Context: the southwest tropical Pacific
The southwest tropical Pacific is a particularly interesting region of the global ocean, identified as a hotspot of nitrogen fixation. Nitrogen fixers (or diazotrophs), such as Trichodesmium, are phytoplankton species that can use atmospheric N2 dissolved in surface waters to get nitrogen (a necessary nutrient), rather than using nitrate and/or ammonium like other species of phytoplankton. Nitrogen fixation gives diazotrophs an advantage in regions where nitrate concentrations are low and other nutrients (mainly phosphate and iron) are in excess.
In this region, phosphate can be brought via vertical supply (winter mixing) and/or via horizontal advection from the east. Iron can be supplied by island mass effects during strong precipitation events, or vertically because subsurface iron concentrations are enhanced by a strong hydrothermal activity. These hypotheses were primarily derived following the 2015 OUTPACE oceanographic campaign. During this cruise, scientists investigated biological variability along an west-to-east transect near 20°S.
SAPPHIRE supported an internship focused on using MODIS satellite chlorophyll data over the 2002-2018 time period. The goal was to put the OUTPACE observations in a longer context and to understand biological variability in the southwest tropical Pacific. More precisely, the main objective was to identify the physical drivers of seasonal and interannual phytoplanktonic enrichments in the region. Ibrahima Afoula Coly, a master 2 student at Aix-Marseille University, spent a few months at the MIO working on these questions. He investigated the impact of vertical mixing, precipitation and sea surface temperature on chlorophyll variability.
Approach and results
The southwest tropical Pacific was split into three boxes representative of oligotrophic to ultraoligotrophic conditions. The two west boxes were also centered on islands. In each box, Ibrahima computed time series of chlorophyll, precipitation, mixed layer depth and sea surface temperature. Observations at the seasonal scale lead to hypotheses that were later verified using monthly data for the period 2002-2018. The results suggest that the processes controlling biological variability differ between austral summer and winter. In winter, vertical mixing increases surface chlorophyll due to entrainment of the deep chlorophyll maximum into surface waters. In summer, strong precipitation events are correlated to increased chlorophyll near islands, suggesting that island runoff increases diazotrophy in the region.