Seasonal Patterns in the Growth of the Giant Kelp, Macrocystis pyrifera

Field surveys revealed seasonal growth changes in the Giant Kelp, Macrocystis pyrifera. Frond counts were used to measure average sporophyte size in the study bed and to follow vegetative growth of a typical plant for about sixteen months. Storm induced wave motions and sea urchin grazing were primary causes of whole plant losses. The importance of fish grazing on kelp was estimated during field surveys. Average damage varied from light to moderate during the study period. Kelp consumption rates by Opaleye Perch, Girella nigricans, were measured in aquarium experiments. Preference for specific kelp tissues was established. Mathematical relationships between kelp growth and environmental conditions were established by multiple linear regression analysis. Regression equations modeled growth patterns observed in field surveys. Average frond number was expressed as a function of sunlight, water temperature, fish grazing, and upwelling index. A regression model was used to hindcast kelp growth fluctuations for conditions at Point Loma, San Diego County, from 1950 to 1969.