Biogeosciences, 8, 3631–3647, 2011 www.biogeosciences.net/8/3631/2011/ Biogeosciences doi:10.5194/bg-8-3631-2011 © Author(s) 2011. CC Attribution 3.0 License. An algorithm for detecting Trichodesmium surface blooms in the South Western Tropical Pacific C. Dupouy1, D. Benielli-Gary2, J. Neveux3, Y. Dandonneau4, and T. K. Westberry5 1LOPB – Laboratoire d’Oceanographie´ Physique et Biogeochimique,´ UMR 6535, CNRS-INSU-IRD-Universite´ de la Mediterran´ ee,´ 1M213, Centre de Noumea,´ B. P. A5, New Caledonia, France 2LAM – Laboratoire d’Astrophysique de Marseille, Poleˆ de l’Etoile´ Site de Chateau-Gombertˆ 38, rue Fred´ eric´ Joliot-Curie 13388, Marseille Cedex 13, France 3UPMC – CNRS, UMR7621, Observatoire Oceanologique´ de Banyuls, Laboratoire d’Oceanographie´ Microbienne, Avenue Fontaule,´ 66651 Banyuls sur Mer, France 4Universite´ de Paris VI-UPMC-LOCEAN and 14 rue de la Victoire, 91740 Chamarande, France 5Dept. Botany Plant Pathology, Oregon State University Corvallis, OR 97331–2902, USA Received: 29 May 2011 – Published in Biogeosciences Discuss.: 15 June 2011 Revised: 22 November 2011 – Accepted: 22 November 2011 – Published: 13 December 2011 Abstract. Trichodesmium, a major colonial cyanobacterial with in situ observations of Trichodesmium surface accumu- nitrogen fixer, forms large blooms in NO3-depleted tropical lations in the Melanesian archipelago around New Caledo- oceans and enhances CO2 sequestration by the ocean due to nia, Vanuatu and Fiji Islands for the same period. its ability to fix dissolved dinitrogen. Thus, its importance in C and N cycles requires better estimates of its distribution at basin to global scales. However, existing algorithms to detect them from satellite have not yet been successful in the South 1 Introduction Western Tropical Pacific (SP). Here, a novel algorithm (TRI- CHOdesmium SATellite) based on radiance anomaly spectra The balance between oceanic N2 fixation and nitrogen losses (RAS) observed in SeaWiFS imagery, is used to detect Tri- (denitrification) in the ocean has been postulated to regulate chodesmium during the austral summertime in the SP (5◦ S– atmospheric CO2 over geological time via the enhancement 25◦ S 160◦ E–170◦ W). Selected pixels are characterized by of biological sequestration of CO2 (Falkowski, 1997; Gru- a restricted range of parameters quantifying RAS spectra ber and Sarmiento, 1997; Deutsch et al., 2007; Capone and (e.g. slope, intercept, curvature). The fraction of valid (non- Knapp, 2007). Unicellular (Zehr et al., 2001; Montoya et cloudy) pixels identified as Trichodesmium surface blooms al., 2004; Church et al., 2008, 2009; Zehr et al., 2011) and in the region is low (between 0.01 and 0.2 %), but is about filamentous cyanobacteria (Carpenter, 1983; Capone et al., 100 times higher than deduced from previous algorithms. At 1997, 2005; LaRoche and Breitbarth, 2005; Bonnet et al., daily scales in the SP, this fraction represents a total ocean 2009; Moisander et al., 2010) incorporate this form of “new” 2 surface area varying from 16 to 48 km in Winter and from nitrogen (N) into the marine food web of tropical and sub- 2 200 to 1000 km in Summer (and at monthly scale, from 500 tropical oceans (Berman-Frank et al., 2004; Mahaffey et al., 2 2 to 1000 km in Winter and from 3100 to 10 890 km in Sum- 2005; Mulholland, 2007). N fixation is considered to be 2 2 mer with a maximum of 26 432 km in January 1999). The the major source of new N in stratified, oligotrophic tropical daily distribution of Trichodesmium surface accumulations oceans (Capone et al., 1997; Karl et al., 2002). Future change in the SP detected by TRICHOSAT is presented for the pe- in sea surface temperature (Breitbarth et al., 2006) or/and riod 1998–2010 which demonstrates that the number of se- CO2 concentration are expected to stimulate photosynthe- lected pixels peaks in November–February each year, con- sis (C fixation) and N2 fixation by filamentous cyanobac- sistent with field observations. This approach was validated teria, particularly by Trichodesmium spp. (Barcelos et al., 2007; Hutchins et al., 2007; Kranz et al., 2009; Levitan et al., 2010). This enhancement of Trichodesmium growth Correspondence to: C. Dupouy could compensate the decreased growth of other phytoplank- ([email protected]) ton owing to a presumed decrease of nitrate supply. Published by Copernicus Publications on behalf of the European Geosciences Union. 3632 C. Dupouy et al.: Trichodesmium surface blooms in the SW Pacific Trichodesmium spp. can form extensive blooms which 2 Material and methods have been observed for a long time in the South Western Pacific Ocean (SP), particularly in austral summer (Dan- 2.1 Data donneau and Gohin, 1984). The presence of three major archipelagos (New Caledonia, Vanuatu and Fiji-Tonga in the 2.1.1 In situ observations SP region (5◦ S–25◦ S, 150◦ E–170◦ W, Fig. 1a) and their Bloom observations (Fig. 2) used here include those col- potential for oceanic iron enrichment from land may trig- lected during the maritime survey of the Economic Exclu- ger these cyanobacterial blooms (Bowman and Lancaster, sive Zone of New Caledonia by the French Navy (aerial and 1965; Mantas et al., 2011) as Trichodesmium blooms require shipboard observations) and those obtained during scientific large quantities of iron (Rubin et al., 2011). The blooms ap- cruises on the R/V Alis between New Caledonia, Vanuatu, pear as brown or orange meandering patterns around those Fiji and Wallis and Futuna Islands (15◦ S to 23◦ S, 160◦ E to archipelagos (Dupouy et al., 1988; Dupouy, 1990; Tenorio,´ 180◦ E; Dupouy et al., 2004a). For most of the observations 2006; Hashihama et al., 2010), are clearly detected from the by the French Navy and the R/V Alis, surface water sam- International Space Shuttle (December 2001 around Tonga ples were collected with a bucket and preserved onboard in a Islands), and were recently highlighted by the NASA Ocean 4 % formalin solution. Identification of diazotroph morpho- Color Website (Feldman et al., 2010). Blooms are also reg- logical groups was made under a Zeiss microscope at IRD ularly observed in waters of the Dampier Archipelago, the Noumea´ (LOCEAN laboratory). In addition, more detailed Arafura Sea (Neveux et al., 2006) and off the Great Bar- identification and abundance of filamentous diazotrophs was rier Reef (Kuchler and Jupp, 1988; Furnas, 1989; Bell et al., obtained from inverted microscopy between October 2001 1999). Trichodesmium was reported in the Western North and October 2003 (nine Diapalis cruises as part of the DI- Pacific (Shiozaki et al., 2009; Kitajima et al., 2009; Konno Azotrophy in a Pacific ZONe program) (Tenorio,´ 2006). In et al., 2010). Nevertheless, observations in the SP contradict these instances, surface sampling was done with 8 L Niskin the recently published global map of Trichodesmium analogs bottles (sometimes with bucket sampling for comparison). based on ecosystem model results that indicate a predom- During the Diapalis cruises, chlorophyll-and phycoerythrin inance of higher densities in the North Pacific than in the measurements were also obtained from spectrofluorometry South Western Pacific Ocean (Monteiro et al., 2010). (Lantoine and Neveux, 1997; Neveux et al., 1999; Neveux Estimating the occurrence of Trichodesmium surface et al., 2006). A slightly higher concentration of chlorophyll- blooms from satellite is a major challenge, but will be re- a (Chl-a) was observed from bucket samples compared to quired for large-scale estimates of nitrogen fixation (e.g., those from Niskin samples. Westberry et al., 2005; Westberry and Siegel, 2006). Re- gional algorithms have been successfully applied on the At- 2.1.2 Satellite ocean color data lantic continental shelf (Subramaniam et al., 2002), off Ca- nary Islands (Ramos et al., 2005) as well as the Indian For the development of the new algorithm, representative coast (Sarangi et al., 2004). Global algorithms have also global SeaWiFS Level3 data (R2010) were selected for sum- been recently developed for estimating phytoplankton com- mer and winter seasons, and included normalized water leav- munity structure in the surface oceans. The PHYtoplankton ing radiance, nLw (λ), at 6 channels (412, 443, 490, 510, SATellite (PHYSAT) algorithm was successful in identifying 555, and 670 nm) as well as SeaWiFS chlorophyll and the Synechococcus-like cyanobacteria with maxima in the trop- diffuse attenuation coefficient at 490 nm (K490 product). ics (Alvain et al., 2005). The Scanning Imaging Absorp- From these data, a Look Up Table (LUT) relating K490 tion Spectrometer for Atmospheric Cartography (SCHIA- to remote sensing reflectance was created (see Sect. 2.2). MACHY) sensor also detects a cyanobacterial signal within However, to avoid compositing artifacts upon application the same latitudinal band (Bracher et al., 2009). However, of the LUT, daily SeaWiFS Level-2 GAC (R2010) between all algorithms generally fail at identifying seasonality of Tri- 1998–2010 and covering the Western Pacific Ocean (160◦ E– chodesmium blooms in the SP and none retrieve the rela- 160◦ W/25◦ N–25◦ S area), were used. tively high abundance of Trichodesmium surface blooms ex- pected and observed in situ during austral summer (Novem- 2.2 Methods ber to March). Here we develop an algorithm to detect Tri- chodesmium surface blooms in the SP which is based on Sea- The general approach was to define a spectral radiance WiFS radiance anomalies (similar to PHYSAT) and apply it anomaly from SeaWiFS nLw(λ) that was specifically related to SeaWiFS data from 1997–2010. to Trichodesmium surface blooms. It aimed at removing the first order variability in ocean color caused by chlorophyll concentration while preserving the variability that may be specifically caused by individual phytoplankton species or other optically active components. This objective is similar to the PHYSAT classification method (Alvain et al., 2005) Biogeosciences, 8, 3631–3647, 2011 www.biogeosciences.net/8/3631/2011/ C. Dupouy et al.: Trichodesmium surface blooms in the SW Pacific 3633 a b 160°E 170°E 180° 170°W 5 Chl-a mg m -3 5°S 1 Hawaii Isl.