Summary
: As part of the VERTIGO research program, samples for nitrogen (d15N) and oxygen (d18O) isotopic signatures in nitrate were analyzed from depth profiles and surface transects from the northwest pacific, near station K2 and the subtropical gyre, near station ALOHA. Contrasting productivity, ecology, nutrient cycling, and particle flux dynamics of these two locations are reflected in the nitrate isotopic signatures. At ALOHA, low nitrate concentrations in near-surface waters were coupled to some of the lowest d15N signatures yet reported for nitrate in seawater. Oxygen isotopic signatures in nitrate also decreased into the shallow layers, most likely due to active cycling of nitrate and production through nitrification. By accessing the isotopic signatures of nitrate at the low concentrations typical of near surface waters at station ALOHA, we are able to observe the "resetting" of nitrogen and oxygen isotopic signatures in nitrate resulting from active cycling and local sources of nitrate within this near-surface layer. In regions where surface nitrate concentrations are higher,and the turnover time of the nitrate pool is longer, nitrate consumption by phytoplankton is the dominant process impacting near-surface nitrate isotopic signatures. Through the datasets compiled simultaneously from the VERTIGO program, we are able to combine isotopic distributions in nitrate with suspended and sinking PN-d15N as well as rate measurements for new production via nitrate uptake and N2 fixation to evaluate different processes involved in nitrogen cycle dynamics.