What does "bulk concentration" mean?

In biogeochemistry/ecosystem models, the concentration of phytoplankton in sea water is often termed "bulk concentration" with units mol P/m^3 (moles of phosphorus per cubic meter, assuming that we know how the stoichiometry of the organic composition of phytoplankton, which is often the case in models). The unit makes sense for "normal" concentrations. What does "bulk" mean in this case?


