An intercomparison of bio-optical techniques for detecting dominant phytoplankton size class from satellite remote sensing Robert J.W. Brewin a,b, ⁎, Nick J. Hardman-Mountford b,c , Samantha J. Lavender a,d , Dionysios E. Raitsos e,f , Takafumi Hirata b,c,1 , Julia Uitz g , Emmanuel Devred h,i , Annick Bricaud j , Aurea Ciotti k , Bernard Gentili j a School of Marine Science and Engineering, University of Plymouth, Drake Circus, Plymouth, PL4 8AA, UK b National Centre for Earth Observation, PML, Plymouth, UK c Plymouth Marine Laboratory (PML), Prospect Place, The Hoe, Plymouth PL1 3DH, UK d ARGANS Ltd, Unit 3, Drake Building, Tamar Science Park, Derriford, Plymouth, PL6 8BY, UK e Hellenic Centre for Marine Research (HCMR), 46.7 km Athens-Sounio, PO Box 712, 190 13 Anavissos, Attica, Greece f Sir Alister Hardy Foundation for Ocean Science (SAHFOS), The Laboratory, Citadel Hill, Plymouth, PL1 2PB, UK g Marine Physical Laboratory (MPL), Scripps Institution of Oceanography, University of California at San Diego, 9500 Gilman Drive, LaJolla, CA 92093-0238, USA h Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, B3H 4J1, Canada i Ocean Science Division, Bedford Institute of Oceanography, Box 1006, Dartmouth, Nova Scotia, B2Y 4A2, Canada j Laboratoire d'Océanographie de Villefranche, Université Pierre et Marie Curie-Paris 6, CNRS, Villefranche-sur-mer, France k UNESP-Campus do Litoral Paulista, Praça Infante Dom Henrique S/N, São Vicente, São Paulo CEP 11220-900, Brazil abstract article info Article history: Received 26 August 2009 Received in revised form 26 August 2010 Accepted 12 September 2010 Keywords: Phytoplankton Size Ocean colour Remote sensing Pigment Chlorophyll-a SeaWiFS Absorption Satellite remote sensing of ocean colour is the only method currently available for synoptically measuring wide-area properties of ocean ecosystems, such as phytoplankton chlorophyll biomass. Recently, a variety of bio- optical and ecological methods have been established that use satellite data to identify and differentiate between either phytoplankton functional types (PFTs) or phytoplankton size classes (PSCs). In this study, several of these techniques were evaluated against in situ observations to determine their ability to detect dominant phytoplankton size classes (micro-, nano- and picoplankton). The techniques are applied to a 10-year ocean-colour data series from the SeaWiFS satellite sensor and compared with in situ data (6504 samples) from a variety of locations in the global ocean. Results show that spectral-response, ecological and abundance-based approaches can all perform with similar accuracy. Detection of microplankton and picoplankton were generally better than detection of nanoplankton. Abundance-based approaches were shown to provide better spatial retrieval of PSCs. Individual model performance varied according to PSC, input satellite data sources and in situ validation data types. Uncertainty in the comparison procedure and data sources was considered. Improved availability of in situ observations would aid ongoing research in this field. © 2010 Elsevier Inc. All rights reserved. 1. Introduction Phytoplankton functional types (PFTs) refer to phytoplankton that have a specific function with regard to the scientific question being addressed (Le Quéré et al., 2005; Nair et al., 2008). For instance, diatoms are responsible for ~20% of global carbon fixation (Nelson et al., 1995) and are major contributors to the biogeochemical cycling of silicon (Falciatore et al., 2000). In terms of primary production and the global carbon cycle, cell size, referred to here as phytoplankton size class (PSC), has previously been adopted to classify the functional groups (Sieburth et al., 1978). According to the conceptual model of Sieburth et al. (1978), the autotrophic pool is split into picoplankton (b 2 μm), nanoplankton (2–20 μm) and microplankton (N 20 μm) contributions. While from a biogeochemical perspective the cell size functional classification may not be fully satisfactory (see Nair et al. (2008)) many ecological and biogeochemical processes are related to cell size. These processes include light absorption, as influenced by the cellular pigment composition and packaging effect (Bricaud et al., 1995, 2004; Duysens, 1956; Kirk, 1975; Morel & Bricaud, 1981; Prieur & Sathyendranath, 1981), nutrient uptake (Probyn, 1985; Sunda & Huntsman, 1997), sinking rate and export (Boyd & Newton, 1999; Laws et al., 2000; Michaels & Silver, 1988). In recent years, a variety of bio-optical methods have been established that use satellite data to identify and differentiate between either PFTs (e.g. Alvain et al., 2005, 2008; Aiken et al., 2007; Raitsos et al., 2008) or PSCs (e.g. Ciotti & Bricaud, 2006; Devred et al., 2006; Uitz et al., 2006; Hirata et al., 2008). Platt et al. (2006) describe the detection of Remote Sensing of Environment 115 (2011) 325–339 ⁎ Corresponding author. A410 Portland Square, Drake Circus, University of Plymouth, Plymouth, PL4 8AA, UK. Tel.: +44 1752 584883. E-mail address: robert.brewin@plymouth.ac.uk (R.J.W. Brewin). 1 Present address: Faculty of Environmental Earth Science, Hokkaido University N10W5, Kita-Ku, Sapporo, 060-0810, Japan. 0034-4257/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.rse.2010.09.004 Contents lists available at ScienceDirect Remote Sensing of Environment journal homepage: www.elsevier.com/locate/rse