Multiyear Trichodesmium increase in the North Atlantic at higher latitudes (38-65N) in the 1980’s
African dust is an important source of iron in the tropical North Atlantic. Ocean fertilization events related to episodes of iron-rich dust input have been extensively described. Traditionally, Trichodesmium distribution is thought to be temperature-dependent and iron-limited, thus typically restricted to tropical and subtropical regions. Due to physiological constraints under lower temperatures, unusual populations found at higher latitudes are thought to be the result of drifting and not actively growing or fixing nitrogen. We analyzed over 200,000 in situ observations from the Continuous Plankton Recorder spanning from 1960-2010. We found increased abundances (up to 10-fold in the eastern side of the basin) from about 1983-1997 that may be related to intense dust activity during that same period. This event may have remained unnoticed because it occurred during the time lapse when ocean color satellites were not available (1986-1997) and discrete observations were strongly biased towards tropical latitudes. The Atlantic is thought not to be iron-limited compared to Pacific (Chappell et al., 2012) but this may not apply to northern Atlantic (latitude >40N). Using NCEP/NCAR reanalysis, we studied wind and pressure anomalies from 1960-2000 as well as the precipitation index in the Sahel Region.
Timing of increased abundances of Trichodesmium coincided with an intense drought in the Sahel region, anomalous pressure systems and strong near surface northward winds (>4m/s) coming out of the Sahel region. Our results show an episodic conduit for iron-rich meridional winds transporting African dust to higher latitudes. There is a long record of dust events over Europe but the mechanism that opens this particular pathway is not well understood. Our results suggest that the enhanced meridional aeolian transport was perhaps mediated by intense ENSO events. In fact, random forest variable importance plots show that ENSO, wind stress and wind direction emerge as key parameters for Trichodesmium abundance and not precipitation anomalies in the Sahel. Moreover, climatological dissolved oxygen (World Ocean Database) may be a better predictor of Trichodesmium distribution than sea-surface-temperature (ICOADS). According to this analysis, diazotroph abundances are maximal under a threshold of dissolved oxygen concentration <5mL/L or a temperature > 26oC. Irrespective of whether these Trichodesmium colonies were actively fixing carbon or nitrogen, this study proposes a new aeolian iron input pathway to be included in Earth System Models and opens a new Trichodesmium latitudinal range to be explored.