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. Plain Language Summary: Microbial nitrogen fixation is the key to carbon sequestration in the ocean. Trichodesmium, being the ubiquitous nitrogen fixing microbe in the Tropical Atlantic Ocean, contributes significantly to nitrogen inputs. Limited availability of data, however, restricts our understanding of environmental parameters in controlling the distribution and abundance of Trichodesmium. To address this, we conducted a comprehensive analysis using large‐scale field‐based data of Trichodesmium abundance to investigate the role of various physical, chemical, and meteorological parameters on the distribution and abundance of Trichodesmium along the zonal and meridional transects of the Tropical Atlantic Ocean. We conclude that Trichodesmium distribution is governed by a complex interplay of environmental factors. Along the zonal transect, Trichodesmium abundance is primarily governed by the availability of PO43− and high sea surface temperatures. Conversely, the inter‐hemispheric variability seems to be influenced by dust deposition (a proxy for iron inputs) and high sea surface temperatures. Furthermore, our estimation of high modeled depth‐integrated nitrogen fixation rates based on Trichodesmium underscores its crucial role in the nitrogen budget. These findings provide valuable insight into the role of environmental factors driving Trichodesmium abundance and its significance toward the global nitrogen budget. Key Points: Trichodesmium distribution across the east‐west continuum in the Tropical North Atlantic Ocean is governed by the availability of phosphate and elevated sea surface temperaturesThe interhemispheric variability in Trichodesmium distribution in the Atlantic Ocean is regulated by the cumulative effect of high temperature and dust depositionTrichodesmium niche may have been overlooked in the past, potentially leading to underestimation of associated N2 fixation rates [ABSTRACT FROM AUTHOR]