Validation of standard and alternative satellite ocean-color chlorophyll products off Western Iberia

C Sá,D D'Alimonte,C.R Mendes,T Kajiyama,V Brotas,J.C.B Da Silva,P.B Oliveira,J Vitorino,A.C Brito

doi:10.1016/j.rse.2015.07.018

Abstract

Abstract Chlorophyll a concentration (Chl) product validation off the Western Iberian coast is here undertaken by directly comparing remote sensing data with in situ surface reference values. Both standard and recently developed alternative algorithms are considered for match-up data analysis. The investigated standard products are those produced by the MERIS (algal 1 and algal 2) and MODIS (OC3M) algorithms. The alternative data products include those generated within the CoastColour Project and Ocean Color Climate Change Initiative (OC-CCI) funded by ESA, as well as a neural net model trained with field measurements collected in the Atlantic off Portugal (MLP ATLP ). Statistical analyses showed that satellite Chl estimates tend to be larger than in situ reference values. The study also revealed that a non-uniform Chl distribution in the water column can be a concurring factor to the documented overestimation tendency when considering larger optical depth match-up stations. Among standard remote sensing products, MODIS OC3M and MERIS algal 2 yield the best agreement with in situ data. The performance of MLP ATLP highlights the capability of regional solutions to further improve Chl retrieval by accounting for environmental specificities. Results also demonstrate the relevance of oceanographic regions such as the Nazare area to evaluate how complex hydrodynamic conditions can influence the quality of Chl products.

Highlights

The chlorophyll a concentration (Chl) obtainable from satellite ocean color imagery is a key quantity for monitoring the spatial and temporal variability of the phytoplankton biomass (e.g. Behrenfeld et al, 2006; Dandonneau et al, 2004; Kahru, Kudela, Manzano-Sarabia, & Mitchell, 2012; Kahru & Mitchell, 2001; Werdell et al, 2009; Yoder, Kennelly, Doney, & Lima, 2010)
It is noted that these findings comply with the results presented in Fig. 3, where coincident match-ups of MODIS and MERIS reveal a higher overestimation tendency of MERIS than MODIS
The objective of this study is to provide an overall validation of space-borne Chl estimates in the Atlantic off the Iberian Peninsula by directly comparing in situ reference measurements with standard and alternative ocean color products

Summary

Introduction

The chlorophyll a concentration (Chl) obtainable from satellite ocean color imagery is a key quantity for monitoring the spatial and temporal variability of the phytoplankton biomass (e.g. Behrenfeld et al, 2006; Dandonneau et al, 2004; Kahru, Kudela, Manzano-Sarabia, & Mitchell, 2012; Kahru & Mitchell, 2001; Werdell et al, 2009; Yoder, Kennelly, Doney, & Lima, 2010). Validation studies have shown an overall tendency of the band-ratio approach to overestimate in situ Chl in the presence of additional CDOM and TSM with respect to their characteristic level for a certain phytoplankton amount (IOCCG, 2000). This can be explained considering that the CDOM and TSM absorptions have an exponential decay as a function of the wavelength. The presence of these optically significant substances above their standard level makes the Rrs spectral slope steeper in the blue-green range, leading the band-ratio inversion scheme to determine higher Chl values than what is in reality

Objectives

Results

Discussion

Conclusion