Søren Nors Nielsen

Emergy, environ, exergy and ecological modelling

Abstract Hitherto, applied calculations of exergy for higher organisms have been based on traditional thermodynamic considerations, which did not take into account the organizational level of organisms. It seems reasonable to include such perspectives in a thermodynamic evaluation of ecosystems. Therefore, two methods that are theoretically more sound for calculations of exergy for higher organisms are proposed in this paper. The first is based upon the thermodynamic information due to genes. The method is rooted in statistical thermodynamics and should be considered the best candidate for exergy calculations of ecosystems including higher organisms. The second method is a parallel to the method used for calculation of emergy, and is based on the cost of free energy computed from an ecological network. Because this method does not consider the increase of information due to evolution, it should be considered theoretically less sound than the first mentioned method. It is, however, interesting to compare the two methods, as they to a certain extent reflect the differences between emergy and exergy. Emergy attempts, as exergy, to account for the quality of energy by the use of a transformity factor. The transformity factors for calculation of emergy are found from the network as the number of solar equivalents that it has cost to construct the considered organism. Emergy is therefore often more easy to compute, provided that the ecological network is known, while exergy after the new methods for calculations presented here seems to have a better theoretical basis. The two methods tested for calculation of exergy give different results, but the results are in the same order of magnitude. The two major problems in development of ecological models are the parameter estimation and the selection of the best model structure. The latter requires that more ecological system properties are incorporated in our models. A procedure based upon recent developments in ecosystem theory is proposed to meet this requirement. It should be considered a first approach to a theoretical improvement of the modelling procedure for development of models with more ecological properties.

Impact of eutrophication and river management within a framework of ecosystem theories

Eutrophication became a dominant process in the Mondego estuarine system in the 1980s, presumably as a result of excessive nutrient release into coastal waters. The main symptoms were the occurrence of seasonal blooms of Enteromorpha spp., green macroalgae, and a drastic reduction of the Zostera noltii meadows. Previous results suggest that this process will determine changes in species composition at other trophic levels. This paper aims at integrating the available information to provide a theoretical interpretation of the recent physicochemical and biological changes in the Mondego estuarine ecosystem, which will be further used as basic framework for the development of a structurally dynamic model. Exergy-based indices, the Exergy Index and Specific Exergy, were applied as ecological indicators (orientors) to describe the state of the ecosystem, taking into account different scenarios along a spatial gradient of eutrophication symptoms. This allowed elucidating the present conditions along the spatial gradient as representing various stages in the temporal evolution of the system, within the framework of bifurcation, Chaos, and Catastrophe theories. Eutrophication appeared as the major driving force behind the gradual shift in primary producers from a community dominated by rooted macrophytes (Z. noltii) to a community dominated by green macroalgae. Through time, concomitant changes at other trophic levels will most probably give origin to a new trophic structure. Moreover, river management emerged as a key question in establishing scenarios in order to determine secondary effects in eutrophied systems. Results suggest that a more conservative river management may be used as a powerful tool to remedy affected areas, including the implementation of ecological engineering principles in different possible management practices. The recent biological changes in the Mondego estuarine ecosystem were found to comply with the framework of the theories considered, while both Exergy-based indices were able to capture the state of the system and distinguish between different scenarios.

Analysis of the properties of exergy and biodiversity along an estuarine gradient of eutrophication

Benthic eutrophication often gives origin to qualitative changes in marine and estuarine ecosystems, for example the shift in primary producers, often followed by changes in species composition and trophic structure at other levels. Through time such modifications may determine a selected new trophic structure. The development of structural dynamic models will allow to simulate such changes, using goal functions to guide ecosystem behaviour and development. The selection of other species and other food web may then be accounted by a continuous optimisation of model parameters according to an ecological goal function. Exergy has been applied in structural dynamic models of shallow lakes, and appears to be one of the most promising approaches. Theoretically, exergy is assumed to become optimised during ecosystems development, and ecosystems are supposed to self organise towards a state of an optimal configuration of this property. Exergy may then constitute not only a suitable system-oriented characteristic to express natural tendencies of ecosystems evolution, but also a good ecological indicator of ecosystems health. Biodiversity is also an important characteristic of ecosystems structure, constituting a powerful and traditional concept, which was found to be suitable to test the intrinsic ecological significance of exergy. We examined the properties of exergy (exergy and specific exergy) and biodiversity (species richness and heterogeneity) along an estuarine gradient of eutrophication, testing the hypothesis that they would follow the same trends in space and time. This hypothesis was validated in a certain extent, with exergy, specific exergy and species richness decreasing as a function of increasing eutrophication, but heterogeneity responding differently. Biodiversity measurements and their interpretation appeared subjective. Exergy and specific exergy may be a suitable alternative, that could be used as goal functions in ecological models and as holistic ecological indicators of ecosystems integrity. Nevertheless, since exergy and specific exergy showed to respond differently to ecosystems seasonal dynamics, it is advisable to use both complementary. The method proposed by Jorgensen et al. (1995) to estimate exergy, which takes into account the biomass of organisms and the thermodynamic information due to genes, appeared to be operational. There is nevertheless an obvious need for the determination of more accurate (discrete) weighing factors to estimate exergy from organisms biomass. We propose to explore the assumption that the dimension of active genomes, which are primarily a function of the required genetic information to build up an organism, are proportional to the relative contents of DNA in different organisms.

Description of the three shallow estuaries: Mondego River (Portugal), Roskilde Fjord (Denmark) and the Lagoon of Venice (Italy)

The paper describes three European estuaries which were compared with respect to the dynamics between autotrophic components under the MUST-project: The Mondego River (M) (Portugal), Roskilde Fjord (R) (Denmark) and Venice Lagoon (V) (Italy). The areas of the three estuaries are (M) 3.4, (R) 125 and (V) 540 km2 and their maximum tidal ranges are (M) 3.3, (R) 0.2 and (V) 2.2 m. They are all eutrophic with high loadings of nitrogen (M) 126 t N/yr, (R) 2500 t N/yr and (V) 7000 t N/yr and high loadings of phosphorus (M) 1 t P/yr, (R) 180 t P/yr and (V) 1000 t P/yr. The dominating phytoplankton species are (M) diatoms and dinoflagellates, (R) Skeletonema sp. and (V) Amphora sp. and Chaetocerus sp. and the dominating macrophytes are (M) Enteromorpha sp., Gracilaria sp. and Zostera noltii, (R) Zostera marina and Ulva lactuca and (V) Ulva rigida and Zostera noltii. All three estuaries are frequently exposed to collapses caused by severe oxygen depletion.

Ecosystem as self-organizing critical systems

Abstract The paper examines the following properties which are often associated with self-organizing critical systems: (1) Is the relationship between body size and abundance for species in ecosystem a power law? (2) Will the frequency with which observed changes exceeds a given change versus the change follow a power law? (3) Will the typical frequencies of ‘avalanches’ follow a power law? (4) Can the occurrences of ‘avalanches’ according to a well-examined ecosystem model be used to explain the underlying causality? As the examinations give the clear answer ‘yes’ to all four questions, and as many other properties of ecosystems also point towards ecosystems as self-organising critical systems, it seems possible to support the hypothesis, that ecosystems are self-organizing systems.

On the consistency between thermodynamical and network approaches to ecosystems

Abstract If one assumes that biological systems develop in the direction of increasing thermodynamic efficiency, one is led to several conclusions about how interactions between parts of the system, as well as the system as a whole, should evolve. For example, one can show that maximizing efficiency will imply that inputs from the exterior should be maximized, internal transfers should be emphasized above negative exogenous links, the costs of structure maintenance should be minimized, and the heaviest imports should be those most easily accessible. In the network approach to ecosystem development, systems are assumed to change over time so as to increase the information inherent in their patterns of flow connections. This has been expressed quantitatively as a rise in the ‘ascendency’ of the flow pattern. That these two different views on ecosystem behaviour are essentially consistent is demonstrated by calculating the sensitivities of the ascendency index to individual types of flows. Those changes in flow patterns that augment the ascendency also contribute to increased thermodynamic efficiency.

Strategies for structural-dynamic modelling

Abstract A new generation of models called structural-dynamic models are needed to describe changes in populations and trophic structures of ecosystems. Some attempts to construct such models have appeared in the literature during the last few years. The conclusions from one of those attempts are summarized and the main conclusions are presented in this paper. The attempts found in the literature reflect four strategies: experimentalistic, empiristic, trophic and holistic. Each strategy is based on its origin in experimental working traditions and paradigms. Differences between these approaches are discussed together with directions for future application of the strategies. A working strategy based on a view of systems in terms of hierarchy together with the implementation of a goal function, such as exergy, is proposed.

Modelling structural dynamical changes in a Danish shallow lake

Abstract Biomanipulation and diversion of nutrients can affect not only the eutrophication level, but also the trophic structure and species composition of the phytoplankton society in lakes. Existing models often fail to describe the reactions of the ecosystem when such qualitative changes are involved. For this purpose structural dynamic models are needed. This paper presents the first attempt to model the qualitative changes that have been observed to occur in a Danish shallow lake as a result of biomanipulation. A structural dynamic model was developed that included nine species or types of algae normally observed in Danish shallow lakes. The high flow ratio of the lake was assumed to justify that summer conditions would represent a steady state of the system. The parameters of the model were calibrated until the model gave adequate results. The available mathematical tools normally used for calibration were found to give unsatisfactory results. The calibration therefore had to be carried out by running the model repetitively, adjusting parameters until it was able to perform according to the changes actually observed in the lake chosen for this case study. The use of parameters from literature to characterize the different types of algae is shown to have a great impact on this phase of model development. The calibration was found to be very time consuming, mainly due to uncertainties connected to the input parameters. The importance and uncertainty of the parameters chosen indicate that we may probably have to choose new strategies for similar modelling approaches in the future, such as the application of goal functions. The implementations would allow parameters to vary during simulations in order to simulate ecosystem properties, such as adaptational processes and buffering capacities which are features possessed by natural ecosystems.

Optimization of exergy in a structural dynamic model

Abstract The hypothesis that ecosystems are optimizing exergy and the effect of applying exergy as optimizing function in modelling are tested in a structural dynamical model. The model used was developed to describe the development of the phytoplankton community in a Danish shallow lake as a result of biomanipulation. Exergy can be seen as the free biogeochemical energy of a system compared to its surroundings. The function has previously been proposed as a goal function for ecosystem development. In other words, exergy is thought to act as a quality indicator and to direct and govern the development of parameters in the ecosystem modelled. Traditional mathematical unconstrained optimization was found not to be adequate in the case described, since unrealistic values were reached during the optimization runs. Constraints on parameters were introduced by transforming them into a hyperbolic cosine function. The effects of changing optimization intervals, i.e. the frequency of optimization during the simulation, and the possible change per interval, i.e. the rate of change, were analysed. The first was to mimic a response time of the system illustrating either memory or resilience of the system. The second was thought to reflect the capability of variation of the genetic pools in the system. Both optimization interval and the allowed change per interval affected the rate with which the model evolved with time. Decreasing intervals and increase of rate speeds up the system. The sequence in which the parameters were manipulated was in accordance with observed dominance in natural ecosystems. The parameter most often changed by the optimizer was the maximum growth rate of the algae.

Models of the structural dynamics in lakes and reservoirs

Abstract This paper presents a possible approach to the construction of ecological models with the ability to predict shifts in species composition. Only little experience has been gained on the applicability of the approach and much more experience is needed before more definite conclusions can be made although the experience obtained up to now looks promising. Two case studies of lakes and reservoirs on the application of the approach are presented to illustrate the results and the shortcomings of the approach in its present form.