Environmental controls of giant-kelp biomass in the Santa Barbara Channel, California

Abstract

Synthesizing long-term observations at multiple scales is vital for understanding the environ- mental drivers of ecosystem dynamics. We assessed the role of several environmental drivers in explaining temporal and spatial patterns in the abundance of giant kelp Macrocystis pyrifera in the Santa Barbara Channel between 1984 and 2009. We developed a novel method for estimating the canopy biomass of giant kelp from Landsat 5 Thematic Mapper satellite imagery, which allowed us to examine the dynamics of giant-kelp biomass on spatial scales ranging from 100s of m 2 to 100s of km 2 and temporal scales ranging from several weeks to 25 yr. Comparisons of changes in canopy biomass with oceanographic and climatic data revealed that winter losses of regional kelp canopy biomass were positively correlated with significant wave height (r 2 = 0.50), while spring recoveries were negatively correlated with sea surface temperature (r 2 = 0.30; used as a proxy for nutrient availability). On interannual timescales, regional kelp-canopy biomass lagged the variations in wave heights, sea surface tem- peratures, and the North Pacific Gyre Oscillation index by 3 yr, indicating that these factors affect cycles of kelp recruitment and mortality. Results from cluster analysis showed that the response of kelp biomass to environmental conditions varied among different sub- regions of the Santa Barbara Channel. The dynamics of kelp biomass in exposed regions were related to wave disturbance, while kelp dynamics in sheltered regions tracked sea surface temperatures more closely. These results depict a high level of regional hetero- geneity in the biomass dynamics of this important foundation species.