As part of the ANACONDAS program, we investigated the role of the Amazon plume in stimulating offshore nitrogen fixation and export production during the river's high‐discharge period (May–June 2010). Using the shipboard underway system, we performed high‐resolution sampling of over 450,000 km2 of surface waters, characterizing the distribution of nutrients, phytoplankton, particulate organic matter (POM), and stable carbon and nitrogen isotopes of POM in the offshore plume. We found distinct regional variations in diazotroph communities, with the Diatom‐Diazotroph Associations (DDA) Hemiaulus hauckii – Richelia intracellularis dominating the low N : P mesohaline waters to the northwest of the plume axis and Trichodesmium spp. primarily occupying oceanic waters to the east. Nutrient availability broadly shaped diazotroph distributions along the salinity gradient, but habitat longevity may also play a role in the finer‐scale distributions of communities, particularly of DDAs. H. hauckii and Trichodesmium spp. affected the nitrogen and carbon budgets in fundamentally different ways within the plume‐influenced regions, with H. hauckii making much greater contributions to the particulate nitrogen pool and to CO2 drawdown than Trichodesmium spp., leading to much higher export fluxes. Our findings provide an important constraint on the role of the Amazon plume in creating distinct niches and roles for diazotrophs in the nutrient and carbon budgets of the western tropical North Atlantic.

The structure of the phytoplankton community in surface waters is the consequence of complex interactions between the physical and chemical properties of the upper water column as well as the interaction within the general biological community. Understanding the structure of phytoplankton communities is especially challenging in highly variable and dynamic marine environments. A variety of strategies have been employed to delineate marine planktonic habitats, including both biogeochemical and water-mass-based approaches. These methods have led to fundamental improvements in our understanding of marine phytoplankton distributions, but they are often difficult to apply to systems with physical and chemical properties and forcings that vary greatly over relatively short spatial or temporal scales. In this study, we have developed a method of dynamic habitat delineation based on environmental variables that are biologically relevant, that integrate over varying time scales, and that are derived from standard oceanographic measurements. As a result, this approach is widely applicable, simple to implement, and effective in resolving the spatial distribution of phytoplankton communities. As a test of our approach, we have applied it to the Amazon River-influenced Western Tropical North Atlantic (WTNA) and to the South China Sea (SCS), which is influenced by both the Mekong River and seasonal coastal upwelling. These two systems differ substantially in their spatial and temporal scales, nutrient sources/sinks, and hydrographic complexity, providing an effective test of the applicability of our analysis. Despite their significant differences in scale and character, our approach generated statistically robust habitat classifications that were clearly relevant to surface phytoplankton communities. Additional analysis of the habitat-defining variables themselves can provide insight into the processes acting to shape phytoplankton communities in each habitat. Finally, by demonstrating the biological relevance of the generated habitats, we gain insights into the conditions promoting the growth of distinct communities and the factors that lead to mismatches between environmental conditions and phytoplankton community structure.

Cyanobacteria are the main autotrophs and N 2 -fixing (diazotrophic) organisms in large parts of the oligotrophic global ocean, where generally all heterotrophic production depends on their activity. Amino acids (AAs) from cyanobacteria are essential macronutrients for these heterotrophic food webs, yet little is known about the de novo synthesis of AAs during N 2 fixation. Through a combination of bulk and amino acid nitrogen (AAN) specific analyses of field based N 2 fixation experiments, we demonstrate that the de novo synthesis of 13 AAs accounted for the majority of bulk N 2 fixation rates at four stations in the central Baltic Sea in July 2015. Slow AA turnover times of 87 6 14 d coincided with low phosphate concentrations and high cellcarbon biomasses of unicellular cyanobacteria. Very fast turnover times of 17 6 3 d coincided with high phosphate concentrations and undecayed Nodularia spumigena cells, but unexpectedly also with phosphate depletion and decayed N. spumigena cells. In a decayed bloom, volumetric N 2 fixation rates into AAN provided a much better estimate of the net incorporation of N 2 into biomass than fixation into bulk nitrogen that rather reflected gross N 2 fixation. In an undecayed bloom, the turnover times of 13 AAs can be predicted from a single bulk N 2 fixation rate. This is the first direct evidence that the very late, decayed stage of a cyanobacteria bloom can be a flashpoint of very fast AA turnover during N 2 fixation with hitherto uncharacterized consequences for heterotrophic food webs and diazotroph N inputs to the global ocean. Highlights 1. Cyanobacteria composition and bloom stage can be critical factors for amino acid (AA) turnover. 2. In a decaying bloom, amino acid nitrogen based volumetric rates reflected net N 2 fixation. 3. Bulk nitrogen based volumetric rates in a decaying bloom reflected gross N 2 fixation. 4. In undecayed blooms, AA turnover can be predicted from single bulk N 2 fixation rates. 5. Decaying cyanobacteria blooms can be flashpoints of AA turnover during N 2 fixation.

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