ROSENTHAL, CLARKE, and DAYTON: ECOLOGY OF A STAND OF GLANT KELP 



by the holdfast. In July 1967, 35 attached Macro- 

 cystis plants which contributed fronds to the sur- 

 face canopy were recorded along the 2 x 100 m 

 transect band (Figure 2). For convenience we 

 have arbitrarily lumped all plants with fronds 

 (stipe and blade) reaching the sea surface into a 

 category as canopy adults. With the passage of 

 time, the number of adult Macrocystis growing 

 within this 100-m transect band was gradually 

 reduced from 35 plants to a single survivor in 

 June 1970. In addition to the plants along this 

 part of the transect we followed 14 adult Macro- 

 cystis that grew along the 50-m shoreward exten- 

 sion of the transect (Figure 3). This shallower 

 portion of the kelp bed displayed a similar reduc- 

 tion in the number of adult Macrocystis. In April 

 1968, 14 plants were recorded along this 2 x 50 m 

 belt, however by April 1970 the last survivor had 

 disappeared. 



The dramatic fluctuations in the number of 

 adult Macrocystis within this kelp stand can be 

 detected from both in situ counts and kelp har- 



FiGURE 2. — Occurrence of adult Macrocystis pyrifera in the 

 2 m X 100 m transect (0-100 m). 



3 



1968 



1969 1970 



TIME 



vesting records over the past 32 yr (Figure 4). 

 North (1971) stated "that beds five and six, lying 

 between Del Mar and Oceanside, California have 

 often fluctuated in this manner. These beds 

 actually consist of approximately eight major 

 kelp areas along about 15 km of coastline. They 

 flourish and disappear more or less in unison, 

 typically but not invariably with a four year 

 period." 



The general pattern for the Del Mar Macro- 

 cystis population seemed to be gradual attrition 

 of the adult plants with little or no replacement 

 over a 3-yr period, followed by recruitment after 

 most of the adult plants had disappeared. With 

 the addition of 40 young adult Macrocystis along 

 the 100-m transect (Figure 2) during the summer 

 of 1971 the study area again supported a density 

 of kelp plants similar to July 1967. 



I970_ 

 STUDY PERIOD 



YEAR 



Figure 3. — Occurrence of adult Macrocystis pyrifera in the 

 2 m X 50 m transect (100-150 m). 



Figure 4. — Relative kelp harvest from bed number 5 I Del 

 Mar). After North (1971) and Kelco Company. 



Causes of Plant Mortality 



Severe grazing of Macrocystis by sea urchins, 

 as described by Leighton (1971) off southern 

 California, has not been observed within this 

 kelp stand. The two conspicuous sea urchins in 

 this area, Strongylocentrotus franciscanus and 

 S. purpuratus were usually observed on consoli- 

 dated sedimentary mounds, under rocks, or 

 within ledges. During both daylight and noc- 

 turnal hours the sea urchins remained in these 

 locations, and in most instances their feeding 

 was restricted to detritus and detached pieces 

 of drift algae. However, S. purpuratus was occa- 

 sionally noted within the deteriorating holdfasts 

 of aging Macrocystis. In general, it appeared that 

 there was enough drift algae for the sea urchin 

 population to make foraging unnecessary, and we 

 gathered no evidence that the urchins were 

 exerting much grazing pressure on Macrocystis. 



673 



