Nitracline Depth (CalCOFI Cruise)

Nitracline depth (NCD, defined as the depth at which nitrate first reaches 1 µM) is a useful proxy for bottom-up ecosystem forcing in nitrate (NO3) limited ecosystems, and has been shown to be strongly correlated with phytoplankton dynamics in the California Current Ecosystem (CCE). Nitracline depth is calculated from nitrate vertical profiles determed from bottle measurements collected by Niskin on CalCOFI cruises. This data is available as: https://doi.org/10.6073/pasta/7f8e5d24e9b27ae695295a8ddc0809d1. Because nitracline depth saturates under strong upwelling conditions in which surface nitrate often exceeds 1 µmol L-1, we also introduce the concept of nitracline depth extended (NCDx). NCDx is defined as the depth at which nitrate first reaches 1 µmol L-1 if surface nitrate is less than one. However, if surface nitrate is greater than one, NCDx is defined as zshallow - nitrateshallow/mNO3z, where zshallow is the shallowest sampling depth for nitrate (typically <5 m), nitrateshallow is the concentration of nitrate at the shallowest sampling depth and mNO3z is the slope of the ordinary least squares regression of nitrate concentration on depth from all CalCOFI samples collected in the upper 100 m of the water column from Aug, 1959 to May, 2021. mNO3z was determined to be 0.1365 µM m-1.