MODIS/Aqua chlorophyll monitoring of the New Caledonia lagoon during the 2008 La Nina event Cécile Dupouy * a,f , Audrey Minghelli-Roman b , Marc Despinoy c , Rudiger Röttgers d , Jacques Neveux e , Christel Pinazo f and Michel Petit c a Centre IRD de Nouméa, BP A5, 98848, Nouméa, New Caledonia; b LE2I, UMR Université Bourgogne, Dijon, France c Unité ESPACE, IRD, Centre IRD de Nouméa, New Caledonia d GKSS, Geesthacht, Hambourg, Germany e UPMC (Paris 6), CNRS, LOBB, 66651 Banyuls sur Mer, France f LOBP, Université de la Méditerranée, 13007, Marseille cedex 09, France ABSTRACT Tropical oligotrophic coral reef lagoons are areas of high biodiversity. Chlorophyll concentration, a proxy for phytoplankton biomass and primary production, is useful to monitor the carbon balance in the context of the climate change and to validate simulations by coupled biogeochemical models. Chlorophyll monitoring by Aqua/MODIS is examined on the large tropical oligo- to mesotrophic lagoon of New Caledonia (23,900 km 2 ). The classical OC3 algorithm developed for MODIS can only be applied in deep waters. In shallow water, when the water is clear with a weak attenuation, the bottom reflectance influences the surface reflectance and then induces an error in the chlorophyll determination. Here, a new OC3-type polynom, relating satellite reflectance ratios and chlorophyll, was determined from bio-optical data collected during a cruise (Valhybio) on the R/V Alis in the frame of the Programme National de Télédétection Spatiale. From the 22th of March to the 9 th of April, data were collected during two surveys of the same network. A total of 170 in situ bio-optical measurements in the South Western and South lagoons of New Caledonia were obtained, within a 2 weeks interval (70 non-cloudy match-ups). Four Modis images were acquired during this cruise with moderate to good atmospheric conditions. The new polynom gives a RMS of 14.8% and a MNB of - 9% and gives a better representation of the “true” water column chlorophyll concentration of the New Caledonia lagoon. Keywords: chlorophyll, algorithm, coral reef, lagoon, ocean color, New Caledonia, tropical Pacific ocean, MODIS, La Nina, sea surface reflectance 1. INTRODUCTION Coral reef lagoon systems are very sensitive to anthropogenic (nutrients, mining) perturbations [1] as well as to long- term climatic changes which are amplified in lagoons [2]. Sea surface chlorophyll is a proxy of phytoplankton biomass and is a direct integrator for the nutrient status of water masses and chlorophyll monitoring by satellite will greatly expand our knowledge of the functioning of coral reef lagoons [3]. Lately, it would allow the validation of simulations of chlorophyll by recently developed coupled biogeochemical models [4]. Tropical coastal environments are characterized by a range of extremely oligo- to eutrophic waters [5-8]. Lagoon waters belong to the class of optically complex waters (classified as Case 2 waters) where mineral particles and colored dissolved organic matter mix with phytoplankton [9]. Indeed, current algorithms such as OC4v4 for SeaWiFS and OC3 for MODIS [10, 11] are suitable for oceanic waters where chlorophyll drives variability of bio-optical properties (absorption and backscattering) of the waters. Attempts have been made to retrieve chlorophyll from remote sensing data in turbid case 2 waters [11-15]. Other algorithms tend to minimize the effect of bottom reflectance which increase surface reflectance values and therefore cause chlorophyll concentrations overestimation using algorithms developed for optically deep data [16]. Remote Sensing of Inland, Coastal, and Oceanic Waters, edited by Robert J. Frouin, Serge Andrefouet, Hiroshi Kawamura, Mervyn J. Lynch, Delu Pan, Trevor Platt, Proc. of SPIE Vol. 7150, 715014 · © 2008 SPIE · CCC code: 0277-786X/08/$18 · doi: 10.1117/12.805310 Proc. of SPIE Vol. 7150 715014-1 2008 SPIE Digital Library -- Subscriber Archive Copy