PISCES biogeochemical model - NEMO

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Si:C ratio can reach one, a value outside the range of the diagnostic equation proposed by bucchiarelli et al. (2002). Thus, to mimic this extremely high values in the Southern Ocean which are associated with the highest silicate surface concentrations, we propose the following parameterization: ( ) Si opt C = ( ) Si opt C = min(1, ( ) Si opt ( ) max(0, Si − Simin ) (1 + 3 C KSi 2 + Si ( Si C ) opt ) (12) In this equation, Si min is a parameter that should be set to a relatively high value, typically 15 µmol/L, to restrict the increase to the Southern Ocean. ) 3.5 Equation for microzooplankton ∂Z ∂t = e micro (g micro (P ) + g micro (D) + g micro (P OC s ))Z 1 − r micro Z K micro + Z Z (13) The grazing on each species N is defined as follows: g micro (N) = g micro K micro G p micro N N + ∑ I (p micro I I) (14) where p micro N is the preference microzooplankton has for N and I are all the reservoirs microzooplankton can graze on. The grazing rate g micro depends on temperature following exactly the same relationship than phytoplankton. It means that we assume a Q 10 for zooplankton (both micro and mesozooplankton) of about 1.9. We have also adopted this dependancy for respiration/mortality. A special treatment is applied on both types of phytoplankton: • For nanophytoplankton, a minimum threshold is assumed based on observations showing that below a certain small chlorophyll concentration, grazing ceases. This threshold is generally of the order of 0.03 mg Chl l −1 . Thus, for nanophytoplankton, the concentration in the grazing equation (see Eq. 14) is max(P-P min ,0) instead of P. • For diatoms, observations show that the biomass increases by the addition of larger cells which escape grazing by microzooplankton. Thus, we assume in PISCES that above a certain concentration, the diatoms excess is unavailable to microzooplanton. The diatoms concentration in Eq. 14 is then min(D max ,D). 3.6 Equation for mesozooplankton ∂M ∂t = e meso (g meso (P ) + g meso (D) + g meso (Z) + g meso (P OC s ) + g meso (P OC b ))M −r meso M K meso + M M − mmeso M 2 (15) 6
The parametrization for the grazing on multiple ressources slightly differs from the one adopted for microzooplankton (compare with equation 14): g meso (N) = g meso p meso N N KG meso + ∑ (p meso I I) I p meso N = γ N N ∑ (γ I I) I This parameterization implies that mesozooplankton preferentially grazes on the more abundant prey. This formulation stabilizes the model. This is the reason it has been adopted for the last trophic level of PISCES. There is one exception to this parameterization of grazing. Grazing on big particles (POC b ) differs from grazing on the other four types of prey. The reason is that it represents flux feeding rather than “conventional” grazing. Flux feeding does not depend on the concentration of the prey but on its flux: g meso (P OC b ) = gF meso F w P OC P OC b b (17) KP F OC F b + P OC b In this equation, there is a michaelis-menten function to avoid an infinite increase of grazing with particles. Thus, for small concentrations of POC b , flux feeding increases linearly with the flux and then smoothly saturates when concentrations become high. The choice for the parameters in this function is rather arbitrary and difficult. In the equation for mesozooplankton, the term with a square dependancy to mesozooplankton does not depict aggregation but grazing by the higher, non-resolved trophic levels. This term depends on temperature with a Q 10 of 1.9, exactly like grazing and respiration/mortality. 3.7 Equation for DOC (16) ∂DOC ∂t = δ nano µ nano P + δ diat µ diat D + (1 − ɛ micro )r micro Z K micro + Z Z +(1 − ɛ meso )r meso M K meso + M M + (1 − σmicro − e micro ) (1 − γ micro )(g micro (P ) + g micro (D) + g micro (P OC s ))Z +(1 − σ meso − e meso )(1 − γ meso )(g meso (P ) + g meso (D) +g meso (Z) + g meso (P OC s ) + g meso (P OC b ))M + λ ⋆ P OCP OC s −λ ⋆ OCs DOC DOC − ΦDOC→P agg − Φ DOC→P OC b agg (18) where the remineralization rate of DOC is parameterized as follows: λ ⋆ DOC = λ DOCL bac 120m lim0.7(Z + M) min(1, ) z L bac Lim = DOC Lnano lim KDOC bac + DOC In the previous equation, 0.7(Z+M) is a proxy for the bacterial concentration. This relationship has been constructed from a version of PISCES that includes an explicite description of the bacterial biomass. Above 120m, this proxy is kept constant and set to its value at 120m. The terms Φ denote aggragation processes and are described hereafter (see Equation 21). 7 (19)
Page 1 and 2: PISCES biogeochemical model Olivier
Page 3 and 4: Figure 1: Architecture of PISCES. T
Page 5: The half-saturation constant of iro
Page 9 and 10: 3.9 Equation for biogenic silica
Page 11 and 12: where ∆(O 2 ) = min ( 1, 0.4 6
Page 13 and 14: Table 1 - continued from previous p
Page 15 and 16: Equation name Table 2 - continued f

PISCES biogeochemical model - NEMO

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