Differences in growth, morphology and tissue carbon and nitrogen of Macrocystis pyrifera within and at the outer edge of a giant kelp forest in California, USA

To investigate the extent to which alteration of physical factors by giant kelp beds affects the growth of kelp within the bed, we conducted analyses of frond morphometrics at 2 mo intervals, elongation rates and tissue chemistry of individuals of Macrocystis pyrifera on the interior and edge of a kelp bed off the coast of Santa Barbara, California, USA. The density of the kelp canopy varied greatly during the 13 mo study, ranging from 0.03 to 8.03 m(2) blade tissue m(-2) sea Surface. Time series analysis of flow velocity, light, temperature and seawater nitrate concentrations inside and outside of the bed indicated that when canopy density was high (May to September) current speeds in the bed were 25% of incident flow, and light below the canopy (at 1 m depth) was reduced to 10% surface irradiance. Nitrate concentrations were highly variable and similar between inside and outside moorings. When the canopy was well developed, kelp fronds on the seaward edge of the bed had faster elongation rates and larger blades, resulting in higher overall growth rates (mass added per day) than interior fronds. Carbon and nitrogen accumulation by edge fronds was also higher during this period, which fueled growth rates of edge fronds that were nearly twice as high as interior fronds when the canopy was densest. Thus, the growth and tissue chemistry of M pyrifera within the kelp bed depended on the extent to which the bed modified ambient physical conditions.