Remote Sensing of Trichodesmium spp. mats in the Western Tropical South Pacific

Type Article
Date 2018-08
Language English
Author(s) Rousset Guillaume1, de Boissieu Florian2, Menkes Christophe E.3, Lefevre Jerome4, Frouin Robert5, Rodier Martine6, Ridoux Vincent7, 8, Laran Sophie7, Bonnet Sophie9, Dupouy Cecile9
Affiliation(s) 1 : IRD, UMR ESPACE DEV, Noumea, New Caledonia.
2 : Inst Natl Rech Sci & Technol Environm Agr IRSTEA, UMR TETIS, Montpellier, France.
3 : Univ Paris 06, Sorbonne Univ, UPMC, IRD,CNRS,MNHN,LOCEAN, IRD Noumea BP A5, Noumea 98848, New Caledonia.
4 : IRD Noumea, LEGOS, Noumea, New Caledonia.
5 : Univ Calif San Diego, Scripps Inst Oceanog, La Jolla, CA 92093 USA.
6 : Univ Polynesie Francaise, Inst Malarme Ifremer, Inst Rech Dev, EIO, Papeete, French Polynesi, Fr Polynesia.
7 : Univ La Rochelle, CNRS, Observ Pelagis, UMS3462, La Rochelle, France.
8 : Univ La Rochelle, CNRS, UMR7273, Ctr Etud Chize, 2 Rue Olympe de Gouges, F-17000 La Rochelle, France.
9 : Aix Marseille Univ, IRD Noumea, UM110, CNRS,INSU,IRD,MIO, BP A5, Noumea 98848, New Caledonia.
Source Biogeosciences (1726-4170) (Copernicus Gesellschaft Mbh), 2018-08 , Vol. 15 , N. 16 , P. 5203-5219
DOI 10.5194/bg-15-5203-2018
WOS© Times Cited 4
Note Special issue Interactions between planktonic organisms and biogeochemical cycles across trophic and N2 fixation gradients in the western tropical South Pacific Ocean: a multidisciplinary approach (OUTPACE experiment) Editor(s): T. Moutin, S. Bonnet, K. Richards, D. G. Capone, E. Marañón, and L. Mémery
Abstract

Trichodesmium is the main nitrogen-fixing species in the South Pacific region, a hotspot for diazotrophy. Due to the paucity of in situ observations, methods for detecting Trichodesmium presence on a large scale have been investigated to assess the regional-to-global impact of these species on primary production and carbon cycling. A number of satellite-derived algorithms have been developed to identify Trichodesmium surface blooms, but determining with confidence their accuracy has been difficult, chiefly because of the scarcity of sea-truth information at time of satellite overpass. Here, we use a series of new cruises as well as airborne observational surveys in the South Pacific to quantify statistically the ability of these algorithms to discern correctly Trichodesmium surface blooms in the satellite imagery. The evaluation, performed on MODIS data at 250 m and 1 km resolution acquired over the South West Pacific, shows limitations due to spatial resolution, clouds, and atmospheric correction. A new satellite-based algorithm is designed to alleviate some of these limitations, by exploiting optimally spectral features in the atmospherically corrected reflectance at 531, 645, 678, 748, and 869 nm. This algorithm outperforms former ones near clouds, limiting false positive detection, and allowing regional scale automation. Compared with observations, 80 % of the detected mats are within a 2 km range, demonstrating the good statistical skill of the new algorithm. Application to MODIS imagery acquired during the February–March 2015 OUTPACE campaign reveals the presence of surface blooms Northwest and East of New Caledonia and near 20° S–172° W in qualitative agreement with measured nitrogen fixation rates. The new algorithm, however, fails to detect sub-surface booms evidenced in trichome counts. Improving Trichodesmium detection requires measuring ocean color at higher spectral and spatial (< 250 m) resolution than MODIS, taking into account environment properties (wind, sea surface temperature, ...), fluorescence, and spatial structure of filaments, and a better understanding of Trichodesmium dynamics, including aggregation processes to generate surface mats. Such sub-mesoscales aggregation processes for Trichodesmium are yet to be understood.

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How to cite 

Rousset Guillaume, de Boissieu Florian, Menkes Christophe E., Lefevre Jerome, Frouin Robert, Rodier Martine, Ridoux Vincent, Laran Sophie, Bonnet Sophie, Dupouy Cecile (2018). Remote Sensing of Trichodesmium spp. mats in the Western Tropical South Pacific. Biogeosciences, 15(16), 5203-5219. Publisher's official version : https://doi.org/10.5194/bg-15-5203-2018 , Open Access version : https://archimer.ifremer.fr/doc/00419/53004/