The Ecosystem Model Structure

For this project, we will use and modify an ecosystem model originally developed for the central and easten equatorial Pacific. The model is uesd to simulate the Low- Silicate, High-Nitrate, Low-Chlorophyll (LSHNLC) conditions, to provide an estimate of ocean-to-atmosphere CO2 flux from this upwelling-dominated region (see Diagram for the model details). This model has been tested against the JGOFS data over the equatorial pacific and is capable of reproducing the Low-Silicate, High-Nitrate, Low-Chlorophyll (LSHNLC) conditions in the equatorial Pacific. The model consists of ten compartments describing two size classes phytoplankton (P1, P2) and zooplankton (Z1, Z2), detritus nitrogen (DN) and detritus silicate (DSi), two forms of dissolved inorganic nitrogen: nitrate (NO3) and ammonium (NH4), Silicate (Si), and total CO2. Nitrate and ammonium are treated as separate nutrients, thus enabling division of primary production into new preduction and regenerated production. Below the euphotic zone, sinking particular organic matter is converted to inorganic nutrients by a regeneration process similar to the one introduced by Sarmiento et al. (1993), in which organic matter decays to ammonium, and then is nitrified to nitrate. The flux of particular material is specified using an empirical function from Karl et al. (1996). The Silicate regeneration is modeled through a similar approach but with deeper regeneration depth profile.

Two size classes of phytoplankton (P1, P2) and zooplankton (Z1, Z2) are included in the model. P1 represents small, easily grazed phytoplankton whose specific growth varies, but whose biomass is regulated by micrograzers (Z1) and whose daily net productivity is largely remineralized (Chavez et al., 1991; Frost, 1991; Murray et al., 1994; Landry et al., 1996, 1997). P2 represents relatively large phytoplankton (> 10 mm) that make up high biomass blooms and contribute disproportionately to sinking flux as ungrazed production or large fecal pellets (Smetacek, 1985; Wefer, 1989; Peiner et al., 1989; Bidigare and Ondrusek, 1996). The P2 phytoplankton class represents the diatom functional group, and has the potential to grow fast under optimal nutrient conditions (N, Si and Fe>>Ks) (Frost, 1996; Coale et al., 1996). Z1 represents small micrograzers whose specific growth rate are similar to P1 phytoplankton whose grazing rate is density dependent (Landry et al., 1997). Z2 represents larger mesozooplankton that graze on P2 and detritus nitrogen (DN) and prey on Z1. The Z2 zooplankton has a feeding threshold with conventional grazing dynamics as described in Frost (1972) and Frost and Franzen (1992).

A diagram of upper ocean ecosystem model