COS 172-6 - Scale dependence of bottom-up versus demographic controls on the dynamics of giant kelp forests

Friday, August 11, 2017: 9:50 AM
D132, Oregon Convention Center
Tom W. Bell1,2 and David A. Siegel1,3, (1)Earth Research Institute, University of California, Santa Barbara, (2)Department of Geography, University of California, Los Angeles, (3)Department of Geography, University of California, Santa Barbara

Examining patterns at a variety of scales is essential for identifying and elucidating ecological processes. High-resolution, spatially explicit data, which are becoming more available with advances in remotely sensed imagery have revolutionized the study of how pattern and variability change across the scale of description. Giant kelp (Macrocystis pyrifera) is a foundation species that supports an ecologically diverse ecosystem on shallow reefs in temperate seas throughout the world. While regional scale control of giant kelp has been linked to environmental drivers, local growth rate and canopy biomass dynamics often defy these regional patterns. We examined changes in giant kelp canopy biomass and the chlorophyll to carbon ratio (Chl:C; a known proxy for the physiological condition of marine autotrophs) in the Santa Barbara Channel using a time series of airborne hyperspectral imagery as part of the NASA Hyperspectral Infrared Imager (HyspIRI) Preparatory Campaign. We asked two overarching questions: What are the regional (kilometer-scale) patterns of Chl:C and how are these related to the spatiotemporal patterns of environmental variables? Are there local (meter-scale) patterns in Chl:C and do these patterns relate to kelp forest growth and decline?


We found that regional patterns of Chl:C were inversely and non-linearly associated with large-scale fluctuations in sea surface temperature. This relationship closely resembles the association between sea surface temperature and ambient nitrate concentration in the California Current upwelling system. Local scale variability in Chl:C across a single kelp forest equaled the regional variability, implying that local scale processes also play a role in the physiological condition of this species. Local scale examples showed that canopy Chl:C was negatively related to the date when kelp canopy first emerged, suggesting that demographic patterns in kelp frond age influence the local physiological condition and persistence of giant kelp canopy. The canopy forming fronds of giant kelp are primarily controlled by programmed senescence, while frond birth rate is related to ambient nitrate concentration. We developed a conceptual model for the progression of a canopy forming giant kelp forest based upon the regional scale nutrient dynamics and local scale demographics to help rectify the differences observed between regional and local scale giant kelp dynamics.