In this study, I aimed to determine whether the genetic signature of Nitrate Reductase (NR) from phytoplankton in the environment is correlated with the in-situ nitrate concentration. To accomplish this, I first generated a phylogeny of phytoplankton NR amino acids sequences to determine if the sequences separate into monophyletic groups that match their taxonomic identification. Secondly, I placed RNA transcripts for NR derived from samples collected off the coast of Hawaii onto the tree to determine which clades expressed NR. Finally, I constructed heat maps to show the abundance of RNA transcripts for each phytoplankton clade by latitude and nitrate concentration to establish trends in phytoplankton phyla distribution. Results indicated that all phytoplankton, no matter phyla or cell size, were more abundant in higher nitrate concentrations. If instead RNA transcript abundance was normalized by chlorophyll concentrations, there was little separation in distribution based on plankton size, and different trends based on phyla emerged. Alveolata, Archaeplastida, and Stramenopiles were found in similar low to moderate nitrate concentrations (0.0023µM-0.8052µM). In contrast, Cryptista did not display a consistent trend across the phylum, as all clades displayed different abundance patterns. Haptophyta, both large and small, made up a significantly higher proportion of phytoplankton found in low nitrate environments (0.0009µM). These results indicate that there is separation of phytoplankton phyla by nitrate concentration, supporting the hypothesis that these phyla have evolved to utilize different ecological niches, however further research is needed with higher taxonomic resolution to fully quantify the factors controlling the distribution of different phytoplankton clades.