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Denitrification in the anoxic sediments of the Santa Barbara Basin has been well documented in the historic and modern record, but the regulation of and frequency with which denitrification occurs in the overlying water column are less understood. Since 2004, the magnitude and speciation of redox active nitrogen species in bottom waters have changed markedly. Most notable are periods of decreased nitrate and increased nitrite concentrations. Here we examine these changes in nitrogen cycling as recorded by the stable isotopes of dissolved nitrate from 2010-2016. When compared to previous studies, our data identify an increase in water column denitrification in the bottom waters of the basin. Observations from inside the basin as well as data from the wider California Current Ecosystem implicate a long-term trend of decreasing oxygen concentrations as the driver for these observed changes, with ramifications for local benthic communities and regional nitrogen loss.

Plain Language Summary The current chemical environment of the deep Santa Barbara Basin is unprecedented in the 30 years since measurements began, suggesting that a regional change in ocean chemistry has occurred over this time period. Here we use stable isotope measurements of nitrate to examine how the nitrogen cycle in the deep part of the basin has changed, documenting a recent increase in the amount of nitrate being removed from the water column at this location. Nitrogen removal in the dark ocean, known as denitrification, occurs when microbes utilize nitrate for respiration in the absence of oxygen. Such a process is consistent with the extremely low or absent oxygen concentrations that accompany the altered nitrate isotope signatures we measured in this study. Denitrification in sediments is common in the Santa Barbara Basin, but extensive water column denitrification has not been previously documented. Loss of nitrate from the water column can have important consequences for the balance of nutrients that support primary production in the ocean. These changes appear to be an effect of decreasing oxygen concentrations observed on a regional scale in the North Pacific Ocean, a trend which is likely to continue as the oceans warm.