There was a large increase in measured primary production rates between the HOT
data (1991 on) compared to results from the 1980s and before, that were less than half
the later results. Some or all of this (there is no way to be certain) was due a change to
very clean carbon-isotope-uptake techniques.
Limiting Nutrients
(^) In recent decades there has been consistent interest in determining which nutrients
limit phytoplankton production in subtropical gyres, and no study has convinced all of
those interested. There are strict “Liebigians” and advocates of limitation by multiple
nutrients. Liebig, an early agricultural chemist, suggested that the one nutrient
compound in least supply relative to plant requirements would be the factor limiting
plant growth. Complexity is added because different phytoplankton have different
requirements, different affinities for any or all nutrients, and are subject to different
pressures from grazing, all of which confuse the results of experiments with
phytoplankton from the field. Moreover, different gyres have different ratios of major