ROSENTHAL. CLARKE, and DAYTON: ECOLOGY OF A STAND OF GIANT KELP 



growing along the transect were frequently re- 

 moved in clumps of two or three plants at a time 

 (Table 2). 



The effect of harvesting on individual plants 

 is still inconclusive. ZoBell (1971) concluded from 

 observations of drift seaweeds on San Diego 

 beaches that there is no consistent relationship 

 between kelp harvesting operations and the 

 amount of seaweed on beaches. However, at Del 

 Mar we found adult Macrocystis pulled free of 

 the substratum following harvesting. On 6 Jan- 

 uary 1969, 10 marked plants were growing 

 along the 100-m transect (Figure 2) and 4 plants 

 remained in the 50-m extension (Figure 3). The 

 seas were calm and there were no loose or drift- 

 ing Macrocystis observed along the entire tran- 

 sect. The kelp bed was harvested on 7-8 Jan- 

 uary 1969. Returning to the study area on 9 

 January 1969 we found unattached Macrocystis 

 which were either drifting in the water column 

 or entangled with other attached Macrocystis. 

 Two of the marked plants on the 100-m transect 

 were detached from the substratum, and one 

 plant from the 50-m extension was missing. 

 Also, one adult Macrocystis from fixed quadrat 

 number 1 was removed. These detached plants 

 probably caused additional mortalities by en- 

 tangling with other plants in the kelp bed. In 

 summary, strong water movement, plant en- 

 tanglement, and harvesting probably act in a 

 synergistic manner on attached Macrocystis. 



Germination, Recruitment, 

 and Survivorship 



Macrocystis undergoes a life cycle that alter- 

 nates between an asexual macroscopic stage 

 termed the sporophyte, and a sexual microscopic 

 gametophyte stage (Brandt, 1923; Neushul and 

 Haxo, 1963). Because of the difficulties inherent 

 in microscopic investigation underwater we re- 

 corded recruitment only when the young sporo- 

 phytes became visible. Although Macrocystis 

 spores and gametophytes were probably present 

 at different times of the year (Neushul, 1959; 

 North, 1964), we were primarily concerned with 

 the plants when they visually became part of 

 the epibenthic community. A few young plants 

 (<1 m in height) were observed in the study area 

 during the early summer of 1967, however by 

 the fall of that same year all of these juvenile 

 sporophytes had disappeared. For the next 23 mo, 

 no Macrocystis recruits were observed in the 

 vicinity of the transect. During this time the over- 

 lying canopy was expanding to a point when, in 

 December 1968, it covered approximately 90% 

 of the 150-m transect line. Thus, insufficient 

 light penetration resulting from shading by adult 

 plants and water turbidity may have been key 

 reasons for the absence of juvenile Macrocystis 

 during 1967 and 1968. However, the bed was 

 harvested in January 1969, which apparently 

 reduced the surface canopy to about 15% cover 



Table 2. — Stipe counts of individually tagged plants through study period. Plant number 1 survived with 7-8 stipes until 

 August 1972. "G" means that the plant was completely gone and "O" signifies that there were no stipes but that the holdfast 

 was still attached. Note the tendency for groups of plants to disappear together; in each case this resulted from mutual 

 entanglement. 



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