floating masses as Sargassum does in the 

 Sargasso Sea (see discussion by North 

 1971b). Although "seas" of Macrocystis 

 have not been found, Moore (1943) and 

 Gerard and Kirkman (1984) described large 

 numbers of living plants with unusual 

 branched fronds drifting on the bottom in 

 quiet bays in southern New Zealand. 



2.3. TEMPERATURE 



Subtidal organisms are continually 

 submerged, and thus are not exposed to the 

 extremes of temperature found in the 

 intertidal zone. However, considerable 

 seasonal and year-to-year differences in 

 temperature occur within the range of 

 giant kelp forests. These differences 

 have been suggested as important to plant 

 distribution, especially on a geographic 

 scale (North 1971b, Murray et al. 1980, 

 Van den Hoek 1982). 



Any discussion of temperature (and 

 other abiotic factors) must be prefaced 

 with warnings about factor covariance and 

 interaction, relationships between tempo- 

 ral and spatial scales of measurement vs. 

 plant response, and differences in 

 response of different life history stages 

 (see Wheeler and Neushul 1981 for review). 

 The effects of temperature and other 

 abiotic factors are often examined alone 

 in field correlations and single-factor 

 laboratory experiments. As pointed out by 

 Hedgpeth and Gonor (1969), however, the 

 effects of temperature can vary depending 

 on other factors such as light and nutri- 

 ents. These interactions have been 

 suggested (Druehl 1978) or demonstrated as 

 important for Macrocystis and other kelps 

 (Luning and Neushul 1978, Dean et al . 

 1983). Moreover, the measurement of 

 temperature in the field may not truly 

 reflect what the organism actually 

 experiences. For example, a shallow 

 thermocline can occur within kelp forests, 

 with bottom temperatures considerably 

 colder than those at the surface. Thus, 

 using surface temperatures as an indica- 

 tion of temperatures within a kelp forest 

 can be inappropriate. The vast majority 

 of seawater temperature measurements are 

 made at the surface. In addition, depend- 

 ing on tides, thermocline position, etc., 

 temperatures on the bottom at kelp forest 

 depths can vary 4 c - 8 °C in less than a 

 day (Quast 1971c, Barilotti and 



Silverthorne 1972, Zimmerman and Kremer 

 1984). Rosenthal et al. (1974) found mean 

 and maximum surface temperatures near 

 their study area over a five-year period 

 to be 16.3 °C and 24.6 °C, respectively, 

 while the mean at 17 m was 13.0 °C, and 

 the maximum 16.0 °C. 



Monthly mean surface temperatures of 

 nearshore waters within the western north 

 Pacific distribution of large stands of 

 Macrocystis pyrifera vary from 12 °- 15 °C 

 near Santa Cruz, California to 18 °- 23 °C 

 in Baja California, Mexico (Sverdrup et 

 al. 1942). It is generally believed that 

 adult giant kelp do not grow well above 20 

 °C, although plants have been found in an 

 area of Baja California, Mexico, where 

 temperatures exceeded this value for 

 several weeks (North 1971b). This latter 

 observation may be exceptional, as plants 

 in Baja California, Mexico generally occur 

 in areas where cool water is upwelled 

 (Dawson 1951). Canopies and entire plants 

 deteriorate in southern California during 

 years when sea water temperatures are 

 elevated ("El Nino" oceanographic condi- 

 tions; see Section 2.5 below) suggesting 

 that high temperatures (or associated low 

 nutrients, see Section 2.5) have deleter- 

 ious effects on adult plants. 



Growth of gametophytes of a variety 

 of kelp species in southern California is 

 generally optimal at 17 °C, while fertili- 

 ty is optimal at 12 °C. Both of these 

 processes were optimal at around 12 °C in 

 gametophytes from central California 

 (Luning and Neushul 1978). Bull kelp 

 ( Nereocystis luetkeana ) does not occur 

 south of Point Conception, and Vadas 

 (1972) concluded that this is because 

 gametophyte fertility and young sporophyte 

 growth occurred at 15 °C but not at 20 °C. 

 However, this conclusion about distribu- 

 tion is questionable, as temperature 

 effects between 15 ° and 20 °C were not 

 evaluated, and temperatures below 20 °C 

 are common for many months of the year 

 south of Point Conception (Barilotti and 

 Silverthorne 1972, Mearns 1978, Dean et 

 al. 1983). One might expect that, if 

 temperature were of great importance to 

 geographic distribution, plants from areas 

 with different temperature characteristics 

 would exhibit different responses to 

 temperature. North (1972b) found that 

 sporophytes transplanted from Baja 



11 



