- Email: [email protected]
Dimensions and scales
e:>
Pergamon
Waf. Sci. Tech. Vol. 37, No.3, pp. 1-7,1998.
PH: S0273-1223(98)OOO50-X
© 1998 fAWQ. Published by Elsevier Science Ltd Printed in Great Britain. 0273-1223/98 $19'00 + 0'00
DIMENSIONS AND SCALES Lambertus Lijklema Department of Water Quality Management and Aquatic Ecology, Agricultural University, P.O. Box 8080, 6700 DD Wageningen, The Netherlands
ABSTRACT Phenomena in the environment occur on a wide range of spatial and temporal scales. This puts certain demands on the ways we perform research and model systems. Transverse mixing in rivers and internal loading of lakes with phosphates are examples illustrating certain features. Time lags in both ecosystems and in society in combination tend to postpone the solution of environmental problems. Eutrophication serves as an example. © 1998 fAWQ. Published by Elsevier Science Ltd
KEYWORDS Costs; dimensions; environment; internal loading; management; nutrients; research; scales; time lags; transverse mixing. INTRODUCTION Environmental problems, the research needed to understand their underlying cause-effect relationships and the management practises needed to solve the problems, are characterized by a wide variety of interacting scales. This variety of scales is not restricted to the different dimensions within which physical processes Occur and the size of the systems we consider, but is also typical for the various mental and social processes which result in our individual and collective perception of the problems, our organisation and planning of the research and the actual management of the environment. This contribution intends to present a number of observations on this issue collected during an academic career in which eutrophication was a major area. PERCEPTION OF SCALES Our apprehension for length and time develops rather naturally and unconsciously during our childhood, except for the more formal aspects of learning to measure in defined units (metre, second etc). The grasp of the arms and the rhythm of awaking and sleeping, of hunger and satiation, become initially the intrinsic measures of space and time within which life is perceived. Gradually the extent of this spatial and temporal perception expands and we start to learn to measure the distances in our home and to school in feet, yards and nowadays even in the UK and the USA in metres. We also learn to watch the clock and most of us manage reasonably well to arrive at our appointment in time. This requires a good coordination between our sense of distance and the time needed to bridge this interval; in other words our sense for velocity has developed.
2
L.LlJKLEMA
There is also an emotional element in our experiences which may arise less gradually: the sudden realization of our smallness in the universe when watching a starry sky, the sensation of infinity and the awe inspiring feeling of solitude in this universe, the wonder at the mystery behind the material reality and the origin of all things. Also the unfolding and manifestation of the microscopic world at the other end of the scale may evoke a sensitivity for the metaphysical reality. Such sensations, sporadic or recurrent, are a fertile substratum and stimulator for curiosity and for both religious and scientific exploration. They may become an element in the shaping of our morals and attitudes, also with respect to the environment, although this 'internalisation' seems mostly not to be a conscious process. There is reason for concern these days that nurturing the philosophical, emotional and religious perceptions of the natural world is disregarded or neglected in the overwhelming light of the apparent successes of the empirical sciences. This is particularly a pitfall within the natural sciences and their application. The value of the 'soft' angle of view is, true enough, recognised formally but does not really count in the hard world of facts, figures, evidence, profit, national and international competition and growth. In dealing with the environment the ethical aspects, the questions of the meaning of life and our ultimate objectives frequently come last as a mere eyewash, an obligatory but ineffective appendix. World conferences on the environment produce mainly lip-service and greenhouse gases. I am not so naive as to believe that a systematic integration of humanities and natural sciences throughout our education will solve our environmental problems, but it is better to try at least to let fundamental questions play their role rather than to wait until the disease bums itself out. As far as the contribution of universities to upholding and furthering intrinsic values concerns, I see as one of the obstacles the diffuse decision making process and the many actors involved. Each actor has his own scales of priorities, including pets and territorial claims. Objectives and educational programs tend to become blurred by compromises and ineffectual addition sums of disconnected courses. Frequently also the government interferes inappropriately and puts directive and bureaucratic burdens on the academic world while stretching its own mission and competence. In terms of 'scales' I see two observations: a) the metaphysical dimension in our individual and collective experiences and consequently in our behaviour and actions, is deprived when compared with the role of physics and other empirical ways of looking at and understanding matter and life. Partly this is because the modem scientific view of life is taught as the true representation of reality whereas philosophy and religion are taught as elements of culture, which is available in a very wide assortment and therefore expelled from the public to the private sector b) the structure of the university and of society as well are not appropriate to address the environmental problems adequately. After this amateurish excursion towards social and philosophical issues it is time to tum to the more physical aspects of scales and dimensions in environmental sciences; particularly water and water quality related subjects. Two examples illustrate the significance of the scale on which processes and phenomena occur and can be analyzed and interpreted. SCALES IN PHYSICAL MIXING The first example deals with transverse mixing in rivers, with the Amazon as an illustration. Near Manaus is the confluence of the Rio Negro (Black River) and the Solimties, the main tributary of the Amazon. Whereas the waters of the Rio Negro are indeed dark due to the high content of humic material, the Solimties has a light colour caused by fine suspended silt. These natural tracers allow to follow the mixing of these two rivers. From the air it can be seen that this mixing is slow; way downstreams the individual flows can still be recognised. Taking a closer look from a boat it can be seen that at the interface of the two rivers fairly large eddies are generated. These vortexes are caused by the friction between the two flows because there is a difference in flow velocity between the two rivers. This is a universal phenomenon for fluids or gases moving with respect to a boundary. The scale of such eddies and their intensity are related among others to the intensity of the friction (the shear stress) and the dimensions of the system, e.g. depth of the river. The frictional energy is dissipated from the large eddies into smaller ones and finally on the molecular scale into heat. At all scales, from the primary seen from a boat towards the molecular scale (Brownian movement),
Dimensions and scales
3
the eddies contribute to mixing. It is however the interaction between the whole range of sizes which controls the end result. Transversal mixing in rivers is rather slow. A long flowing distance is needed before transversal concentration gradients vanish. This mixing length may be in the order of 100 km for a river like the Rhine. At the border between Germany and The Netherlands there are still small differences in the concentration of substances discharged way upstream along one of the banks. Such phenomena may be relevant in water quality management for the proper location of water intakes or their temporal closing down after an accidental spill upstream.
It can be envisaged that when measuring somewhere in a cross section of the Amazon the colour intensity or another variable associated with the difference in quality of the two contributing rivers, will show a signal going up and down in a cyclic way. Near the middle of the river the amplitude in the variation will decline first while mixing proceeds. Near the banks initially there will be no variation at all as the waters of either tributary have not yet reached the opposite bank. Therefore there will be a pattern with the higher amplitudes moving gradually towards both banks while declining with distance. A more or less exact mathematical description of these phenomena (a model) and the associated mixing and dilution of the silt and the humics, could be generated. However, it is customary to model transverse mixing analogous to Fickian diffusion, albeit with a dispersion coefficient several orders of magnitude greater than the diffusion coefficient. The simulation result is again a gradual spreading in lateral direction but without the cyclic variations discussed above. The concentration profiles obtained are more smoothed and actually an approximation of the time averaged values. For most purposes this is adequate even though the model does highly simplify reality. In fact the mixing is a complicated interplay between dispersion in three dimensions on all scales below the dimensions of the river; superimposed upon a non uniform velocity field over the cross section. Conclusion: lumping of the mixing phenomena acting on a range of scales into one (lateral) dispersion coefficient is adequate to describe lateral mixing. SCALES IN INTERNAL LOADING A second example of the interaction between different temporal and spatial scales is derived from eutrophication: the enrichment of soils or waters with nutrients, originating inter alia from fertilisation of croplands and sewage or effluents from waste water treatment plants. The negative impacts, especially in stagnant waters, include dense algal blooms, disturbance of the structure and functioning of the ecosystem, lower biodiversity etc. In terms of costs there are impacts on recreation, fisheries, water supply, natural values etc. The intensity of the loading of (stagnant) aquatic systems with nutrients is general expressed in grams of phosphorus or nitrogen per square metre of water surface per day or year. The temporal dimension, the variation of this loading in time, is of particular interest. At certain times the input may not be consumed directly by the algae because the conditions are not suitable for growth. such as during winter. Nutrients then may be flushed from the system. However, nutrients especially phosphate, also can be stored in the system in the sediments and recycled to the water during the growing season. The quantity accumulated in the active toplayer of the sediments generally is much higher than the quantity present in the overlying water; e.g. a more than three-hundred fold quantity per square metre in the top 10 cm as compared to a 5 m column of overlying water. This stored quantity in the sediment may be sufficient for what is needed to support algal growth during 10 to 25 years in a fairly productive lake. Such figures make plausible why scientists and managers are so interested in the sediment-water interaction and the so-called internal loading: the release of phosphate or nitrogen from the sediments. Here again scales are important. First there are the scales on which the physical, chemical and microbiological processes occur and which, in their interaction. control the release of dissolved species from the sediments. The spatial scales are from less than a millimetre or even the molecular scale, to a few centimetres. For instance, the depth of the oxygen penetration into the sediments for summer conditions in a eutrophic system may be calculated or measured
4
L.LUKLEMA
as I mm or less. This is smaller than the grain size of larger particles. The customary model concept used in calculations of a homogeneous. continuous sediment phase thus is biassed. Measurement with a micro• electrode may be biassed as well, because only a local concentration is registered and inserting the electrode can create an artefact. Also inhomogeneity may need consideration. This is certainly the case when quantifying and modelling denitrification in unsaturated soils, in which small, actively denitrifying anoxic sites occur in an otherwise oxic environment. The spatial distribution in number. size and activity of such hot spots will control to what extent experiments with a sample of a certain size may be representative for a plot, a field or even a catchment. Moreover this distribution will change with the moisture content of the soil: a higher content will increase the anoxic fraction but in a non-uniform way in space. Thus there is an intricate and complex interaction between the temporal and spatial scales of denitrification as driven by soil characteristics and climatic factors. In saturated sediments inhomogeneity can be enhanced by rooting macrophytes, which affect the environmental conditions among others by their ability to transport oxygen downward during photosynthesis. which is also variable in time. Even without roots the system is complex. In the fixation of phosphates in sediments the redox sensitive iron frequently plays an essential role. Its chemistry of oxidation. hydrolysis, oxolation and phosphate adsorption is complicated by non-stoichiometric ratios, pH sensitivity. hysteresis and aging effects. Time constants cover a wide range: the adsorption of phosphate has been shown by Portielje (1994) to run to the order of a year due to solid phase diffusion. The reduction of iron-phosphate complexes seems to decrease with the P:Fe ratio (de Best et al.• 1995). and thus may tend to become incomplete in phosphate rich sediments. The mobility of both iron and phosphate in the sediment boundary layer are thus interrelated and difficult to model. In a PhD study by Hieltjes (1980). nearly 20 years ago, we found the vertical profiles (figure I) of phosphate: (circles, left hand side) and iron• oxide (triangles. right hand side) in the solid phase. The resolution of I mm was obtained by deep freezing and grinding 2 cores taken at the same location in a lake.
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Figure I. Depth profiles of phosphorus (as P20S) and iron (as Fe:z03) content in two sediment cores. Circles: phosphate. Triangles: iron. Open symbols: anoxic column. Closed symbols: oxic overlying water.
Two observations are: - the difference in the phosphate and iron concentrations. at maybe a few metrEs distance. is considerable;
Dimensions and scales
.5
- the black circles and triangles, which concern columns exposed to oxygenated overlying water, show that the top 10-11 mm is enriched with phosphate due to migration from the anaerobic layer below I em towards the surface and adsorption on the ferric hydroxides. Near the interface, the top 5 mm, a lower concentration occurs due to slow desorption and diffusion into the lake water. The open circles apply to the phosphate content in the solid phase of a column which for the last three weeks has been anoxic. It can be seen that phosphate has concentrated progressively near the interface due to solubilization and diffusion. The concomitant release rate of the dissolved species accordingly was much higher. SCALES AND EXPERIMENTAL APPROACHES Comparing the scales of these processes at the sediment-water interface with the transverse mixing in the Amazon, we see that the relevant spatial dimensions are very small in the sediment and rather large in the river. The reverse applies to the temporal scales: sediment dilution takes years or decades whereas the transverse mixing is complete within about one day. These scales bear upon the opportunities and limitations to study processes under controlled conditions. Experiments with sediment columns in the laboratory, although afflicted with certain artefacts and spatial variability (Lijklema, 1993), still may represent the current conditions within a lake with respect to, for instance: sediment oxygen demand, ammonification, nitrification and denitrification and phosphate release. However, extending the experiment beyond a few days will introduce divergences from the field conditions because certain processes with longer time constants are discontinued: e.g. sediment mixing by macrofauna and sedimentation of fresh detritus. Transverse mixing in a river however essentially cannot be mimicked at the scale of laboratory vessels and, in the case of the Amazon it is not needed either. In ecosystems there is a tight coupling between physical, chemical and biological processes, each with their specific spatial and temporal scales; see for example Figure 2. time scale decade
accumulation In sediments
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Figure 2. Relationship between process rates and spatial scale of effects.
Individual processes or interactions, such as grazing on one specific prey organism by one single predator, are very difficult to copy faithfully in laboratory systems and/or the results cannot be extrapolated to the
6
L.LIJKLEMA
field scale. This means that a true, realistic analysis of water systems generally requires field work. For the case of the mixing process in the Amazon this entails mainly physical measurements of the flow field on different scales; which is complex but attainable. The research can benefit considerably from mathematical modelling. In ecosystem research the number of interacting processes is so large, there are so many state variables and such a range of temporal and spatial scales involved, that in view of the limited analytical capacity, frequently empirical observations fall short to elucidate quantitatively ecosystem behaviour. And for management objectives we need such quantitative descriptions, not only concepts. RESEARCH COSTS This brings us to another dimension, which frequently controls the way we do our research: the costs. Extending the size of our experimental vessels and the duration of the research increases the costs. Mathematical modelling generally is cheap, next comes the work in the laboratory, than mesOCosms and our department had the opportunity to work on the scale of artificial test ditches in cooperation with research institutes of DLO (Ministry of Agriculture). The investments alone for these facilities were over one million guilders. Along with the spatial dimensions also the duration of experiments increases (see also Figure 2) provided that the potential at each experimental scale is exploited properly. However, there is always a pressure for quick results from funding agencies and also from shallow researchers with a lack of patience or perseverance. The costs and time required for full field scale observations, if properly sampled in time and space, can be very high; especially in terms of manpower. Lack of funding within the Universities becomes a serious motive to abandon research on the proper scale. Further the over-emphasized pressure to publish is discouraging the long term, system oriented approaches which do not generate quick results. The mound of papers published in all environmental sciences as another product of this pressure becomes ridiculous anyway; an increasing proportion of the manuscripts is never cited and probably remains unread as well. Ideally a Water Quality Management oriented department of a university should be able to integrate the physical, chemical and biological aspects while using a modeling framework. For instance quantitative research on the physical, chemical and biological conditions and the boundaries which render the habitat suitable for certain communities can be tackled. To often such studies lack expertise in one or more areas. A broad, mutual commitment to a joint long term task in an interdisciplinary team is required. Much of the needed progress in environmental sciences and technology is in this interdisciplinary level. TIME SCALES IN MANAGEMENT In most countries it took many years of excessive nutrient loading of aquatic systems before negative impacts were observed. Firstly this is a consequence of the inertia in the loading of the systems, especially for phosphate. It takes time to saturate the buffer capacity of the sediments. Besides this chemical resilience, there is also a resistance in the biological sub-system. A certain stress can be absorbed before the stability is lost and the ecosystem shifts to a new equilibrium. In eutrophication this is a change from clear water with, in the shallow parts, rooting macrophytes and a high diversity of fish etc. into turbid waters with high concentrations of a limited number of algal species (frequently the objectionable Cyanobacteria), a dominance of bream and a lack of pike and macrophytes. These phenomena became apparent after most of the harm had been done. Initially, as customary with public problems, much time was lost in denying and playing down the gravity of the problem. Further the need to identify the causes, initially through rather haphazard and intuitive approaches, took time and delayed actions by the authorities. Finally it became apparent that nutrient enrichment by sewage and effluents of treatment works and runoff from agriculture were the wrong-doers. In The Netherlands at this point in time the agricultural sector had been stimulated to develop intensive cattle breeding systems with import of fodder and the associated surplusses of nutrients. This is not changed easily; there is understandably a strong social resistance in this sector to reverse the trends. Even in the public sector controlling sewage treatment, nutrient removal could not be introduced without overcoming considerable opposition: against the introduction of (higher) levies due to the expected uselessness unless agriculture would take appropriate measures as well etc. Moreover, the realization that the tardy process of loading of farm lands and of
Dimensions and scales
7
sediments is mirrored in long term leaching and internal loading after reducing external inputs, was not stimulating the political decision making. Also degraded aquatic ecosystems have their resilience in the biological subsystem which delays recovery (Hosper, 1997). The time horizon for politicians frequently coincides with the duration of their mandate which is short compared to ecosystem time constants. In The Netherlands the persistence of the swine plague of 1997, which certainly is related to the high density of pigs held in certain areas, probably will contribute to the determination to take further steps to reduce nutrient loading and to control eutrophication. The foregoing illustrates that time constants in social processes may be at least equally significant in controlling the rates of environmental changes as the time constants of the inherent physical processes associated with the environmental change. Another example is the climate change. The change itself is slow but the social action seems to be even slower if not non-existent. Of course this problem is further complicated by its global scale, whereas eutrophication of (fresh) waters in principle can be tackled on the scale of a catchment. Yet there is also a worldwide dimension to eutrophication. The intensive cattle breeding in a number of developed countries is an attempt to generate a reasonable income at comparatively small scale farms, especially on poor soils. For this goal cheap fodder is bought in developing countries and a huge nutrient flux from these countries towards especially W. Europe has been generated. The consequences are eutrophication in Europe and loss of fertility, erosion and desertification in Africa. It is evident that economic interests on a worldwide scale (GATT), regionally (EU), nationally and even on a local (land use planning etc.) and individual scale (farmers) interact in a complex way and that the management of this complex system is extremely difficult. CONCLUSION A number of conclusions can be drawn. From an academic point of view it seems obvious that within the field of environmental sciences and technology it is essential that: students learn to appreciate the various spatial and temporal scales on which physical and biological processes occur and the interactions between all levels awareness of students for the interrelationship between social processes in environmental management is stimulated, especially for those oriented towards engineering and practise interdisciplinarity is needed for university departments responsible for research and education in environmental issues in order to prevent myopia, ineffective one-sided views and narrowly educated students research should be pursued as much as possible on all scales, including the field staff, individually and collectively, should try to contribute to social awareness of long time changes in environmental degradation and restoration, including their driving forces REFERENCES de Best, J.• Danen Louwerse, H. J.• Portielje, R. and Lijklema, L. (1995). Reduction of artificial iron(lII)phosphate complexes by hydroxylammonnium chloride. Wat. Res., 29(8), 1885-1894. Hie1tjes, A. H. M. (1980). Properties and Behaviour of phosphate in sediments. PhD Thesis (in dutch). Twente University of Technology, The Netherlands. Hosper, S. H. (1997). Clearing Lakes: An ecosystem approach to the restoration and management of shallow lakes in The Netherlands. PhD Thesis. Agricultural University Wageningen The Netherlands. ISBN 90-5485-682-3. Lijklema, L. (1993). Considerations in modeling the sediment water exchange of phosphorus. Hydrobiologia, 253, 219-231. Portielje, R. (1994). Response of shallow aquatic ecosystems to different nutrient loading levels. PhD Thesis. Agricultural University Wageningen, The Netherlands. ISBN 90-5485-275-5.
Pergamon
Waf. Sci. Tech. Vol. 37, No.3, pp. 1-7,1998.
PH: S0273-1223(98)OOO50-X
© 1998 fAWQ. Published by Elsevier Science Ltd Printed in Great Britain. 0273-1223/98 $19'00 + 0'00
DIMENSIONS AND SCALES Lambertus Lijklema Department of Water Quality Management and Aquatic Ecology, Agricultural University, P.O. Box 8080, 6700 DD Wageningen, The Netherlands
ABSTRACT Phenomena in the environment occur on a wide range of spatial and temporal scales. This puts certain demands on the ways we perform research and model systems. Transverse mixing in rivers and internal loading of lakes with phosphates are examples illustrating certain features. Time lags in both ecosystems and in society in combination tend to postpone the solution of environmental problems. Eutrophication serves as an example. © 1998 fAWQ. Published by Elsevier Science Ltd
KEYWORDS Costs; dimensions; environment; internal loading; management; nutrients; research; scales; time lags; transverse mixing. INTRODUCTION Environmental problems, the research needed to understand their underlying cause-effect relationships and the management practises needed to solve the problems, are characterized by a wide variety of interacting scales. This variety of scales is not restricted to the different dimensions within which physical processes Occur and the size of the systems we consider, but is also typical for the various mental and social processes which result in our individual and collective perception of the problems, our organisation and planning of the research and the actual management of the environment. This contribution intends to present a number of observations on this issue collected during an academic career in which eutrophication was a major area. PERCEPTION OF SCALES Our apprehension for length and time develops rather naturally and unconsciously during our childhood, except for the more formal aspects of learning to measure in defined units (metre, second etc). The grasp of the arms and the rhythm of awaking and sleeping, of hunger and satiation, become initially the intrinsic measures of space and time within which life is perceived. Gradually the extent of this spatial and temporal perception expands and we start to learn to measure the distances in our home and to school in feet, yards and nowadays even in the UK and the USA in metres. We also learn to watch the clock and most of us manage reasonably well to arrive at our appointment in time. This requires a good coordination between our sense of distance and the time needed to bridge this interval; in other words our sense for velocity has developed.
2
L.LlJKLEMA
There is also an emotional element in our experiences which may arise less gradually: the sudden realization of our smallness in the universe when watching a starry sky, the sensation of infinity and the awe inspiring feeling of solitude in this universe, the wonder at the mystery behind the material reality and the origin of all things. Also the unfolding and manifestation of the microscopic world at the other end of the scale may evoke a sensitivity for the metaphysical reality. Such sensations, sporadic or recurrent, are a fertile substratum and stimulator for curiosity and for both religious and scientific exploration. They may become an element in the shaping of our morals and attitudes, also with respect to the environment, although this 'internalisation' seems mostly not to be a conscious process. There is reason for concern these days that nurturing the philosophical, emotional and religious perceptions of the natural world is disregarded or neglected in the overwhelming light of the apparent successes of the empirical sciences. This is particularly a pitfall within the natural sciences and their application. The value of the 'soft' angle of view is, true enough, recognised formally but does not really count in the hard world of facts, figures, evidence, profit, national and international competition and growth. In dealing with the environment the ethical aspects, the questions of the meaning of life and our ultimate objectives frequently come last as a mere eyewash, an obligatory but ineffective appendix. World conferences on the environment produce mainly lip-service and greenhouse gases. I am not so naive as to believe that a systematic integration of humanities and natural sciences throughout our education will solve our environmental problems, but it is better to try at least to let fundamental questions play their role rather than to wait until the disease bums itself out. As far as the contribution of universities to upholding and furthering intrinsic values concerns, I see as one of the obstacles the diffuse decision making process and the many actors involved. Each actor has his own scales of priorities, including pets and territorial claims. Objectives and educational programs tend to become blurred by compromises and ineffectual addition sums of disconnected courses. Frequently also the government interferes inappropriately and puts directive and bureaucratic burdens on the academic world while stretching its own mission and competence. In terms of 'scales' I see two observations: a) the metaphysical dimension in our individual and collective experiences and consequently in our behaviour and actions, is deprived when compared with the role of physics and other empirical ways of looking at and understanding matter and life. Partly this is because the modem scientific view of life is taught as the true representation of reality whereas philosophy and religion are taught as elements of culture, which is available in a very wide assortment and therefore expelled from the public to the private sector b) the structure of the university and of society as well are not appropriate to address the environmental problems adequately. After this amateurish excursion towards social and philosophical issues it is time to tum to the more physical aspects of scales and dimensions in environmental sciences; particularly water and water quality related subjects. Two examples illustrate the significance of the scale on which processes and phenomena occur and can be analyzed and interpreted. SCALES IN PHYSICAL MIXING The first example deals with transverse mixing in rivers, with the Amazon as an illustration. Near Manaus is the confluence of the Rio Negro (Black River) and the Solimties, the main tributary of the Amazon. Whereas the waters of the Rio Negro are indeed dark due to the high content of humic material, the Solimties has a light colour caused by fine suspended silt. These natural tracers allow to follow the mixing of these two rivers. From the air it can be seen that this mixing is slow; way downstreams the individual flows can still be recognised. Taking a closer look from a boat it can be seen that at the interface of the two rivers fairly large eddies are generated. These vortexes are caused by the friction between the two flows because there is a difference in flow velocity between the two rivers. This is a universal phenomenon for fluids or gases moving with respect to a boundary. The scale of such eddies and their intensity are related among others to the intensity of the friction (the shear stress) and the dimensions of the system, e.g. depth of the river. The frictional energy is dissipated from the large eddies into smaller ones and finally on the molecular scale into heat. At all scales, from the primary seen from a boat towards the molecular scale (Brownian movement),
Dimensions and scales
3
the eddies contribute to mixing. It is however the interaction between the whole range of sizes which controls the end result. Transversal mixing in rivers is rather slow. A long flowing distance is needed before transversal concentration gradients vanish. This mixing length may be in the order of 100 km for a river like the Rhine. At the border between Germany and The Netherlands there are still small differences in the concentration of substances discharged way upstream along one of the banks. Such phenomena may be relevant in water quality management for the proper location of water intakes or their temporal closing down after an accidental spill upstream.
It can be envisaged that when measuring somewhere in a cross section of the Amazon the colour intensity or another variable associated with the difference in quality of the two contributing rivers, will show a signal going up and down in a cyclic way. Near the middle of the river the amplitude in the variation will decline first while mixing proceeds. Near the banks initially there will be no variation at all as the waters of either tributary have not yet reached the opposite bank. Therefore there will be a pattern with the higher amplitudes moving gradually towards both banks while declining with distance. A more or less exact mathematical description of these phenomena (a model) and the associated mixing and dilution of the silt and the humics, could be generated. However, it is customary to model transverse mixing analogous to Fickian diffusion, albeit with a dispersion coefficient several orders of magnitude greater than the diffusion coefficient. The simulation result is again a gradual spreading in lateral direction but without the cyclic variations discussed above. The concentration profiles obtained are more smoothed and actually an approximation of the time averaged values. For most purposes this is adequate even though the model does highly simplify reality. In fact the mixing is a complicated interplay between dispersion in three dimensions on all scales below the dimensions of the river; superimposed upon a non uniform velocity field over the cross section. Conclusion: lumping of the mixing phenomena acting on a range of scales into one (lateral) dispersion coefficient is adequate to describe lateral mixing. SCALES IN INTERNAL LOADING A second example of the interaction between different temporal and spatial scales is derived from eutrophication: the enrichment of soils or waters with nutrients, originating inter alia from fertilisation of croplands and sewage or effluents from waste water treatment plants. The negative impacts, especially in stagnant waters, include dense algal blooms, disturbance of the structure and functioning of the ecosystem, lower biodiversity etc. In terms of costs there are impacts on recreation, fisheries, water supply, natural values etc. The intensity of the loading of (stagnant) aquatic systems with nutrients is general expressed in grams of phosphorus or nitrogen per square metre of water surface per day or year. The temporal dimension, the variation of this loading in time, is of particular interest. At certain times the input may not be consumed directly by the algae because the conditions are not suitable for growth. such as during winter. Nutrients then may be flushed from the system. However, nutrients especially phosphate, also can be stored in the system in the sediments and recycled to the water during the growing season. The quantity accumulated in the active toplayer of the sediments generally is much higher than the quantity present in the overlying water; e.g. a more than three-hundred fold quantity per square metre in the top 10 cm as compared to a 5 m column of overlying water. This stored quantity in the sediment may be sufficient for what is needed to support algal growth during 10 to 25 years in a fairly productive lake. Such figures make plausible why scientists and managers are so interested in the sediment-water interaction and the so-called internal loading: the release of phosphate or nitrogen from the sediments. Here again scales are important. First there are the scales on which the physical, chemical and microbiological processes occur and which, in their interaction. control the release of dissolved species from the sediments. The spatial scales are from less than a millimetre or even the molecular scale, to a few centimetres. For instance, the depth of the oxygen penetration into the sediments for summer conditions in a eutrophic system may be calculated or measured
4
L.LUKLEMA
as I mm or less. This is smaller than the grain size of larger particles. The customary model concept used in calculations of a homogeneous. continuous sediment phase thus is biassed. Measurement with a micro• electrode may be biassed as well, because only a local concentration is registered and inserting the electrode can create an artefact. Also inhomogeneity may need consideration. This is certainly the case when quantifying and modelling denitrification in unsaturated soils, in which small, actively denitrifying anoxic sites occur in an otherwise oxic environment. The spatial distribution in number. size and activity of such hot spots will control to what extent experiments with a sample of a certain size may be representative for a plot, a field or even a catchment. Moreover this distribution will change with the moisture content of the soil: a higher content will increase the anoxic fraction but in a non-uniform way in space. Thus there is an intricate and complex interaction between the temporal and spatial scales of denitrification as driven by soil characteristics and climatic factors. In saturated sediments inhomogeneity can be enhanced by rooting macrophytes, which affect the environmental conditions among others by their ability to transport oxygen downward during photosynthesis. which is also variable in time. Even without roots the system is complex. In the fixation of phosphates in sediments the redox sensitive iron frequently plays an essential role. Its chemistry of oxidation. hydrolysis, oxolation and phosphate adsorption is complicated by non-stoichiometric ratios, pH sensitivity. hysteresis and aging effects. Time constants cover a wide range: the adsorption of phosphate has been shown by Portielje (1994) to run to the order of a year due to solid phase diffusion. The reduction of iron-phosphate complexes seems to decrease with the P:Fe ratio (de Best et al.• 1995). and thus may tend to become incomplete in phosphate rich sediments. The mobility of both iron and phosphate in the sediment boundary layer are thus interrelated and difficult to model. In a PhD study by Hieltjes (1980). nearly 20 years ago, we found the vertical profiles (figure I) of phosphate: (circles, left hand side) and iron• oxide (triangles. right hand side) in the solid phase. The resolution of I mm was obtained by deep freezing and grinding 2 cores taken at the same location in a lake.
0
A ~
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r-
4
~
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Figure I. Depth profiles of phosphorus (as P20S) and iron (as Fe:z03) content in two sediment cores. Circles: phosphate. Triangles: iron. Open symbols: anoxic column. Closed symbols: oxic overlying water.
Two observations are: - the difference in the phosphate and iron concentrations. at maybe a few metrEs distance. is considerable;
Dimensions and scales
.5
- the black circles and triangles, which concern columns exposed to oxygenated overlying water, show that the top 10-11 mm is enriched with phosphate due to migration from the anaerobic layer below I em towards the surface and adsorption on the ferric hydroxides. Near the interface, the top 5 mm, a lower concentration occurs due to slow desorption and diffusion into the lake water. The open circles apply to the phosphate content in the solid phase of a column which for the last three weeks has been anoxic. It can be seen that phosphate has concentrated progressively near the interface due to solubilization and diffusion. The concomitant release rate of the dissolved species accordingly was much higher. SCALES AND EXPERIMENTAL APPROACHES Comparing the scales of these processes at the sediment-water interface with the transverse mixing in the Amazon, we see that the relevant spatial dimensions are very small in the sediment and rather large in the river. The reverse applies to the temporal scales: sediment dilution takes years or decades whereas the transverse mixing is complete within about one day. These scales bear upon the opportunities and limitations to study processes under controlled conditions. Experiments with sediment columns in the laboratory, although afflicted with certain artefacts and spatial variability (Lijklema, 1993), still may represent the current conditions within a lake with respect to, for instance: sediment oxygen demand, ammonification, nitrification and denitrification and phosphate release. However, extending the experiment beyond a few days will introduce divergences from the field conditions because certain processes with longer time constants are discontinued: e.g. sediment mixing by macrofauna and sedimentation of fresh detritus. Transverse mixing in a river however essentially cannot be mimicked at the scale of laboratory vessels and, in the case of the Amazon it is not needed either. In ecosystems there is a tight coupling between physical, chemical and biological processes, each with their specific spatial and temporal scales; see for example Figure 2. time scale decade
accumulation In sediments
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Figure 2. Relationship between process rates and spatial scale of effects.
Individual processes or interactions, such as grazing on one specific prey organism by one single predator, are very difficult to copy faithfully in laboratory systems and/or the results cannot be extrapolated to the
6
L.LIJKLEMA
field scale. This means that a true, realistic analysis of water systems generally requires field work. For the case of the mixing process in the Amazon this entails mainly physical measurements of the flow field on different scales; which is complex but attainable. The research can benefit considerably from mathematical modelling. In ecosystem research the number of interacting processes is so large, there are so many state variables and such a range of temporal and spatial scales involved, that in view of the limited analytical capacity, frequently empirical observations fall short to elucidate quantitatively ecosystem behaviour. And for management objectives we need such quantitative descriptions, not only concepts. RESEARCH COSTS This brings us to another dimension, which frequently controls the way we do our research: the costs. Extending the size of our experimental vessels and the duration of the research increases the costs. Mathematical modelling generally is cheap, next comes the work in the laboratory, than mesOCosms and our department had the opportunity to work on the scale of artificial test ditches in cooperation with research institutes of DLO (Ministry of Agriculture). The investments alone for these facilities were over one million guilders. Along with the spatial dimensions also the duration of experiments increases (see also Figure 2) provided that the potential at each experimental scale is exploited properly. However, there is always a pressure for quick results from funding agencies and also from shallow researchers with a lack of patience or perseverance. The costs and time required for full field scale observations, if properly sampled in time and space, can be very high; especially in terms of manpower. Lack of funding within the Universities becomes a serious motive to abandon research on the proper scale. Further the over-emphasized pressure to publish is discouraging the long term, system oriented approaches which do not generate quick results. The mound of papers published in all environmental sciences as another product of this pressure becomes ridiculous anyway; an increasing proportion of the manuscripts is never cited and probably remains unread as well. Ideally a Water Quality Management oriented department of a university should be able to integrate the physical, chemical and biological aspects while using a modeling framework. For instance quantitative research on the physical, chemical and biological conditions and the boundaries which render the habitat suitable for certain communities can be tackled. To often such studies lack expertise in one or more areas. A broad, mutual commitment to a joint long term task in an interdisciplinary team is required. Much of the needed progress in environmental sciences and technology is in this interdisciplinary level. TIME SCALES IN MANAGEMENT In most countries it took many years of excessive nutrient loading of aquatic systems before negative impacts were observed. Firstly this is a consequence of the inertia in the loading of the systems, especially for phosphate. It takes time to saturate the buffer capacity of the sediments. Besides this chemical resilience, there is also a resistance in the biological sub-system. A certain stress can be absorbed before the stability is lost and the ecosystem shifts to a new equilibrium. In eutrophication this is a change from clear water with, in the shallow parts, rooting macrophytes and a high diversity of fish etc. into turbid waters with high concentrations of a limited number of algal species (frequently the objectionable Cyanobacteria), a dominance of bream and a lack of pike and macrophytes. These phenomena became apparent after most of the harm had been done. Initially, as customary with public problems, much time was lost in denying and playing down the gravity of the problem. Further the need to identify the causes, initially through rather haphazard and intuitive approaches, took time and delayed actions by the authorities. Finally it became apparent that nutrient enrichment by sewage and effluents of treatment works and runoff from agriculture were the wrong-doers. In The Netherlands at this point in time the agricultural sector had been stimulated to develop intensive cattle breeding systems with import of fodder and the associated surplusses of nutrients. This is not changed easily; there is understandably a strong social resistance in this sector to reverse the trends. Even in the public sector controlling sewage treatment, nutrient removal could not be introduced without overcoming considerable opposition: against the introduction of (higher) levies due to the expected uselessness unless agriculture would take appropriate measures as well etc. Moreover, the realization that the tardy process of loading of farm lands and of
Dimensions and scales
7
sediments is mirrored in long term leaching and internal loading after reducing external inputs, was not stimulating the political decision making. Also degraded aquatic ecosystems have their resilience in the biological subsystem which delays recovery (Hosper, 1997). The time horizon for politicians frequently coincides with the duration of their mandate which is short compared to ecosystem time constants. In The Netherlands the persistence of the swine plague of 1997, which certainly is related to the high density of pigs held in certain areas, probably will contribute to the determination to take further steps to reduce nutrient loading and to control eutrophication. The foregoing illustrates that time constants in social processes may be at least equally significant in controlling the rates of environmental changes as the time constants of the inherent physical processes associated with the environmental change. Another example is the climate change. The change itself is slow but the social action seems to be even slower if not non-existent. Of course this problem is further complicated by its global scale, whereas eutrophication of (fresh) waters in principle can be tackled on the scale of a catchment. Yet there is also a worldwide dimension to eutrophication. The intensive cattle breeding in a number of developed countries is an attempt to generate a reasonable income at comparatively small scale farms, especially on poor soils. For this goal cheap fodder is bought in developing countries and a huge nutrient flux from these countries towards especially W. Europe has been generated. The consequences are eutrophication in Europe and loss of fertility, erosion and desertification in Africa. It is evident that economic interests on a worldwide scale (GATT), regionally (EU), nationally and even on a local (land use planning etc.) and individual scale (farmers) interact in a complex way and that the management of this complex system is extremely difficult. CONCLUSION A number of conclusions can be drawn. From an academic point of view it seems obvious that within the field of environmental sciences and technology it is essential that: students learn to appreciate the various spatial and temporal scales on which physical and biological processes occur and the interactions between all levels awareness of students for the interrelationship between social processes in environmental management is stimulated, especially for those oriented towards engineering and practise interdisciplinarity is needed for university departments responsible for research and education in environmental issues in order to prevent myopia, ineffective one-sided views and narrowly educated students research should be pursued as much as possible on all scales, including the field staff, individually and collectively, should try to contribute to social awareness of long time changes in environmental degradation and restoration, including their driving forces REFERENCES de Best, J.• Danen Louwerse, H. J.• Portielje, R. and Lijklema, L. (1995). Reduction of artificial iron(lII)phosphate complexes by hydroxylammonnium chloride. Wat. Res., 29(8), 1885-1894. Hie1tjes, A. H. M. (1980). Properties and Behaviour of phosphate in sediments. PhD Thesis (in dutch). Twente University of Technology, The Netherlands. Hosper, S. H. (1997). Clearing Lakes: An ecosystem approach to the restoration and management of shallow lakes in The Netherlands. PhD Thesis. Agricultural University Wageningen The Netherlands. ISBN 90-5485-682-3. Lijklema, L. (1993). Considerations in modeling the sediment water exchange of phosphorus. Hydrobiologia, 253, 219-231. Portielje, R. (1994). Response of shallow aquatic ecosystems to different nutrient loading levels. PhD Thesis. Agricultural University Wageningen, The Netherlands. ISBN 90-5485-275-5.