Carolyn Buchwald, Marine Chemistry & Geochemistry


Understanding Nitrogen Cycling in Low Oxygen Coastal Waters. Using Natural Abundance Isotopes and Functional Gene Quantitative PCR

Biologically available forms of nitrogen such as nitrate (NO3-), nitrite (NO2-), and ammonia (NH4+), are important nutrients in the ocean. The availability of these nutrients can control the amount of productivity and subsequent carbon export from the surface ocean, which is one of the largest sinks of atmospheric carbon dioxide and has a large impact on climate.

Certain areas of the ocean have high levels of nutrient upwelling, which stimulates high productivity in the surface ocean. The sinking material from the increased productivity stimulates microbial respiration and depletes oxygen in the middle of the water column. The largest and most extreme of such oxygen minima occur in areas of the ocean known as Oxygen Deficient Zones (ODZ’s). ODZ’s are also analogous to coastal regions in which anthropogenic nutrients, from wastewater and agricultural runoff enter the ocean and create large sub-oxic and anoxic “dead zones” where the oxygen is too low for marine life to exist. These low oxygen regions are important places to study oceanic nitrogen cycling because they account for 30% of all the bioavailable nitrogen losses although they are only 0.1% of the area of the ocean. These are also areas of nitrous oxide (N2O) formation, a potent green house gas. Coastal runoff induced oxygen deficiency is likely to expand with increasing human population on coasts. This emphasizes the importance of studying nitrogen cycling in low oxygen regions, highlighting their role as a fixed nitrogen sink, their control on climate, and the need to better assess how they might respond to future anthropogenic changes.

The two main goals of my graduate thesis are: 1) To determine when, where and at what rates nitrogen cycling processes are happening in the water column in oxygen deficient zones by interpreting d15N and d18O isotope profiles of nitrite (NO2-) and nitrate (NO3-); and 2) To make a global ocean fixed nitrogen budget using the d18O of deep-water nitrate.

To address these goals I was able to participate in three research cruises, two to the Eastern Tropical South Pacific (ETSP) and one to the Costa Rica Upwelling Dome (CRD).  The cruises to the ETSP were part of a NSF chemical oceanography grant in which multiple research groups were working together to understand the nitrogen cycling in this major ODZ. I collected samples to measure the natural abundance isotopes (15N and 18O) of nitrite and nitrate, collected samples for DNA and RNA analysis, and set up incubations to measure nitrification rates. Using all these measurements together we are able to determine what organisms are present, where and what rates nitrification was happening in the water column, which will help the overall goal of constructing a nitrogen budget for these areas of the ocean.

I am grateful for the support of the Ocean Institutes Support for Students and Post Doctoral Fellows endowed fund for making this research possible.