Simple models of steady deep maxima in chlorophyll and biomass

Possible mechanisms behind the observed deep maxima in chlorophyll and phytoplankton biomass in the open ocean are investigated with simple, one-dimensional ecosystem models. Sinking of organic matter is shown to be critical to the formation of a deep maximum in biomass in these models. However, the form of the sinking material is not of primary importance to the system: in models with sinking of detritus, sinking of one phytoplankton species, and sinking of all phytoplankton, the effect is qualitatively the same. In the two-compartment nutrient-phytoplankton model, the magnitude of the deep biomass maximum depends more strongly on sinking rate and diffusivity than on growth and death rates, while the depth of the maximum is influenced by all four parameters. A model with two phytoplankton groups which exhibit distinct growth rate characteristics and chlorophyll contents shows how a deep chlorophyll maximum could form in the absence of sinking. In this model, when separate compartments are included for nitrate and ammonia, it is possible to distinguish between new and regenerated production, and the phytoplankton group which makes up the deep chlorophyll maximum is found to carry out almost all of the new production. Variation of eddy diffusivity with depth is also investigated, and is found not to fundamentally alter results from models with constant diffusivity.