Bio-Optical Assessment of Giant Kelp Dynamics

1. Bio-Optical Assessment of Giant Kelp Dynamics Richard.C. Zimmerman1, W. Paul Bissett2, Daniel C. Reed3 1Dept. Ocean Earth & Atmospheric Sciences, Old Dominion University, Norfolk, VA 2Florida Environmental Research Institute, Tampa FL 3University of California, Santa Barbara, CA Macrocystis pyrifera (giant kelp) NPP = 14.7 x Standing Crop Kelp Density (Kg DW m-2) 0.54 – 0.61 0.62 - 0.68 0.69 – 0.75 0.75 – 0.82 0.83 – 0.88 0.89 – 0.95 Kelp Productivity (g DW m-2 d-1) 8 – 9 9.1 – 10 10.1 – 11 11.1 – 12 12.1 – 13 13.1 - 14 Pescadero Rocks Carmel Bay Pt. Lobos ACKNOWLEDGEMENTSThis research is supported by the National Oceanic and Atmospheric Administration (NOAA) National Aeronautics and Space Administration (NASA) and National Science Foundation (NSF) INTRODUCTION RETRIEVAL OF GIANT KELP BIOMASS AND PRODUCTIVITY FROM OPTICAL MEASUREMENTS The productivity of giant kelp forests is highly variable across time and space. Winter storms and summer periods of nutrient limitation act as bottom-up regulators of kelp abundance and growth in a geography-dependent manner. The goal of this research is to develop to a predictive understanding of giant kelp forest dynamics s in the nearshore waters of California using a combination of (i) bio-optical modeling of kelp productivity, (ii) high-resolution remote sensing of kelp cover, biomass & its physiological state, and (iii) metapopulation modeling of kelp patch dynamics. Here we present progress on objectives (i) and (ii). The consistent optical signature of the kelp canopy produces a Blade Area Index (BAI, identical to Leaf Area Index used in terrestrial vegetation studies) that is linearly related to diver counts of kelp abundance. The slope (0.1) indicates that the canopy structure exerts a strong package effect on the optical efficiency of light absorption. The ability to predict BAI allows retrieval of standing biomass and productivity from measurements of below-canopy irradiance. OPTICAL PROPERTIES OF GIANT KELP BLADES Perhaps more importantly, the strong reflectance signal in the NIR allows absolute kelp abundance and productivity to be calculated and mapped across the habitat from remotely sensed hyperspectral imagery using the normalized difference vegetation index (NDVI). Optical properties of kelp blades show age dependent differences that may provide useful signals for understanding how the age structure of kelp populations affects the submarine light environment and remote sensing reflectance. Reflectances are highest in senescent tissue, particularly in the NIR. Spectral slopes of NIR reflectance increase with tissue age. Mature tissues have the highest optical density and the lowest reflectance. • Converting NDVI into absolute kelp abundance and productivity: • Optical BAI = NDVI/0.71 • True BAI = Optical BAI * 9.04 • Biomass = True BAI/13.3 • Productivity = Biomass * 14.7 CANOPY IMPACT ON THE SUBMARINE LIGHT ENVIRONMENT The giant kelp canopy significantly alters the light environment relative to adjacent open water. Example spectral coefficients of diffuse attenuation for downwelling plane irradiance (Kd) in open water adjacent to the kelp forests at Mohawk Reef and Arroyo Quemada range from 0.1 to 0.6 m-1, with strongly defined minima in the green. Attenuation coefficients under the kelp canopies (including water) averaged 0.8 m-1 and were spectrally flat. Canopy absorbances retrieved from these measurements were quantitatively consistent with laboratory measurements of individual blades.