Two‑Layer Vertical Partitioning of Phytoplankton Biomass and Stoichiometry in the North Pacific Subtropical Gyre

Phytoplankton habitat can be separated vertically into two distinct layers in stratified waters. These two layers present distinct ecological niches and nutrient regimes that govern primary production and biogeochemical cycles. Here, we present an approach for estimating in situ phytoplankton biomass and stoichiometry by partitioning vertical profiles of chlorophyll, particulate carbon and particulate nutrients into two phytoplankton communities that dominate these distinct layers, using a conceptual model. The model outputs the surface and subsurface specific phytoplankton carbon-to-chlorophyll (C:Chl), nitrogen-to-chlorophyll (N:Chl) and carbon-to-nitrogen-to-phosphorus (C:N:P) ratios, with associated uncertainties. Applied to Station ALOHA in the North Pacific Subtropical Gyre, the approach yields C:Chl ratios and C:N stoichiometric estimates for the surface consistent with known ranges, while revealing distinct values for the subsurface. Subsurface communities dominated total water column Chl-a stocks of Chl-a (74%) and contributed substantially (31-36%) to phytoplankton carbon, nitrogen and phosphorus depth-integrated stocks, underscoring their important role in sustaining total water-column biomass in this permanently stratified system. Results highlight how phytoplankton-specific stoichiometric estimates can reveal vertical structure that bulk particulate metrics may mask. The approach is intended for use with discrete bottle datasets collected on research ships but has the potential to retrieve additional stoichiometric ratios from particulate profiles of other elements. Advantages and limitations of our approach are discussed in the context of existing methods.