While 15N-labeled nitrogen (N) compounds have been used to quantify N uptake rates by plankton communities for decades, accurately ascribing those rates to individual populations or species has been a challenge. Here, we apply sorting flow cytometry combined with species-specific immuno-detection of a harmful alga, Aureococcus anophagefferens, to contrast the nutritional ecology of this alga with co-occurring picoplankton (picoeukaryotes, cyanobacteria, heterotrophic bacteria) during brown tides. The method was iteratively refined to yield close agreement (85–101%) between plankton community 15N uptake quantified via traditional filtration and this novel sorting method. Sorting of plankton revealed that the ?15N values of A. anophagefferens and phycocyanin-containing cyanobacteria were more enriched (? 10‰) than the values of other picoeukaryotes and heterotrophic bacteria that decreased to?