Integrated Ecology and Biogeochemistry Models

Biogeochemical models are most useful when integrated into larger biogeochemical cycling models, which requires integration with physical and ecological models. Here, the integration with ecological models is discussed in some detail.

Role of Ecological Models in Biogeochemical Research

Ecological models play an important role in biogeochemical research, as illustrated in Figure 5. Often, the starting point of biogeochemical research projects are field studies where certain spatial or temporal patterns are noticed. Those studies prompt controlled laboratory experiments that often more clearly demonstrate functional relationships and provide kinetic data on processes. Based on those studies, biogeochemical process models are developed. In this sequence of events, the laboratory experiments and process model development were motivated by the desire to understand the field data, and they do provide a qualitative understanding of the field data. However, a more quantitative understanding of the field data is often desired, which can be achieved by integrating the biogeochemical process model into a full ecological model and used it to simulate the field data. This, in turn, will identify further knowledge gaps and the cycle continues. Therefore, ecological models, and their integration with biogeochemical process models, are essential for biogeochemical research.

Integrating Ecological and Biogeochemical Models

The integration of ecological and biogeochemical models can be challenging, because they typically use different

Figure 5 Role of ecological models in biogeochemical research.

Figure 4 Temperature dependence of the endogenous respiration rate of Anabaena circinalis akinetes. Data from Fay P (1988) Viability of akinetes of the planktonic cyanobacterium Anabaena circinalis. Proceedings of the Royal Society of London B 234: 283-301.

modeling approaches. Although ecological models have traditionally used a population-level approach, individual-based approaches are becoming more common. In individual-based models (IBMs) (also called agent-based models, ABMs), the individual members of the population are simulated separately. Each wolf or moose is an independent entity, that moves, eats, reproduces, dies, etc., and a population-level behavior emerges as a result of the action of individuals. This is in contrast to populationlevel models that modify population-level properties, like the total number of wolfs and moose, directly. Chemical and biogeochemical models typically use a populationlevel modeling approach, although there is now a movement of individual-based modeling for microorganisms, like algae and bacteria.

Example 1: Aquatic Environment

An important practical management problem that has led modelers to construct linked ecological-biogeochemical models is cultural eutrophication, the excessive growth of algae due to anthropogenic input of nutrients. The cause of eutrophication is typically increased levels of nutrients (phosphorus, nitrogen), which are chemical quantities - a chemical modeling problem. However, the problem manifests itself by increased number of algae, the dynamics of which are often controlled by zooplankton - an ecological modeling problem. Models that address this problem have traditionally been constructed by extending the chemistry concepts to phytoplankton and zooplankton. That is, the algae are quantified as concentration and their growth is conceptualized as a chemical reaction between them and the nutrients. A typical flow diagram for a lake eutrophication model is presented in Figure 6, which shows the state variables (boxes) and processes (arrows) in the various spatial compartments. The spatial segmentation consists of two layers in the water column and two layers in the sediment bed. In the water column, the surface layer (epilimnion) is separated from the bottom layer (hypolimnion) by the seasonal thermocline. In the sediment bed the aerobic layer is separated from the anaerobic layer by the depth of oxygen penetration. The

Epilimnion

Hypo-limnion

Uptake and excretion

Phyto-plankton

Nutrients

Uptake and excretion

Graz.

Zooplankton

Predation and excretion

Settling

Settling

Nutrients

DOM

POM

Min.

Phyto-plankton

Graz.

Zooplankton

Deposition

Nutrients

Min.

Hydr.

Predation and excretion e S

Deposition

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