El caso de las eléctricas

Following the pioneering development of ecosystem models used to trace the flow of material between various components in an ecosystem [Evans & Parslow, 1985; Fasham et al., 1990; Frost, 1987; Steele & Frost, 1977], several marine plankton models with different complexity have been developed. For the Southern Ocean, one-dimensional plankton models like SWAMCO [Lancelot et al., 1991], the KERFIX simulation of Pondaven et al. [1998; 2000], the AESOPS modelling program [Fennel et al., 2003a; b], or the real-time forecasting during the CROZEX experiment [Popova et al., 2007], have been used to investigate ecosystem dynamics in the context of research cruises or station time series. The physical environment in these models has been reduced to the variability of the mixed layer depth.

On the other hand, coupled ocean-ecosystem models with simple parameterizations of biogeochemical fluxes between four and five compartments have been developed for global [Doney et al., 2009; Le Quéré et al., 2005; Moore et al., 2002a; 2002b; Tagliabue et al., 2009a] and regional applications [Chai et al., 2007;

Fennel et al., 2003a; Findlay et al., 2006; Ji et al., 2008; Machu et al., 2005;

Oschlies, 2001; Tagliabue & Arrigo, 2003].

The Oschlies & Garçon [1999] (hereafter OG99) model of the marine ecosystem is one of several which aim to describe the seasonal dynamics of the open-ocean planktonic ecosystem [Aumont et al., 2003; Evans & Parslow, 1985; Fasham et al., 1990; Frost, 1993]. Although originally proposed in [Oschlies & Garçon, 1998], several refinements have subsequently been made, leading to the choice of OG99 as the reference model upon which the research detailed in this thesis is based.

Two versions of the simple nitrogen-based NPZD pelagic model are used in this study. The original version of the model is identical to the one described by OG99, and the optimised model is that described by Schartau & Oschlies [2003a] (hereafter SO03a). It’s differences with respect to the original version are the optimised parameter values, the inclusion of a temperature dependence of all remineralisation rates, a quadratic phytoplankton mortality and a linear loss from phytoplankton back to the dissolved inorganic nitrogen (DIN) pool (i.e. exudation rate).

While the models are complex enough to cover many aspects of the nonlinear ecosystem dynamics, they are simple and efficient to investigate the general principles of biological-physical processes and to predict potential outcomes based on idealized scenarios. These models are built on certain assumptions (see Chapter 1), so they are useful to explore the driving mechanism under specific conditions.

The models track the biogeochemical cycle of nitrogen. Following the nitrogen- based ecosystem model developed by Fasham et al. [1990], nitrogen is distributed among the compartments phytoplankton, zooplankton, nitrogen-containing detritus, and the nutrient nitrate. The focus is on the phytoplankton production and subsequent consumption by zooplankton. There is not explicit modelization of bacterial pools or processes, instead this is simply described by a remineralisation rate that returns nitrogen to the nitrate pool. This approach is appropriate for the focus on seasonal production responses to iron, and particularly so for the Southern Ocean where nitrate is not depleted and thus the details of nitrogen regeneration are not significant. For work in other oceans or with other emphasis, for example nitrogen regeneration [Bopp et al., 2001], or dimethylsulphide (DMS) production by bacteria [Bonner-Knowles et al., 2005], more explicit modelling of the bacterial loop is required [Aumont et al., 2002; Gabric et al., 1993].

The OG99 model has been used in a series of model studies performed in the North Atlantic [Garcon et al., 2001; Oschlies et al., 2000; Oschlies, 2001] and in global applications [Pasquero et al., 2005; Schmittner et al., 2008]. For their simulations they relied on parameter values similar to those published by Sarmiento et al. [1993] and Fasham et al. [1993], which were adopted from published laboratory experiments or approximated from rates derived from in situ measurements.

The SO03a version of the model is the result of an attempt to identify a single set of parameter values that improves the performance of the preliminary NPZD-model version (i.e., OG99) at three different locations where time-series data were available (i.e., BATS site, NABE site, and OWS-INDIA). By assimilating observations, Schartau & Oschlies [2003a; 2003b] provided optimal parameter estimates for the

Although the OG99 model has several failings, some of which have been addressed in subsequent papers [Oschlies, 2002; Oschlies & Schartau, 2005], it has several features which recommend it:

(i) It is a relatively detailed model of the ecosystem, incorporating the most important biotic compartments plus abiotic and detritus compartments;

(ii) It takes account of two of the more significant aspects of the ocean, vertical mixing and solar irradiance (mesoscale features that have been the focus of research in biogeochemical modelling for the last ten years);

(iii) Despite having four compartments and around twenty parameters (many of them have not been evaluated in the field or by experimental work, and some of which cannot reasonably be estimated at all), it is simpler than some of the more detailed ecosystem models [Aumont & Bopp, 2006; Fennel et al., 2003a; Fujii et al., 2005; Lancelot et al., 2000;

Mongin et al., 2006]

In this chapter, the OG99 and the SO03 models are introduced. Each of the model’s four equations and their derivations are detailed. The models are also compared to each other. This comparison is important because the paucity of data and the lack of any mechanistic theories underlying the ecological interactions under study, often makes the choice of model appear more a case of personal preference than objective merit. The chapter is then concluded by a Southern Ocean application (i.e., HNLC-reference simulation) that uses data from the Kerguelen Ocean and Plateau compared Study (KEOPS), and from the French Kerguelen Point Fix Station (KERFIX), both located in the Indian sector of the Southern Ocean.