The overarching goal of this project is to mechanistically connect euphotic zone processes with mesopelagic zone processes. We plan 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 (25 years) at the Oceanic Flux Program (OFP) and euphotic zone measurements (10+ years) at the Bermuda Atlantic Time-series Site (BATS). In order to realize this goal, we have derived a mesopelagic ecosystem structure. Furthermore, we have coupled this ecosystem structure with an epipelagic ecosystem and now model the flux of biogeochemically active constituents (carbon, nitrogen, phosphorus, silica and iron) from the surface to 4000 m. In this second phase, we present the joined ecosystem models competing over the entire water column in an one-dimensional framework. Schematics and initial results of this modeling effort as well as a discussion of the interplay between epi- and mesopelagic ecosystems are presented.
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.
The model under construction is actually a combination of three models. An epipelagic zone ecosystem model (Moore et al., 2002) has been modified and extended to 4000 m. It is forced with output from an advanced physical model of the upper ocean mixing regime (Large et al., 1994) of buoyancy flux, wind stress, and surface irradiance. This combination is used to drive the mesopelagic portion of the model. The epipelagic portion of the model is shown in detail in Figure 1. The overall linkage between the epipelagic and mesopelagic models is accomplisghed through the flux of large detritus on a one-dimensional (to start with) grid. Aggregation and disaggregation (Boehm and Grant, 1998; Jackson and Burd, 2002) transform detrital particles from one pool to the other and back again. The nutrients at each depth gain from detrital remineralization and zooplankton excretion. The model yields a particulate flux between levels, at the trap levels this flux can be directly compared to the data collected at the OFP traps.
The results of the modeled sinking detritus pools are shown in Figure 2. While this current model is still lacking important deep ocean processes, it can be observed that large detritus is produced mainly in the euphotic zone over the course of a year, and sinks rapidly through the water column while aggregating and disaggregating with small detritus.
Comparisons between data from the OFP-site and modeled detrital flux as (Fig. 4) shows that the modeled organic carbon flux is of the same magnitude as the observed OFP data on organic carbon, however the timing of large flux events is off by 60-80 days at all depths. At the same time the CaCO3 flux in the model is much smaller than the observed flux.
It is apparent from the above model-data comparison that important mesopleagic processes are still missing in this model, although the organic carbon flux is close to the observed carbon flux at all three sampling depths. 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. This mesopelagic portion of the model is still under development and will eventually consist of a predator (active feeding habit zooplankton), a scavenger (passive feeding habit zooplankton), large detritus (sinks), small detritus (non-sinking), and a nutrient pool as shown in Figure 4. With detritus as the primary source of food moving through the water column, it will be fed upon by the predator/scavenger pair and will also undergo bacterially mediated remineralization into nutrients. The large detrital pool at depth will gain material from the formation of fecal pellets from the scavenger and predatory zooplankton.
add two deep zooplankton classes (active feeder,
scavenger).
add feeding preferences for all zooplankton
classes.
bacteria are not explicitely modeld, add as state
variable.
improve CaCO3 chemistry.