New production in the equatorial Pacific: a comparison of field data with estimates derived from empirical and theoretical models

Measurements of new production based on uptake of 15N-labeled nitrate in the equatorial Pacific are compared with estimates derived from empirical and theoretical models. Average f-ratios and new production are predicted to within 10–20% by a theoretical steady-state model in which temperature and primary production are the independent variables that determine new production. Examination of the results reveals that the theoretical model gives a very accurate representation of the pattern in new production at primary production rates below ∼60–70 mmol C m−2 d−1 but systematically underestimates new production at higher primary production rates. The discrepancy between measured and predicted new production rates is significantly (p<0.005) correlated with mean euphotic zone nitrate concentrations and drops to zero at nitrate concentrations less than 3 μM. A likely explanation for the bias is the imbalance between primary production and herbivore grazing that occurs in recently upwelled water. This imbalance cannot be taken into account in a steady-state model. At nitrate concentrations less than 3 μM, the new production characteristics of the system closely resemble those predicted by the steady-state model. An empirical model, based on data collected prior to 1979, significantly overestimates new production in the equatorial Pacific at primary production rates above roughly 20 mmol C m−2 d−1. Likely causes of the bias in the empirical model are the need to take temperature effects into account and artifacts in rate measurements made prior to the widespread acceptance of the need to use clean sampling and incubation techniques.