The overarching goal of this project is to mechanistically connect euphotic zone processes with meso- and bathypelagic zone processes. It is our long term goal to accomplish this by means of a prognostic model that can be used to further our understanding of unparalleled time-series of deep-water sediment traps (21+ years) at the Oceanic Flux Program (OFP), euphotic zone measurements (10+ years) at the Bermuda Atlantic Time-series Site (BATS). In order to realize this goal we will derive a meso/bathypelagic ecosystem structure and use it to model the flux of biogeochemically active constituents (carbon, nitrogen and silica) through the water column. In this initial phase, we present the kernel of the mesopelagic ecosystem in a zero-dimensional, nitrogen only form. The equations and initial parameters are presented along with insight as to how this kernel fits into the over all scheme.
Of considerable scientific interest is the role remineralization plays in the global carbon cycle. It is the ``biological pump'' that fixes carbon in the upper water column and exports it for long time periods to the deep ocean. Yet the biological pump can only respond to changing climate indirectly (Denman et al., 1996) via modification of remineralization processes. From a global carbon cycle point-of-view, it is the processes that govern remineralization in the mid- to deep-ocean waters that provide the feedback to the biogeochemical carbon cycle. And yet, our understanding of these processes is very limited. This modeling study will allow mechanistic, prognostic experimentation of these remineralization processes through formulation of the ecosystem structure. In this manner we will be able to elucidate the importance of ( e.g. bacterial mediated decomposition vs. zooplankton repackaging) processes,for example, on the over all remineralization of organic matter at this site and, by extension, the globe.
A number of deep-sea ecosystem models have been proposed in recent years (Dadou et al., 2001; Boehm and Grant, 2001; Armstrong et al., 2002; Jackson and Burd, 2002) in an one-dimensional context. Armstrong et al. (2002) challenge the Martin et al. (1987) concept that to a large extent remineraliztion is a function of depth only. Dadou et al. (2001) found that remineralization of DON and zooplankton excretion were the dominate processes in remineralization in the Northeastern Tropical Atlantic. Boehm and Grant (2001) found steady state solutions that do not require bacterially mediated remineralization to explain the exponential decay of POC flux in the mesopelagic zone. Jackson and Burd (2002) also produced steady state solutions that predict an exponential decay of organic matter flux with depth, but perturbations of these steady state solutions can create oscillations in the populations and could explain the observed large swings in deep flux. Here we create an ecosystem model that tries to explicitly model all processes believed to be important in the regulation of organic matter flux during the long fall from the euphotic to the mud at the bottom of the sea.
The model under construction is actually a combination of three models. An epipelagic zone ecosystem model (Moore et al., 2002) will be coupled with an advanced physical model of the upper ocean mixing regime (Large et al., 1994) forced by buoyancy flux, wind stress, and surface irradiance. This combination will, in turn, be used to drive the meso/bathypelagic portion of the model. Figure 1 shows a comparison of the large (sinking) detritus from the epipelagic model to the organic nitrogen flux intercepted by the PIT traps deployed at BATS, the agreement is reasonably good. The overall linkage between the epipelagic and mesopelagic models is shown in Fig. 2, through the flux of large detritus on a one-dimensional (to start with) grid. Figure 2 is only meant to be schematic and the vertical resolution is actually 10 m or better from the surface to the bottom.
In Fig. 4 we show a ten-year trace of the five state variables from the mesopelagic ecosystem model (nominal depth 500m). Figure 5 is year ten of a ten year model run repeatedly forced with one year's worth of large detritus flux from Moore et al's (2002) model. While this is only a zero-dimensional simulation, some of the features seen agree qualitatively with data published in Deevey and Brooks (1971) and Conte et al. (2001). In particular we see the same sort of timing in the populations of scavenger and predator with the scavengers preceeding the predators. The large detrital pool follows a similar seasonal cycle as do the OFP sediment traps, including the double peak.
Nevertheless, in order to address some of the more interesting issues brought to light by the analysis of the OFP time-series (increased homogenization of particle composition and the increasing C/N ratios with depth) the model will have to address the stoichiometry of the consumer---food relationship. We will extend the approach of Geider et al. (1998) to the meso/bathypelagic zone by assuming that zooplankton also have cell quotas for nitrogen, carbon, and phosphorus. In this manner, the composition of the falling detrital material will change {\em via} the repackaging of the two zooplankton classes, the scavengers directly and the predators indirectly.
Additionally there are other issues that need to be addressed and these issues point the way for the future work with this model. The following features are planned to be added:
predators migrate vertically (Steinberg et al., 2000);
embed zero-D ecosystem into a one-D vertical structure;
ecosystem still needs nitrification;
sloppy predation COULD be split between Dl and Ds;
bacteria are not explicitly modeled, add as a state variable;
only two size classes for detritus;
particle sinking velocity is constant with depth, needs to be
variable.