Nitrogen and Oxygen Isotopes of Nitrate as Indicators of Nitrogen Utilization in Monterey Bay

As nitrate is utilized by phytoplankton in the euphotic zone, the isotopic compositions of both oxygen and nitrogen in the residual nitrate become enriched in the heavier isotopes (¹⁵N and ¹⁸O). Recent evidence from phytoplankton culture experiments showed that the enrichments in ¹⁵N and ¹⁸O occur intracellularly during the reduction of nitrate to nitrite. Furthermore, the isotopic composition of the external nitrate pool reflects the balance between intracellular reduction and efflux of the intracellular nitrate. It has also been shown that while ¹⁵N and ¹⁸O fractionation factors (ɛ ) can vary widely depending on growth conditions, there is nonetheless a strong coupling (1:1) of these isotope effects during reduction of nitrate by phytoplankton. In this study, the isotopic composition of nitrate in the upper 200m from three stations in Monterey Bay was measured monthly over an annual cycle. Preliminary results reveal relatively invariable isotopic compositions (δ ¹⁵N = +7 to +8 permil; δ ¹⁸O = 0 to +1 permil) in the deeper portions of the water column, with strong enrichments generally occurring shallower than 60m (δ ¹⁵N up to +14 permil; δ ¹⁸O up to +15 permil). Initial results also suggest some degree of spatial and temporal variability in the relationship between δ ¹⁵N and δ ¹⁸O. While it is unclear exactly what may be controlling this relationship, results from the full suite of monthly samples will provide additional insight on nitrogen utilization and its effects on nitrate isotopic composition. We discuss the results in relation to temperature, salinity, oxygen and nutrient concentrations, primary productivity and phytoplankton species.