Suspended particulate nitrogen delta N-15 versus nitrate utilization: observations in Monterey Bay, CA

Over a four-year period the delta(15)N 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 parts per thousand a significant portion of the known oceanic delta(15)N(PN) range. delta(15)N(PN) generally increased as [NO3-] declined, in keeping with the hypothesis that delta(15)N(PN) rises w ith increased NO3- utilization. Previously reported measurements conducted elsewhere in the Pacific and Indian oceans generally match this trend. The distinctly non-linear relationship between delta(15)N(PN) and In[NO3-] is c onsistent with a closed-system Rayleigh fractionation model where: NO3- is the reactant, PN is the accumulating end product, initial [NO3-] = 30 mu M, initial delta(15)N(NO3-) = 7 parts per thousand and biological fractionati on, epsilon = 9 parts per thousand. There is, however, considerable scatter in the data about this trend (r(2) = 0.37), with the uncertainty ( +/- 1 s tandard deviation) of empirically estimating nitrate utilization from delta (15)N(PN) of +/- 4.3 mu M. Also, the a required to fit the model to observe d delta(15)N(NP) variations produces delta(15)N(NO3-) at intermediate and l ow [NO3-] that is much higher than has thus far been observed in the ocean. One way to rectify this discrepancy would be if epsilon declined as [NO3-] decreased. Lack of significant linear correlation between delta(15)N(PN) a nd phytoplankton growth rate, cell size, and 1/[NO3-] argues against delta being influenced by limitations imposed by diffusive NO3- transport. Howeve r, logarithmic functions of these factors can explain up to 50% of the obse rved delta(15)N(PN) variability, suggesting that active NO3- transport or s pecies-specific effects may be influencing epsilon and hence delta(15)N(PN) in Monterey Bay. Published by Elsevier Science Ltd.