Supplementary data files

We measured N2 fixation along the Western Antarctic Peninsula (WAP) and at Palmer Station over two austral summer months. N2 fixation rates measured by 15N2 assays were below conservative detection limits, despite being detected when using less stringent detection thresholds, suggesting a potential for N2 fixation. This was confirmed by our continuous acetylene reduction method (FARACAS). nifH gene amplicon sequencing revealed that Gammaproteobacteria were the dominant class of identified diazotrophs in the WAP’s coastal waters. However, Epsilonproteobacteria contributed disproportionally to nifH expression, particularly when putative N2 fixation rates were at the highest. Combining diazotroph communities with environmental observations, we hypothesize that vertical water mixing resuspended sediments into the water column and contributed to the limited N2 fixation in Antarctic waters.