Abstract
Abstract. Trichodesmium is the major nitrogen-fixing species in the western tropical South Pacific (WTSP) region, a hot spot of diazotrophy. Due to the paucity of in situ observations, remote-sensing methods for detecting Trichodesmium presence on a large scale have been investigated to assess the regional-to-global impact of this organism on primary production and carbon cycling. A number of algorithms have been developed to identify Trichodesmium surface blooms from space, but determining with confidence their accuracy has been difficult, chiefly because of the scarcity of sea-truth information at the time of satellite overpass. Here, we use a series of new cruises as well as airborne surveys over the WTSP to evaluate their ability to detect Trichodesmium surface blooms in the satellite imagery. The evaluation, performed on MODIS data at 250 m and 1 km resolution acquired over the region, 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. Improving Trichodesmium detection requires measuring ocean color at higher spectral and spatial (<250 m) resolution than MODIS, taking into account environment properties (e.g., 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-mesoscale aggregation processes for Trichodesmium are yet to be understood.
Highlights
The western tropical South Pacific (WTSP) is a low nutrient, low chlorophyll (LNLC) region, harboring surface nitrate concentrations close to detection limits of standard analytical methods and limiting for the growth of the majority of phytoplankton species (Le Borgne et al, 2011)
The aim of this study is to provide a systematic detection of Trichodesmium blooms in the vast WTSP ocean between latitudes 26 and 10◦ S and longitudes 155 and 190◦ E, building on previously published algorithms and using marine reflectances measured by Moderate Resolution Imaging Spectroradiometer (MODIS) onboard Aqua
The detection results of the three Trichodesmium detection algorithms are illustrated in Fig. 5 on the MODIS tile A2007290.0355, used in McKinna et al (2011)
Summary
The western tropical South Pacific (WTSP) is a low nutrient, low chlorophyll (LNLC) region, harboring surface nitrate concentrations close to detection limits of standard analytical methods and limiting for the growth of the majority of phytoplankton species (Le Borgne et al, 2011). This lack of inorganic nitrogen favors the growth of dinitrogen (N2)-fixing organisms (or diazotrophs), which have the ability to use the inexhaustible pool of N2 dissolved in seawater and convert it into bioavailable ammonia. During the southern austral summer, Trichodesmium blooms have long been detected by satellites in the region, mostly around New Caledonia and Vanuatu (Dupouy et al, 2000, 2011), and later confirmed by microscopic enumerations (Shiozaki et al, 2014)
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