Assessment of Giant Kelp Physiological State Using Airborne Hyperspectral Imagery

Giant kelp is a highly dynamic foundation species that supports an ecologically and economically important ecosystem found throughout the globe. Currently, multispectral sensors (Landsat) provide valuable time series of emergent kelp canopy biomass that are useful for many applications. Hyperspectral sensors can provide information that quantify the quality or physiological condition of the kelp canopy, which can be linked to characteristics such as canopy age and morphology, light exposure, nutrient stress and photosynthetic yield. The HyspIRI Preparatory Airborne Campaign delivered near seasonal hyperspectral imagery of giant kelp canopy using the AVIRIS sensor (~20 m spatial resolution; 10 nm spectral resolution), to support the proposed spaceborne hyperspectral imager mission. These images, combined with additional AVIRIS imagery, were used to assess giant kelp canopy condition across several years and biogeographical regions, including Monterey Bay, the Santa Barbara Channel, and the Southern California coast. Specifically, we developed novel techniques to infer the chlorophyll a to carbon ratio (Chl:C) from the AVIRIS imagery, derived from field observations of canopy blade reflectance, pigment concentrations and carbon content, and these determinations of Chl:C are used as measures of the physiological state of the kelp canopy. We found that the spatial and temporal variability in physiological condition of the kelp canopy varied with light exposure and timing of nutrient pulses due to coastal upwelling. These observations are consistent with photophysiological theory and field observations. Physiological state dynamics gleaned from airborne sensors and proposed spaceborne hyperspectral sensors enhance our understanding of this important ecosystem engineer, and provide useful information for marine scientists and ecosystem managers.