An Ecosystem Model Including C, N and P Cycles Applied to Stn ALOHA: Simulating Primary Production, Organic Matter Stoichiometries and Export

We have applied a one dimensional model to simulate the production, recycling and export of organic matter at Station ALOHA, Hawaii. Our ecosystem model is a modified version of the NEMURO formulation, with modifications including addition of nitrogen fixation, carbon and phosphorous cycles and variable stoichiometries. We also compared versions of the model with and without a formulation for the cycling of dissolved organic matter (DOM) via the Microbial Food Web (MFW). Stoichiometries of all living organisms are constant (although differing significantly by type of organism), but stoichiometries vary for non-living organic matter. Using differential rates for remineralization of C:N:P in DOM and POM, the model can simulate the observed mean stoichiometry of POM in traps at 150 meters depth and some of the observed variations with depth in the stoichiometry of DOM. Differential remineralization allows the ecosystem to recycle nutrients and increase net production, but still only to about 20% of the observed primary productivity (mean of 480 mg C m\textsuperscript{-2} d\textsuperscript{-1}). High ratios of C:N uptake by phytoplankton and especially diazotrophs have been cited as a possible explanation for high productivity in oligotrophic regions. Using high uptake ratios of C:N and allowing diazotrophs to take up ammonium, mean simulated production increased to 640 (standard model) and 470 (MFW model) mg C m\textsuperscript{-2} d\textsuperscript{-1}. If only half of the DOC is counted in primary production, consistent with experimental evidence, the simulated production is roughly 20% lower. Thus the simulated primary production approaches the measurments (but is still lower for the MFW model), while the model simulates reasonably well the mean fluxes of POM and the profiles of DIN, DIP and DOM.