A comparison of the variability of biological nutrients against depth and potential density

The main biogeochemical nutrient distributions, along with ambient ocean temperature and the light field, control ocean biological productivity. Observations of nutrients are much sparser than physical observations of temperature and salinity, yet it is critical to validate biogeochemical models against these sparse observations if we are to successfully model biological variability and trends. Here we use data from the Bermuda Atlantic Time-series Study and from the World Ocean Database 2005, to demonstrate quantitatively that over the entire globe a significant fraction of the temporal variability of phosphate, silicate and nitrate within the oceans is correlated with water density. The variability of these nutrients with respect to depth and neutral density is estimated and it is shown that in most regions variability against density is significantly reduced. The largest reductions in variability were found within the main pycnocline. This in principle allows nutrient distributions to be inferred from physical hydrographic measurements, a fact that can usefully be applied to modeling, assimilating, and, in the long term, for biogeochemical forecasting.