On the use of the empirical relationships of Aumont et al. (2002) and Belviso et al. (2004) in the MIRO-DMS biogeochemical model.
By N. Gypens and S. Belviso

In the last part of the discussion Gypens et al. [1] compared DMS concentration and atmospheric DMS emissions (FDMS) modelled for the Belgian coastal waters by the MIRO-DMS mechanistic model with those obtained from empirical relationships (Anderson et al. [2]; Aumont et al. [3]; Simó and Dachs [4]; Belviso et al. [5]; Lana et al. [6]). Most of these relationships estimate DMS concentration and FDMS as a function of environmental variables (e.g. nutrient concentration, SST, solar radiation, mixed layer depth) and total Chl a (TChl a).

Among tested relationships, those of Aumont et al. [3] and Belviso et al. [5] use TChl a, a community structure index (Fp, corresponding to the proportion of microphytoplankton (mainly diatoms) within the whole phytoplankton community) calculated either independently of TChl a [3] or directly from TChl a [4], diagnostics of the microplanktonic and nanoplanktonic pools of DMSP and a diagnostic of the DMS-to-DMSP ratio itself Fp-dependent. The sea surface DMSP and DMS concentration deduced directly from surface TChl a measurements compare very well with the field observations carried out in April 1998 during an intense Phaeocystis globosa bloom off the Dutch coast [5]. Implemented in the MIRO-DMS model, these relationships however largely overestimate DMS concentrations in the area during Phaeocystis bloom (this work).

The discrepancy does not arise from differences in the magnitude of simulated and observed TChl a levels but in the way both approaches assess the contribution of diatoms (Fp) and non-diatoms (1-Fp) to TChl a. Indeed, in MIRO-DMS, the contribution of diatoms and non-diatoms is not computed as a function of TChl a, but is fully prognostic. There is a much better agreement between DMS simulations and observations when the diatom/non-diatom contribution to TChl a is estimated using the Fp method of Belviso et al. [5] applied to TChl a prognosticated by the model (and computed as a function of Fp*TChl a and (1-Fp)*(TChl a) respectively) (Fig. 1). However, when the model simulates a massive spring bloom of nanophytoplankton (Phaeocystis colonies), the Fp method diagnose a bloom of diatoms because Fp= 0.92 for TChl a concentrations > 3.5 mg.m-3 in [5] and is not in conjunction with the simulated community structure.

As previously pointed by Halloran et al. [7], the use of empirical relationships for future prediction at the regional or global scale need to be done carefully and results interpreted knowing the limits of use of these relations.

FIGURE 1:
http://www.co2.ulg.ac.be/...

Reference
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