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 (85101%) 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?0 after A. anophagefferens abundance declined, suggesting that these plankton utilized isotopically lighter nitrogen sources (e.g., recycled nutrients or fertilizer). A. anophagefferens utilized multiple forms of nitrogen (e.g., nitrate, ammonium, urea) during blooms and their uptake rates of ammonium and urea were highest during blooms. However, A. anophagefferens urea uptake rates on a per cell basis were fivefold faster than all other groups, affirming the nutritionally strategic uptake of urea to fuel brown tides. This study presents a novel approach to successfully sort a single algal species from a plankton community for the purpose of assessing nitrogen uptake and highlights a promising and powerful approach for investigating and contrasting the nutritional ecology of bloom-causing species and co-occurring plankton populations during harmful algal blooms.