An algorithm for detecting &amp;lt;i&amp;gt;Trichodesmium&amp;lt;/i&amp;gt; surface blooms in the South Western Tropical Pacific

C Dupouy,J Neveux,T K Westberry,Y Dandonneau,D Benielli-Gary

doi:10.5194/bg-8-3631-2011

Abstract

Abstract. Trichodesmium, a major colonial cyanobacterial nitrogen fixer, forms large blooms in NO3-depleted tropical oceans and enhances CO2 sequestration by the ocean due to 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 Western Tropical Pacific (SP). Here, a novel algorithm (TRICHOdesmium SATellite) based on radiance anomaly spectra (RAS) observed in SeaWiFS imagery, is used to detect Trichodesmium during the austral summertime in the SP (5° S–25° S 160° E–170° W). Selected pixels are characterized by a restricted range of parameters quantifying RAS spectra (e.g. slope, intercept, curvature). The fraction of valid (non-cloudy) pixels identified as Trichodesmium surface blooms in the region is low (between 0.01 and 0.2 %), but is about 100 times higher than deduced from previous algorithms. At daily scales in the SP, this fraction represents a total ocean surface area varying from 16 to 48 km2 in Winter and from 200 to 1000 km2 in Summer (and at monthly scale, from 500 to 1000 km2 in Winter and from 3100 to 10 890 km2 in Summer with a maximum of 26 432 km2 in January 1999). The daily distribution of Trichodesmium surface accumulations in the SP detected by TRICHOSAT is presented for the period 1998–2010 which demonstrates that the number of selected pixels peaks in November–February each year, consistent with field observations. This approach was validated with in situ observations of Trichodesmium surface accumulations in the Melanesian archipelago around New Caledonia, Vanuatu and Fiji Islands for the same period.

Highlights

The balance between oceanic N2 fixation and nitrogen losses in the ocean has been postulated to regulate atmospheric CO2 over geological time via the enhancement of biological sequestration of CO2 (Falkowski, 1997; Gruber and Sarmiento, 1997; Deutsch et al, 2007; Capone and Knapp, 2007)
Twice in the series (November 1998, November 1999) large areas of 30 km2 were covered by slicks, and once in February 2004, the whole area between New Caledonia and Vanuatu (300 km2) was covered by numerous elongated slicks well detected by longline aircraft
TRICHOSAT identifies pixels for which the radiance anomaly spectra (RAS) is characterized by a narrow range of S and of Yt which implies a relatively linear RAS spectrum compared with the total range of S and Yt found within the whole Pacific Ocean

Summary

Introduction

The balance between oceanic N2 fixation and nitrogen losses (denitrification) in the ocean has been postulated to regulate atmospheric CO2 over geological time via the enhancement of biological sequestration of CO2 (Falkowski, 1997; Gruber and Sarmiento, 1997; Deutsch et al, 2007; Capone and Knapp, 2007). Unicellular (Zehr et al, 2001; Montoya et al, 2004; Church et al, 2008, 2009; Zehr et al, 2011) and filamentous cyanobacteria (Carpenter, 1983; Capone et al, 1997, 2005; LaRoche and Breitbarth, 2005; Bonnet et al, 2009; Moisander et al, 2010) incorporate this form of “new” nitrogen (N) into the marine food web of tropical and subtropical oceans (Berman-Frank et al, 2004; Mahaffey et al, 2005; Mulholland, 2007). (Barcelos et al, 2007; Hutchins et al, 2007; Kranz et al, 2009; Levitan et al, 2010) This enhancement of Trichodesmium growth could compensate the decreased growth of other phytoplankton owing to a presumed decrease of nitrate supply

Methods

Results

Discussion

Conclusion