The biogeochemical structuring role of horizontal stirring: Lagrangian perspectives on iron delivery downstream of the Kerguelen Plateau

Abstract

International audienceField campaigns are instrumental in providing ground truth for understanding and modeling global ocean biogeochemical budgets. A survey however can only inspect a fraction of the global oceans, typically a region hundreds of kilometers wide for a temporal window of the order of (at most) several weeks. This spatiotemporal domain is also the one in which the mesoscale activity induces through horizontal stirring a strong variability in the biogeochemical tracers, with ephemeral, local contrasts which can easily mask the regional and seasonal gradients. Therefore, whenever local in situ measures are used to infer larger-scale budgets, one faces the challenge of identifying the mesoscale structuring effect, if not simply to filter it out. In the case of the KEOPS2 investigation of biogeochemical responses to natural iron fertilization, this problem was tackled by designing an adaptive sampling strategy based on regionally optimized multisatellite products analyzed in real time by specifically designed Lagrangian diagnostics. This strategy identified the different mesoscale and stirring structures present in the region and tracked the dynamical frontiers among them. It also enabled back trajectories for the ship-sampled stations to be estimated, providing important insights into the timing and pathways of iron supply, which were explored further using a model based on first-order iron removal. This context was essential for the interpretation of the field results. The mesoscale circulation-based strategy was also validated post-cruise by comparing the Lagrangian maps derived from satellites with the patterns of more than one hundred drifters, including some adaptively released during KEOPS2 and a subsequent research voyage. The KEOPS2 strategy was adapted to the specific biogeochemical characteristics of the region, but its principles are general and will be useful for future in situ biogeochemical surveys

Role of Iron in the Marquesas Island Mass Effect

“…Contrary to macronutrient concentrations, weak dissolved iron concentrations (Fe) have been reported in the Marquesas (0.15 < Fe < 0.41 nmol/L) suggesting a possible limitation of phytoplankton growth by Fe (Blain et al, 2008). Indeed, similar IME have been investigated notably around islands in the Southern Ocean (i.e., Kerguelen, South Georgia, and Crozet Islands) where phytoplankton blooms regularly occur D'Ovidio et al, 2015;Maraldi et al, 2009;Planquette et al, 2007;Robinson et al, 2016). In those cases, Fe fertilization by the islands has been incriminated to explain the observed intense blooms, whereas these islands are localized in HNLC areas.…”

DUACS DT2018: 25 years of reprocessed sea level altimetry products

“…The multi-mission mapping procedure in DUACS is based on an optimal interpolation (OI) technique derived from Le Traon and Ogor (1998), Ducet et al (2000), and Le Traon et al (2003). This method is designed to generate regularly gridded products for sea level anomalies by combining measurements from different altimeters.…”

“…Ever since, it has been producing altimetry products for the scientific community in either near real time (NRT), with a delay ranging from a few hours to 1 d, or delayed time (DT), with a delay of a few months. The processing unit has been redesigned and regularly upgraded as knowledge of altimetry processing has been refined (Le Traon and Ogor, 1998;Ducet et al, 2000;Dibarboure et al, 2011;Pujol et al, 2016). Every few years, a complete reprocessing is performed through DUACS that includes all altimetry missions and that uses up-to-date improvements and recommendations from the international altimetry community.…”

Preface: The Oligotrophy to the UlTra-oligotrophy PACific Experiment (OUTPACE cruise, 18 February to 3 April 2015)

“…The western tropical South Pacific (WTSP) is an ideal location to study the fate of N fixed by N 2 fixation, as it is considered a hot spot of N 2 fixation in the world ocean . While average N 2 fixation rates range from 20 to 200 µmol N m −2 d −1 in the tropical North Atlantic (Benavides and Voss, 2015) and Pacific (Böttjer et al, 2017;Dore et al, 2002), they reach 30-5400 µmol N m −2 d −1 (average ∼ 800 µmol N m −2 d −1 ) in the Solomon Sea (western part of the WTSP) (Bonnet et al, 2009, which is in the upper range of rates reported in the global N 2 fixation MAREDAT database and even surpassed its upper rates (100-1000 µmol N m −2 d −1 ) (Luo et al, 2012). Very high rates have also been recently reported in the Arabian Sea (Gandhi et al, 2011;Kumar et al, 2017).…”