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Ocean deoxygenation due to climate change may alter redox-sensitive nutrientcycles in the marine environment. The productive eastern tropical NorthAtlantic (ETNA) upwelling region may be particularly affected when therelatively moderate oxygen minimum zone (OMZ) deoxygenates further andmicrobially driven nitrogen (N) loss processes are promoted. Consequently,water masses with a low nitrogen to phosphorus (N : P) ratio could reach theeuphotic layer, possibly influencing primary production in those waters.Previous mesocosm studies in the oligotrophic Atlantic Ocean identifiednitrate availability as a control of primary production, while a possibleco-limitation of nitrate and phosphate could not be ruled out. To betterunderstand the impact of changing N : P ratios on primary production andN2 fixation in the ETNA surface ocean, we conducted land-based mesocosmexperiments with natural plankton communities and applied a broad range ofN : P ratios (2.67–48). Silicic acid was supplied at 15 µmol L−1 in all mesocosms. We monitored nutrient drawdown, biomassaccumulation and nitrogen fixation in response to variable nutrientstoichiometry. Our results confirmed nitrate to be the key factordetermining primary production. We found that excess phosphate was channeledthrough particulate organic matter (POP) into the dissolved organic matter(DOP) pool. In mesocosms with low inorganic phosphate availability, DOP wasutilized while N2 fixation increased, suggesting a link between thosetwo processes. Interestingly this observation was most pronounced inmesocosms where nitrate was still available, indicating that bioavailable Ndoes not necessarily suppress N2 fixation. We observed a shift from amixed cyanobacteria–proteobacteria dominated active diazotrophic communitytowards a diatom-diazotrophic association of the Richelia-Rhizosolenia symbiosis. We hypothesizethat a potential change in nutrient stoichiometry in the ETNA might lead toa general shift within the diazotrophic community, potentially influencingprimary productivity and carbon export

biogeosciences

Changing nutrient stoichiometry affects phytoplankton production, DOP accumulation and dinitrogen fixation – a mesocosm experiment in the eastern tropical North Atlantic