During the DeepDOM cruise, we transited through the Amazon plume. The Amazon river is responsible for ~15% of all fresh water input to the world’s oceans. Consequently, this region represents a unique biogeochemical environment, characterized by low salinity and rich in terrestrial carbon and nutrients. In the low salinity lens of the plume, we discovered a blooming species of Coscinodiscus, a centric diatom. We were able to monitor its physiological status in situ, perform field incubation experiments, and recover and isolate from this bloom to study in culture. Using these data we hope to address three questions: 1) What are the metabolic underpinnings of the Coscinodiscus response to N, P, or Si stress? 2) Do N, P, or Si responses suggest a control on the Coscinodiscus bloom in the Amazon Plume? and 3) How does competition affect physiological ecology?