Suspended particulate nitrogen δ15N versus nitrate utilization: observations in Monterey Bay, CA

Over a four-year period the δ15N values of particulate nitrogen, PN, suspended in surface waters at several sites in Monterey Bay were observed to vary between 1.3 and 7.6‰, a significant portion of the known oceanic δ15NPN range. δ15NPN generally increased as [NO3-] declined, in keeping with the hypothesis that δ15NPN rises with increased NO3- utilization. Previously reported measurements conducted elsewhere in the Pacific and Indian oceans generally match this trend. The distinctly non-linear relationship between δ15NPN and ln[NO3-] is consistent with a closed-system, Rayleigh fractionation model where: NO3- is the reactant, PN is the accumulating end product, initial [NO3-]=30 μM, initial δ15NNO3-=7‰, and biological fractionation, ε=9‰. There is, however, considerable scatter in the data about this trend (r2=0.37), with the uncertainty (±1 standard deviation) of empirically estimating nitrate utilization from δ15NPN of ±4.3 μM. Also, the ε required to fit the model to observed δ15NPN variations produces δ15NNO3- at intermediate and low [NO3-] that is much higher than has thus far been observed in the ocean. One way to rectify this discrepancy would be if ε declined as [NO3-] decreased. Lack of significant linear correlation between δ15NPN and phytoplankton growth rate, cell size, and 1/[NO3-] argues against δ being influenced by limitations imposed by diffusive NO3- transport. However, logarithmic functions of these factors can explain up to 50% of the observed δ15NPN variability, suggesting that active NO3- transport or species-specific effects may be influencing ε and hence δ15NPN in Monterey Bay.