EFFECTS OF SHADING BY ADULTS ON THE GROWTH OF BLADE-STAGE MACROCYSTIS-PYRIFERA (PHAEOPHYTA) DURING AND AFTER THE 1982-1984 EL-NINO

The effect of shading by an adult canopy on blade-stage Macrocystis py rifera (L.) C. A. Agardh was estimated by comparing the average growth rate of individuals under a canopy to that of individuals in a canopy gap. This comparison was made in 1983 during a strong El Nino and aga in in 1986 after the El Nino. Estimated nutrient concentrations in 198 3 were two orders of magnitude below those in 1986, whereas ambient li ght levels were over 3 times higher. The kelp canopy caused similar pr oportional light reductions (20-30%) during both years. Blades grew 18 % slower under the canopy than in the clearing in 1983 and about 77% s lower under the canopy in 1986. Blade-stage individuals grew at the sa me rates in clearings in. 1983 and 1986. Regardless of shading, the av erage growth rate of blade-stage kelp under the ambient, low-nutrient conditions of 1983 was higher than that later observed for multifronde d juveniles during the same El Nino. The growth of blade-stage kelp wa s more like that of larger juveniles growing under high-nutrient condi tions. The difference may be due to greater concentrations of nutrient s very near the sea floor where single blades are growing compared to concentrations higher in the water column where larger kelp have most of their tissues.