ECOLOGY AND NATURAL HISTORY OF A STAND OF 



GIANT KELP, MACROCYSTIS PYRIFERA, 



OFF DEL MAR, CALIFORNIA 



Richard J. Rosenthal,^ William D. Clarke,^ and Paul K. Dayton^ 



ABSTRACT 



The assemblage of plants and animals living within a stand of Macrocystis pyrifera off the coast 

 of southern California was studied from July 1967 through February 1973. Macrocystis is a perennial 

 kelp, with some individuals living as long as 7 yr; however, the average life span in this bed was 

 approximately 3 to 4 yr. Physical disturbances associated with storms were the major mortality 

 causes of adult Macrocystis in this area. Once detached, these plants drift through the bed and 

 become entangled with other plants which results in extensive mortality. The fact that germination 

 was greatest after the surface canopy was thinned by natural attrition and commercial harvesting 

 suggests that light is a critical factor influencing the recruitment o{ Macrocystis. There was little 

 indication to show that sea urchin grazing contributed to kelp mortality. 



Faunal species identified included 38 fish species and 98 invertebrate species. Of these, 14 species 

 of macroinvertebrates were chosen for more intensive study as they represented common or char- 

 acteristic species in the kelp bed. Patterns of distribution and abundance were recorded during 

 the study period. Most species had aggregated distribution patterns and the populations of most 

 remained reasonably constant over 4.25 yr. Styela montereyensis (ascidian) fluctuated annually 

 and the Muricea californica (octocoral) population slowly decreased during this time. Conversely, 

 Diopatra ornata (polychaete) displayed a numerical increase, such that in August 1972 it was the 

 most abundant macroinvertebrate in the Del Mar kelp bed. A qualitative food web is presented 

 based on limited trophic information. 



Many large kelp stands historically have either 

 undergone dramatic oscillations in areal cover and 

 standing crop or completely disappeared (see 

 North, 1971, for history and data). Nearshore 

 kelp stands are found in many scattered locations 

 along the mainland of California and fringe most 

 of the state's offshore islands; in southern Cali- 

 fornia kelps are often conspicuous when the 

 dominant plant, Macrocystis pyrifera (Linnaeus) 

 C. Agardh, forms a floating canopy or bed along 

 the sea surface. This kelp community contains 

 many plant and animal species which contribute 

 aesthetic as well as diverse recreational and com- 

 mercial resources. The loss and deterioration of 

 these stands is correlated with many man-caused 

 perturbations and natural events such as reduced 

 water quality, the over-harvest of numerous im- 

 portant component animal populations, and fluc- 

 tuations in seawater temperature. 



'Scripps Institution of Oceanography, University of California 

 at San Diego, La Jolla, CA 92037; present address: Dames and 

 Moore, 711 "H" Street, Suite 500, Anchorage, AK 99501. 



^Westinghouse Ocean Research Laboratory, Annapolis, MD 

 21404. Deceased. 



'Scripps Institution of Oceanography, University of California 

 at San Diego, La Jolla, CA 92037. 



There is widespread concern regarding effects 

 of increasing human perturbations to these near- 

 shore kelp communities which include increas- 

 ing recreational and commercial usage as well as 

 many projected sewer and thermal outfalls into or 

 in the proximity of kelp beds. Despite this concern, 

 there is little information regarding natural 

 temporal variation of populations inhabiting 

 these assemblages. Furthermore, natural history 

 data such as food web interactions critical to a 

 functional understanding of this community are 

 in a very rudimentary state. This information is 

 obviously vital to the proper management of 

 this resource. 



The objectives of this paper are to: 1) describe 

 patterns of distribution and abundance, 2) record 

 long term population fluxes, and 3) note food web 

 and other natural history observations of con- 

 spicuous members of a relatively undisturbed kelp 

 association. Such data are essential to the growth 

 of a functional understanding of this community. 



The study site (Figure 1) was located in a bed 

 oi Macrocystis pyrifera about 1 km offshore from 

 Del Mar, Calif, (lat. 32°57'N, long. 117°16'W). 

 The majority of the observations were made at 



Manuscript accepted December 1973. 

 FISHERY BULLETIN: VOL. 72, NO. 3, 1974. 



670 



