Environmental Engineering Reference
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the lake, i.e. the lowering of the nutrient load through the establishment of sewage
treatment, the species composition considerably changed. This also influenced the
characteristics of the growth dynamics and the nutrient budgets, and thus the overall
ecology (Scheffer et al. 1993). The lake responded differently to a comparable
impact when the biocoenosis consisted of different organisms.
19.3.2 Structural Dynamic Modelling
A further approach where the lake model helped to advance ecological theory
was structural dynamic modelling . Only a short time after measures had been
undertaken,LakeGlumsøshowedthefirstsignsofashiftinthespecies
composition of the phytoplankton and zooplankton community. In the first
period this appeared not to be important to the quantitative predictions of the
model and the validation of the prognosis was performed in an acceptable manner
(Jørgensen et al. 1986). This underlines the achievement of the approach as it was
the first lake model where it was possible to validate a prognosis. However, the
subsequent shift was not captured well by the initial model. This refers to the fact
that conventional differential equations have a fixed model structure which
describe only the quantitative change of particular variables. What if the number
of variables and/or the structure of the equations changes during the simulated
time interval? This was investigated in context of structural dynamic modelling.
In fact, this approach was useful, since the dominant species had changed during
the lake restoration that reduced the nutrient load combating eutrophication.
The overall approach of structural dynamic modelling was described in (Jørgen-
sen 1986, 1992). Specific results were also presented in Jørgensen (1995) and
Nielsen (1995).
19.3.3 Goal Functions
As an important issue in ecological theory, it was discussed whether certain
characteristic features develop towards specific (optimal?) values on the level
of the entire ecosystem. Are there coherent trends resulting from the interaction
of the components, in form of a specific ecosystem state, that was optimized
during succession? At a very early state the exergy function derived from a
thermodynamic analysis of the ecosystem (Jørgensen and Mejer 1979, 1981,
1983; Mejer and Jørgensen 1979) was considered as a possible candidate to
govern the evolution of ecosystems with time. The function was found to be
closely correlated to the buffer capacity of the system (Jørgensen 1982) and it
was tested as a goal function for Lake Glumsø by Jørgensen (1986) and later
Salomonsen and Jensen (1996). Likewise, its importance was analysed for other
similar Danish lakes (Jørgensen 1995; Nielsen 1995, 1997) where the role in
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