Physiological response of the stipitate understory kelp, Pterygophora californica Ruprecht, to shading by the giant kelp, Macrocystis pyrifera C. Agardh

Abstract

A canopy removal experiment was performed in Monterey Bay, California, USA, to determine the physiological response of the understory kelp, Pterygophora californica Ruprecht, to shading by the giant kelp, Macrocystis pyrifera C. Agardh. The 28-day experiment was carried out at the end of the summer of 1989 when surface canopy development was at a maximum. Benthic light levels (PAR) increased 4–5 fold with thinning or removal of the surface canopy. Tissue samples collected during the experiment showed significant increases of mannitol (the sugar alcohol product of brown algal photosynthesis) within 14 days in thinned and cleared sites but not in controls. Concentrations of laminarin (carbohydrate storage polymer), free amino acids, tissue nitrate, and photosynthetic pigments (chlorophyll a, chlorophyll c, and fucoxanthin) showed no response within 28 days to surface canopy manipulations. Growth of new stipe and lateral sporophyll tissue was observed in thinned and cleared sites by the end of the experiment. Photosynthesis and growth of adult Pterygphora can be inhibited by a well-developed surface canopy of Macrocystis and plants can respond quickly once the canopy is removed, Since disturbance from storms is the primary natural mechanism of surface canopy removal, degree of development and persistence of an understory canopy may be indirectly controlled by wave exposure in forests of giant kelp.