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.
Impact of Monsoon-Transported Anthropogenic Aerosols and Sun-Glint on the Satellite-Derived Spectral Remote Sensing Reflectance in the Indian Ocean
