Carbon and nitrogen flows during a bloom of the coccolithophore Emiliania huxleyi: Modelling a mesocosm experiment

Abstract

A dynamic model has been developed to represent biogeochemical variables and processes observed during experimental blooms of the coccolithophore Emiliania huxleyi induced in mesocosms over a period of 23 days. The model describes carbon (C), nitrogen (N), and phosphorus (P) cycling through E. huxleyi and the microbial loop, and computes pH and the partial pressure of carbon dioxide (pCO 2 ) from dissolved inorganic carbon (DIC) and total alkalinity (TA). The main innovations are: 1) the representation of E. huxleyi dynamics using an unbalanced growth model in carbon and nitrogen, 2) the gathering of formulations describing typical processes involved in the export of carbon such as primary production, calcification, cellular dissolved organic carbon (DOC) excretion, transparent exopolymer (TEP) formation and viral lyses, and 3) an original and validated representation of the calcification process as a function of the net primary production with a modulation by the intra-cellular N:C ratio mimicking the effect of nutrients limitation on the onset of calcification. It is shown that this new mathematical formulation of calcification provides a better representation of the dynamics of TA, DIC and calcification rates derived from experimental data compared to classicaly used formulations (e.g. function of biomass or of net primary production without any modulation term). In a first step, the model has been applied to the simulations of present pCO 2 conditions. It adequately reproduces the observations for chemical and biological variables and provides an overall view of carbon and nitrogen dynamics. Carbon and nitrogen budgets are derived from the model for the different phases of the bloom, highlighting three distinct phases, reflecting the evolution of the cellular C:N ratio and the interaction between hosts and viruses. During the first phase, inorganic nutrients are massively consumed by E. huxleyi increasing its biomass. Uptakes of carbon and nitrogen are maintained at a constant ratio. The second phase is triggered by the exhaustion of phosphate (PO 4 3− ). Uptake of carbon and nitrogen being uncoupled, the cellular C:N ratio of E. huxleyi increases. This stimulates the active release of DOC, acting as precursors for TEP. The third phase is characterised by an enhancement of the phytoplankton mortality due to viral lysis. A huge amount of DOC has been accumulated in the mesocosm.

A data–model synthesis to explain variability in calcification observed during a CO<sub>2</sub> perturbation mesocosm experiment

“…Due to the design of the PeECE-I experiment, our model includes some additional features. The first is that we consider an explicit representation of dissolved combined carbohydrates (dCCHO) that act as precursors for carbon content of transparent exopolymer particles (TEPC), similar to Schartau et al (2007) and Joassin et al (2011). Since our model resolves changes in TA along with DIC so that we can also derive pH val- Figure 2.…”

“…We resolve neither viral infections nor bacterial biomass explicitly, as done in Joassin et al (2011). Microbial activity is implicitly considered by parameterisations of hydrolysis and remineralisation.…”

“…For this we take a modelling approach to simulate environmental conditions and the predominant dynamics of nine individual mesocosms as described in Engel et al (2005) and in Delille et al (2005). Joassin et al (2011) presented a dynamical model to simulate the mass flux of carbon (C), nitrogen (N), and phosphorus (P) for the same PeECE-I experiment. Their model resolves growth and losses of E. huxleyi together with interdependencies between bacteria, viruses, detritus, and dissolved organic matter (DOM).…”

“…Their model resolves growth and losses of E. huxleyi together with interdependencies between bacteria, viruses, detritus, and dissolved organic matter (DOM). The model of Joassin et al (2011) also features the exudation and coagulation process of dissolved polysaccharides (here referred to as dissolved combined carbohydrates, dCCHO) to form transparent exopolymer particles (TEP). In the study of Joassin et al (2011) some emphasis is put on the enhanced mortality of E. huxleyi due to viral lysis and on the variable stoichiometry (C : N ratio) of the particulate organic matter (POC : PON ratio).…”

“…The model of Joassin et al (2011) also features the exudation and coagulation process of dissolved polysaccharides (here referred to as dissolved combined carbohydrates, dCCHO) to form transparent exopolymer particles (TEP). In the study of Joassin et al (2011) some emphasis is put on the enhanced mortality of E. huxleyi due to viral lysis and on the variable stoichiometry (C : N ratio) of the particulate organic matter (POC : PON ratio). They did not attempt to resolve a dependency between calcification and CO 2 concentration and therefore restricted their simulations to one treatment with three replicate mesocosms that were exposed to present-day CO 2 concentrations.…”