The productivity of ocean regions near Hawai’i and US territories in the Pacific Ocean is controlled by the amount of essential nutrients in seawater. Nutrients are consumed by phytoplankton that form the base of the food web, including fisheries that deliver economic and nutritional benefit to the United States. The nitrogen-containing molecule urea is an example of a nutrient that is present at very low concentrations and cannot be measured confidently in open ocean waters. This project improves methods used to measure the amount and turnover of urea within surface waters. Advancing understanding of this potentially large pool of bioavailable nitrogen will provide an important step toward predicting the health and productivity of open ocean ecosystems. The project will also provide training in laboratory and oceanographic methods to a postdoctoral researcher and multiple undergraduate students. The science team will participate in research cruises funded by the NSF-supported Hawaii Ocean Timeseries (HOT) program. Numerous lines of evidence collected over several decades have implicated the urea molecule as a source of recycled nitrogen to oligotrophic gyre ecosystems, but this importance has escaped direct quantification at open ocean observatories like the Hawaii Ocean Timeseries site Station ALOHA. This project will rigorously investigate the accuracy of low-level urea analyses by optimizing and comparing several alternative methods and then conduct a series of measur