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The effect of phytoplankton pigment composition and packaging · PDF file The effect of phytoplankton pigment composition and packaging on the retrieval of chlorophyll-a concentration

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  • TECHNICAL NOTE

    The effect of phytoplankton pigment composition and packaging on the retrieval of chlorophyll-a concentration from satellite observations in the Southern Ocean Babula Jena

    Polar Sciences, ESSO, National Centre for Antarctic and Ocean Research, Ministry of Earth Sciences (MoES), Goa, India

    ABSTRACT The Antarctic waters are known to be optically unique and the standard empirical ocean colour algorithms applied to these waters may not address the regional bio-optical characteristics. This article sheds light on the performance of current empirical algorithms and a regionally optimized algorithm (ROA) for the retrieval of chlorophyll-a (chl-a) concentration from Aqua- Moderate Resolution Imaging Spectroradiometer (Aqua-MODIS) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) in the Indian Ocean Sector of Southern Ocean (IOSO). Analysis indicated that empirical algorithms used for the retrieval of chl-a concentra- tion from Aqua-MODIS and SeaWiFS underestimate by a factor varying from 2 to 2.9, resulting in underestimation when in situ chl-a exceeds about 0.3 mg m−3. To explain these uncertainties, a study was carried out to understand the effect of phytoplankton pigment composition and pigment packaging on remote-sensing reflectance (Rrs,λ), from the analysis of phytoplankton-specific absorption coefficient (aph,*λ). The spatial variation of phytoplank- ton groups analysed using diagnostics pigments (DP) indicated shifting of the phytoplankton community structure from offshore to coastal Antarctic, with a significant increasing trend for diatoms and a decreasing trend for haptophytes population. The diatom- dominated population exhibits lower aph,*λ in the 405–510 nm region (with relative flattening in 443–489 nm) compared with the aph,*λ spectra of the haptophytes-dominated population that peaks near 443 nm. The flattening of aph,*λ spectra for the dia- tom-dominated population was attributed to its larger cell size, which leads to pigment packaging (intracellular shading) and in turn results in higher Rrs,λ. The relationship between pigment composition (normalized by chl-a) and blue:green absorption band ratios (aph,*443:aph,*555 and aph,*489:aph,*555) corresponding to the Aqua-MODIS and SeaWiFS bands showed in-phase associa- tions with most of the pigments such as 19ʹ-hexanoyloxyfucox- anthin, 19ʹ-butanoyloxyfucoxanthin, peridinin, and zeaxanthin. In contrast, the out-of-phase association observed between the blue: green absorption ratios and fucoxanthin indicated apparent devia- tions from the general pigment retrieval algorithms, which assumes that blue:green ratios vary in a systematic form with chl-a. The out-of-phase correspondence suggests that the increas- ing trend of fucoxanthin pigments towards the Antarctic coast was

    ARTICLE HISTORY Received 24 August 2016 Accepted 9 March 2017

    CONTACT Babula Jena [email protected] ESSO, National Centre for Antarctic and Ocean Research, Ministry of Earth Sciences (MoES), Headland Sada, Goa 403804, India

    INTERNATIONAL JOURNAL OF REMOTE SENSING, 2017 http://dx.doi.org/10.1080/01431161.2017.1308034

    © 2017 Informa UK Limited, trading as Taylor & Francis Group

    http://www.tandfonline.com http://crossmark.crossref.org/dialog/?doi=10.1080/01431161.2017.1308034&domain=pdf

  • associated with the decreasing trend of blue:green absorption ratios and in turn results in higher Rrs,λ. Therefore, an increase in Rrs,λ leads to underestimation of chl-a from Aqua-MODIS and SeaWiFS in the IOSO region.

    1. Introduction

    Sea-surface chlorophyll-a (chl-a) concentration is considered one of the essential para- meters for the estimation of primary production and study of carbon dioxide (CO2) dynamics. The observations are important to link the role of the Southern Ocean pelagic ecosystem in the global biogeochemical cycle. In situ observations are sparse around the Southern Ocean because of adverse weather condition, navigational hazards, remote loca- tion, and inhospitable environment with high sea states driven by strong winds. Satellite remote sensing of the Southern Ocean colour provides synoptic and time-series coverage of near-surface chl-a concentration dynamics affected by seasonality (Johnson et al. 2013). The synoptic coverage of chl-a concentration is widely used to investigate the dynamics of regional oceanographic features such as fronts, eddies, gyres, upwelling zones, plumes, and surface current patterns. These oceanographic features are useful to study the Southern Ocean ecosystem, which supports large assemblages of phytoplankton, zooplankton, sea- birds, seals, and whales. This signifies the need for the retrieval of chl-a from ocean colour sensors with greater accuracy over the Southern Ocean. However, the retrieval is compli- cated at the regional and local scales as the spectral inherent optical properties (IOPs) of the ocean influencing the ocean colour are complex (Sauer et al. 2012).

    The satellite-derived chl-a concentration in the Indian Ocean of Southern Ocean (IOSO) may have large uncertainties owing to its different bio-optical characteristics, in addition to sparse matchup observations (in situ and satellite), being used for algo- rithm development and validation. For example, in order to address the regional bio- optical characteristics of the IOSO region (60° E to 100° E and 40° S to the Antarctic coast) for retrieval of chl-a concentration, only 15 in situ bio-optical profiles are available at National Aeronautics and Space Administration (NASA), bio-Optical Marine Algorithm Data set (NOMAD). Mitchell and Holm-Hansen (1991) applied the Coastal Zone Colour Scanner (CZCS) global algorithm (GA; Gordon et al. 1983) in the coastal waters of the western Antarctic Peninsula and the adjacent open ocean waters of Drake Passage. They noticed underestimation of CZCS-derived chl-a and the study highlighted the need for regional algorithm development and validation. In Ross Sea, Moore et al. (1999) validated Sea-viewing Wide Field-of-view Sensor (SeaWiFS)-derived chl-a with in situ-extracted concentration for a range of 0.1–1.5 mg m−3 and the results revealed an underestimation of the SeaWiFS-derived chl-a value. Carder et al. (2003) analysed the Southern Ocean chl-a concentration using the SeaWiFS OC4 algorithm (O’reilly 2000) and revealed significant underestimation of the values between 0.2 and 10.0 mg m−3. Similarly, Korb, Whitehouse, and Ward (2004) reported that the SeaWiFS OC4 algorithm output values were only 87% of the in situ chl-aflu for concentrations lower than 1.0 mg m−3 and only 30% for concentrations above 5.0 mg m−3 in the South Georgia area, Southern Ocean.

    2 B. JENA

  • In contrast to earlier studies, Marrari, Hu, and Daly (2006) reported that high-resolution (1 km) daily SeaWiFS chl-a data generated using the global OC4v4 algorithm was an accurate measure over the waters of Antarctic Peninsula, if concurrent in situ data were generated using high-performance liquid chromatography (HPLC). Further investigations by Szeto et al. (2011) showed that SeaWiFS chl-a using the global OC4v4 algorithm consistently underestimates the chl-a value by 50%, although HPLC-based measurements were considered as suggested by Marrari, Hu, and Daly (2006). A recent study by Johnson et al. (2013) reported that the relationship between the maximum band ratio of remote- sensing reflectance (Rrs,λ) and in situ chl-a concentration in the Southern Ocean was poorly described by global empirical algorithms and concluded that the current algorithms significantly underestimated the chl-a concentration. They attempted to develop site- specific algorithms over the entire Southern Ocean for Aqua-Moderate Resolution Imaging Spectroradiometer (Aqua-MODIS) and SeaWiFS, yet most of the matchup bio- optical observations were located approximately along the 150° E meridian. The developed algorithms could improve the coefficients of determination (R2) from 0.26 to 0.51 for Aqua- MODIS and from 0.27 to 0.46 for SeaWiFS. The algorithm development process conducted by Johnson et al. (2013) has not addressed the issues associated with atmospheric correc- tions. In addition, the algorithm is yet to be validated in the IOSO waters where the regional and seasonal variations in the environment may lead to different bio-optical characteristics. Hence, the present study was carried out over the IOSO waters (60° E to 100° E and 40° S to Antarctic coast, Figure 1) with the detailed objectives as follows: (i) employ the current GA (O’reilly 2000) and the southern ocean algorithm (SOA) (Johnson et al. 2013) for the retrieval and validation of chl-a concentration from SeaWiFS and Aqua-MODIS; (ii) analyse

    Figure 1. Study area shows ETOPO 1 (Earth Topography One Arc-Minute Global Relief Model, 2009) bathymetric information along with locations of in situ bio-optical observations and HPLC phyto- plankton pigments from station 1 to station 10 (filled circles) collected on cruise on board the R/V Roger Revelle (I8SI9 N), used for the development of regional optimized algorithm (ROA). In situ chlorophyll-a concentration collected on cruise on board the R/V Roger Revelle (filled triangles) and world ocean database (squares) were used for validation of ROA.

    INTERNATIONAL JOURNAL OF REMOTE SENSING 3

  • the possibility of developing a regionally optimized algorithm (ROA) for the estimation of chl-a concentration; (iii) validate the robustness of the developed ROA, and (iv) diagnose the possible causes of uncertainties in satellite retrieval, particularly the effect of phyto- plankton pigment composition on specific absorption coefficient (aph,*λ) through the pigment package effect.

    2. Data analysis and methodology

    2.1. Satellite measurements of chl-a c

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