Examining the regeneration of nitrogen by assimilating data from incubations into a multi-element ecosystem model

We examined the flows of nitrogen in two batch incubations of plankton assemblages under controlled conditions, using mesopelagic seawater from different depths. Observations included concentrations of nutrients, organic matter (particulate and dissolved) and plankton (biomass by species). Because nitrogen flows were not observed directly (as is generally the case), we examined them using a multi-element ecosystem model developed to simulate these experiments. Dynamic changes in the observed bulk POC/PON ratio (C/N ratio of particulate organic matter, POM) were consistent with a quota model for phytoplankton. To relate these changes to the regeneration of nitrogen, we included such a quota model and all major planktonic groups observed. By assimilating the data (all observations of carbon and nitrogen concentrations, including living carbon biomasses) into the model using the Monte Carlo Markov Chain, we accurately simulated the changes in bulk POC/PON ratio and its difference between incubations. Changes in phytoplankton composition dominated the changes in bulk POC/PON ratio. Bacteria dominated the regeneration of nitrogen in both incubations, and changes in bacterial regeneration strongly affected the bulk POC/PON ratio. After nitrate depletion, gross nitrogen regeneration varied inversely with observed POC/PON ratio and was constrained by this ratio. Thus, we have demonstrated a powerful new approach to constraining the flows of nutrients through ecosystems. Our results suggest that simultaneous observations of POC and PON could place an additional constraint on the supply of inorganic nitrogen in models of the oligotrophic ocean. © 2006 Elsevier B.V. All rights reserved.