zones different from where adult plants 

 normally occur has been addressed in a few 

 studies. 



In a study in New Zealand, Schiel 

 (1981) settled Sargassum sinclairii 

 germlings onto plates, and placed them on 

 a shallow reef (5 m) where adult plants 

 were abundant and on a deep reef (15 m) 

 where adult plants were scarce. Initial 

 survival of germlings was better in the 

 deep area. Once the young plants began to 

 form blades, however, shallow plants grew 

 at a much faster rate, while most deep 

 plants grew slowly and eventually died. 

 Schiel (1981) speculated that the major 

 reason for few adult Sargassum plants in 

 deep areas was that germlings rarely 

 reached these habitats in great abundance, 

 and that the few which did settle had a 

 low probability of growth and survival. 

 As in the studies of Schonbeck and Norton 

 (1978, 1980) and Kennelly (1983), small 

 Crustacea were probably important sources 

 of mortality for germlings which grew 

 slowly. 



Adult distribution may also be a 

 reflection of spore distribution. This is 

 a persistent problem in algal research, 

 and one which is only just beginning to be 

 addressed (e.g., Kennelly 1983). Spore 

 fall is probably not evenly distributed in 

 natural situations, and may be affected by 

 current and surge conditions in much the 

 same way as "seed shadows" occur in some 

 terrestrial situations (Harper 1977). 

 Dense aggregations of spores or germlings 

 may also be important to the production of 

 large algal stands (Fletcher and Fletcher 

 1975, Fletcher 1980). Denley and Dayton 

 (in press) suggest ways in which spore 

 fall may be examined using settlement 

 plates and microscopic examinations, and 

 techniques are now available for j_n situ 

 microscopic examination of substrata 

 (Kennelly and Underwood 1984). In such an 

 experiment, Chapman (1984) found that the 

 greatest proportional mortality for 

 Laminaria longicruris and Jl. digitata 

 occurred between the time the microscopic 

 plants attached to the substratum and when 

 they became visible, a period of about 6 

 weeks (see Section 4.3.3.1 for a 

 description of his experiments). This is 

 a unique study in that it assesses the 

 number of spores produced for adult plants 

 per square meter, the recruitment of 



microscopic sporophytes that results from 

 these spores, the visible recruitment of 

 macroscopic sporophytes to natural 

 substrata in the field, and the subsequent 

 survivorship of the plants. This kind of 

 innovative study is essential to clarify 

 the distribution histories of species, and 

 to assess whether competition among algal 

 species is an important structuring force 

 in algal communities. 



5.6 EFFECTS OF GRAZING 



General Hypothesis: The activities 

 of grazers affect the distribution and 

 abundance of large brown algae. 



Many studies mention that the 

 activities of grazers affect the 

 distribution and abundance of large brown 

 algae in subtidal regions, but there have 

 been relatively few experimental studies 

 which assess the nature of their effects. 

 Most of our present knowledge about the 

 effects of grazers on algal assemblages 

 comes from experimental studies in 

 intertidal areas, where herbivorous 

 gastropods are usually the most abundant 

 and important grazers (Underwood 1979, for 

 review). Much of the small-scale 

 patchiness in the abundances of intertidal 

 algae is caused by grazing on filamentous 

 and foliose plants, as well as on algal 

 spores (e.g., Dayton 1971; Underwood and 

 Jernakoff 1981, 1984). Algal cover and 

 diversity may be dependent on the density 

 of grazers in a given area (Lubchenco 

 1978, Underwood et al. 1983). 



The regime of grazing generally 

 changes abruptly in the boundary between 

 intertidal and subtidal regions. Even on 

 shores where the abundances of herbivorous 

 gastropods such as limpets and trochids 

 are great, their distribution tends to end 

 where the zone of dense algae (normally 

 fucoids) begins in the immediate subtidal. 

 Herbivorous gastropods tend to be less 

 abundant in the subtidal , where sea 

 urchins are normally the major grazing 

 invertebrate. Herbivorous fish may also 

 affect subtidal algal assemblages (Choat 

 1982, Gaines and Lubchenco 1982). 



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