Inter-annual variability of nutrient distribution in the Eastern Tropical North Pacific oxygen deficient zone

Abstract

In Oxygen Deficient Zones (ODZs) in the ocean, anaerobic respiratory processes remove fixed nitrogen from the marine system as nitrogen gas. The expansion of ODZs over time in response to accelerated anthropogenic climate change is of concern, so understanding their unique chemistry is critical. The Eastern Tropical North Pacific (ETNP) is home to the world's largest naturally occurring ODZ, and has been sampled by seven oceanographic transects from 1965 to early 2017. The equatorial Pacific is also subject to El Niño and La Niña phenomena, making the ETNP a physically, chemically, and biologically important system to study. This project’s purpose is to use nutrient data from research cruises dating back to 1965 to examine changes in the vertical extent of the ODZ and in nutrient distributions in the ETNP over both short and long timescales. The most recent addition of data to a long-term study was collected on the R/V Sikuliaq from December 30th, 2016 to January 15th, 2017. Temporal variability between ODZ vertical extent, nutrient distribution patterns, and El Niño/La Niña were analyzed. The ODZ has increased in thickness by 300 m since 1965. Each year, the nitrite maximum appeared to be permanently bound by the oxycline, while varying in latitudinal extent throughout the years. Current datasets lack the statistical power to determine whether or not ENSO state is driving an increase in the ODZ vertical extent or nutrient distributions, though the observations from this study indicate notable inter-annual variability therein.