment are related. 



5.4 OTHER ABIOTIC FACTORS 



5.4.1 Depth Distribution 



General Hypothesis: The depth 

 distribution of kelp plants is limited by 

 the abiotic factors of light and 

 nutrients. 



Some of the main problems faced by 

 subtidal workers are the differences 

 between areas and depths in physical and 

 biological factors, and also the 

 differences in levels of these factors 

 among areas. Intertidal studies provide 

 the best examples of patchiness in the 

 occurrence of organisms within and between 

 areas, and experiments to determine how 

 this patchiness originates and is 

 maintained. A particularly thorough 

 quantitative survey of intertidal areas in 

 eastern Australia showed that there were 

 broadly overlapping vertical distributions 

 of species, which were partially 

 determined by exposure and substratum 

 heterogeneity (Underwood et al. 1983). 

 Species from different levels on the shore 



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