lowest ropes were on the bottom, with 

 replicates at 2, 4, and 6 m above the 

 bottom. They recorded the density of 

 sporophytes produced from these cultures, 

 and measured temperature and irradiance 

 daily and total nitrogen weekly. Nitrogen 

 levels were generally higher on the bot- 

 tom, and lowest at 6 m off the bottom. 

 Temperatures during August - October were 

 lowest on the bottom and highest at 6 m, 

 but differences were slight between depths 

 during October - January. Irradiance was 

 invariably lowest on the bottom, and pro- 

 gressively higher at the shallower depths. 

 Temperature alone accounted for a rela- 

 tively high proportion of the variance in 

 sporophyte density at all depths (48%). 

 Irradiance explained a significant portion 

 of the variance in sporophyte density for 

 all treatments combined, but was more 

 important for bottom substrata. This was 

 because light regimes on the bottom were, 

 at times, below critical irradiance 

 levels. Sedimentation levels also had a 

 significant effect, being more pronounced 

 on the bottom treatments. 



A surprising result of these 

 experiments was that there were not 

 significant correlations between nitrogen 

 levels and the production of sporophytes. 

 Several outplants of gametophytes produced 

 high densities of sporophytes despite low 

 levels of nitrogen (0.2 - 1.0 ug-at/1). 

 This lack of correlation was attributed to 

 the fact that the outplants were within a 

 narrow range of critical levels at all 

 times, and that weekly measurements of 

 total nitrogen may be too coarse a 

 measurement to detect effects on settled 

 substrata. 



These experiments indicate that even 

 relatively small differences in depth may 

 have important effects on gametophyte 

 growth, fertility, and the production of 

 sporophytes. Critical thresholds in 

 temperature and irradiance may be 

 approached, while levels of sedimentation 

 may be correlated with particular depths. 

 These sorts of experiments which assess 

 the performance of small plants while 

 monitoring the physical environment are 

 critical to an understanding of broad 

 zonational patterns. The fact that larger 

 sporophytes grow, and apparently do well 

 in deeper areas at some sites, reveals 

 little about how plants come to be in 



those larger size categories. Gameto- 

 phytes and young sporophytes must cope 

 with the physical regimes of the 

 micro-sites in which they settle, in much 

 the same way as do terrestrial plants 

 (e.g., Harper 1977). The logistics of 

 altering and controlling physical regimes 

 at this scale have not been worked out for 

 field studies, but the experimental/ 

 correlative approach used by Dean et al . 

 (1983) holds much promise. 



5.4.2 Variability Within Depths 



General Hypothesis: The variability 

 in levels of temperature, light and nutri- 

 ents at the same depth within a locality 

 results in the patchy distribution and 

 variable abundance of algae. 



There are no field studies that 

 conclusively show the effects of 

 temperature, light, and nutrients on any 

 life-history stage of Macrocystis . 

 Experimental studies have not been 

 feasible because of the difficulty in 

 controlling the levels of these correlated 

 factors in the field. For example, Luning 

 (1980) found that the total amount of 

 light needed for gametogenesis in 

 Laminaria increased exponentially as 

 temperature increased. Dean et al . (1983) 

 were able to quantify this in field 

 experiments. Macrocystis spores were 

 settled onto small ropes in the 

 laboratory, and grown to the gametophyte 

 stage before outplanting to the field. 

 The densities of the sporophytes that 

 eventually appeared were determined, while 

 temperature and irradiance were measured 

 with jijn situ sensors and integrating 

 recorders. They found that there were 

 threshold levels of light and temperature 

 for the production of different densities 

 of sporophytes (Figure 29). For example, 

 no sporophytes were produced above a 

 temperature of 17.6 °C or below 

 irradiance levels of 0.3 E/m 2 /day. To get 

 dense recruitment (> 50 sporophytes/cm 2 ) , 

 temperatures were below 16.3 °C, and 

 irradiance above 0.4 E/m 2 /day. Higher 

 irradiance levels (1 E/m 2 /day) were 

 required for production of higher 

 densities of sporophytes as temperatures 

 approached 16 °C. However, recent 

 laboratory experiments indicate there is 

 little interaction between light and 

 temperature if nutrients are adequate. 



95 



