Phosphate Influx and Dust Deposition Create Zonal and Meridional Biogeochemical Gradients in Trichodesmium Abundance

Trichodesmium plays a key role in the biogeochemical cycling of carbon, nitrogen and phosphorus in the Tropical Atlantic Ocean. A complex interplay of physicochemical factors control the growth of Trichodesmium. However, owing to the large spatial and temporal variability, the relative influence of these factors in controlling Trichodesmium distribution and abundance remains unclear. In this study, we examined the basin-scale distribution pattern of Trichodesmium in the upper 200 m water column of the Atlantic Ocean (25°N–30°S and 70°W–20°E) using a large data set (n = 33,235) and tried to constrain the distribution based on various physicochemical parameters. We suggest that the combined effect of warm temperatures and phosphate (PO₄³⁻) availability determines the zonal spatial extent and the abundance of Trichodesmium in the Tropical North Atlantic Ocean. However, the availability of dissolved iron, along with high sea surface temperatures and meteorological parameters such as the wind direction and precipitation, likely govern the meridional distribution of Trichodesmium across the Atlantic Ocean. Excess PO₄³⁻ at the surface rules out the possibility of PO₄³⁻ limitation in regulating the meridional distribution of the Trichodesmium. Depth-integrated nitrogen fixation rates, based on a multiple linear regression, vary from 0.07 to 306 μmol N m⁻² d⁻¹. The presence of Trichodesmium colonies down to a depth of 200 m and the depth-integrated nitrogen fixation rates reflect the pivotal role of Trichodesmium in the nitrogen budget of this region.