Attempting Consistent Simulations of Stn. ALOHA with a Multi-Element Ecosystem Model

Abstract

We used a one dimensional, multi-element model to simulate the primary production (PP), recycling and export of organic matter at Stn. ALOHA, near Hawaii. We compared versions of the model with and without the cycling of dissolved organic matter (DOM) via the Microbial Food Web (MFW). We incorporated recently published measurements of high C:N ratios for uptake by diazotrophs. For other phytoplankton we included a formulation for overflow production of dissolved organic carbon (DOC), which occurs under nutrient-limited, light-replete conditions. We were able to match the observed mean DOC profile near the surface with both models, by tuning only the fraction of overflow DOC that is labile. The simulated bulk C:N remineralization ratio from the MFW model agreed well with a data-based estimate for the North Pacific subtropical gyre, but that from the Base model was too low. This is because the MFW model includes bacteria, with their low-C:N biomass. Simulated mean PP was lower than observed by 10% (Base) and 27% (MFW). This is consistent with the expectation that the 14C-method measures something greater than net production. DOC accounted for approximately half of simulated PP, most of this being overflow DOC. We find that overflow production and the MFW are key processes for reconciling the various data and PP measurements at this oligotrophic site. The impact of bacteria on the C:N remineralization ratio is an important link between ecosystem structure and the cycling of carbon.