The large number and complicated nature of the processes in a lake ecosystem described in the previous section makes lake ecosystem modeling a very demanding task. However, there are more reasons that even increase the difficulty of building up a lake ecosystem model:
• A lake is a heterogeneous environment consisting of quite different, interconnected habitats (e.g., environmental conditions in the epilimnion, hypolimnion, littoral, and the sediment are significantly different).
• Within each of the trophic levels of the lake ecosystem shown in Figure 1, there are a large number and a high diversity of species that are difficult to describe collectively as well as individually (e.g., there is a huge number of different phytoplankton species which all have the same essential function in Figure 1 but differ considerably in their properties).
• The occurrence of many individual species in a lake is highly irregular (some species occur at a high density in particular years and form only a minor contribution to the biomass in other years even under similar driving conditions).
• The properties of species are difficult to extract from lake data and field and laboratory experiments (easily measurable properties such as size and volume are not strongly related to properties relevant for growth and occurrence).
• The species are adaptable so that they can change their properties to some degree in response to the environmental conditions they encountered in the past (e.g., different size, different light dependence, or different elemental composition). This can make even measured properties unreliable when applied to a situation with different environmental condition history.
• There is a strong interaction between mixing on one side and oxygen, nutrient, and biological population dynamics on the other side. These processes are difficult to describe and hard to separate using lake data.
• The interactions between sediment and water column are very important for the lake ecosystem, especially the nutrient release from the sediment. The processes governing these interactions are difficult to quantify.
• Populations of higher organisms have an age distribution. Different age groups of the same species can play entirely different roles in the food web.
Most of these difficulties listed above occur similarly when modeling other ecosystems, particularly aquatic ecosystems. But the generic difficulties related to heterogeneity, adaptability, species richness, and interactions at various levels are even similar when modeling terrestrial ecosystems.
Different lake ecosystem models differ in the degree of simplification or refinement of the food web and transport processes, in the mathematical formulation used for quantifying process rates, and in the way they cope with the difficulties listed above. In the following section, we give an overview of typical approaches grouped according to different components of lake ecosystem models.
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