COMMENTS ON RIVERS AND MODELS
R. Margalef
Dept. Ecology, Fac. Biology, Univ. Barcelona. Diagonal, 635, 08028 Barcelona.
Introduction know that the amount of organic carbon carried from the continents to the sea through the rivers. amounts to The epicontinental aquatic ecosystems have been and the one por cent of the net primary production of the are traditionally the object of study of Limnology. This continental part of the biosphere. Rivers themselves are science has been inordinately fond of lakes. seen as mi- in a large fraction heterotrophic. This <(vocationnfor he- niature oceans, able to provide much stimulation com- terotrophy has been abused by mankind that has always bined with a relatively easy access and conditions of found it expeditive to drop organic residues from itself work. They were supposed to provide at a small scale, and from its civilization into the water. appropriate for study, the same basic phenomena asso- As the influence and transport works one way, the ciated with aquatic life generally. Flowing waters have oceans are heterotrophic to a degree that has surely attracted less attention that they deserve, as their study changed through geologic times. As the carbon com- posed serious problems, conceptual and otherwise, but pounds of terrestrial organisms -plants- are often no- now it is time to reverse this tradition, and think of the tably recalcitrant to decomposition as a result of the bio- epicontinental waters mainly as a transport and accu- chemical evolution for resistence of their system of mulation system, functioning as a bridge or link bet- transport and support (wood), one could suspect that a ween continental and oceanic ecosystems. From the considerable fraction, above that expected, of the orga- point of view of the practical problems of everyday life, nic matter dissolved in the ocean water may ultimately water supply and water quality, flowing waters are wit- be of terrestrial or continental origin. In fact, in fresh- hout doubt the most important. Think only of cultural water. macrophytes (with the associated action of water distribution systems, and of sewers, two subjects terrestrial fungi) are relatively more important than al- of study that biologically have been almost neglected. gae, as suppliers of the dissolved dark organic matter Limnologists have been biaised against rivers and in that characterizes several bodies of continental water favour of lakes and one deep reason for this, was the (black water rivers, etc.). conceptual difficulties associated with the modelling of The epicontinental aquatic system should be studied rivers, and even of how to conceive rivers in the frame in all its extension. From the drop of rainwater, that of traditional ecology. The problems concern the physi- stays or glides over the surface of a leaf, to the wet- cal nature of rivers and other flowing waters connected lands joining the sea. Now there are supplementary mo- to them, and ways to possible quantitative descriptions, tives of interest that originate in recent human action; the regime of the flow of waters through the whole for instance. the changes in the composition of rain-wa- system, and a reasonable attempt at modelling. ter as it washes an altered atmosphere and comes out with more acid ions, or with varied neutral substances of very diverse effects compared with former times. An epicontinental transport system Rain not only clears air of noxious substances, but also takes out pheromones and other materials with poten- The epicontinental (freshwater or limnic) part of the tial biological significance. On the surface of leaves, biosphere is not only a section of the global water cycle, rain-water provides a substratum for bacterial growth, but a most important part of it. The disproportion bet- disperses diaspores of fungi and other organisms and ween the surfaces covered by oceans and by land, and supports the development of small animals, including the easier evaporation over water, means that land re- species of ciliates that are very typical of such situations ceives a relative excess of rain, so that annually 30000 (Colpodu). In adaptation to the short period of presen- km" must flow through the rivers from the continents ce of liquid water, after excystation and feeding for a to the sea. This water carries a load of very diverse ma- few hours on bacteria, they encyst again and experien- terials, in part as a result of chemical and physical ero- ce cell division inside the cysts in preparation of the next sion, in part as a result of the activity of organisms. We rain. In areas in which water remains more time, a hig- her number of species find access and a more diverse fauna develops, as in the sheats of leaves of grass, in the small pools on the leaves of Dipsacus and on the tro- Limnetlca, 7: 185-190 (1991) pical Bromeliaceue. Part of the same system is the wa- 0Asoctacion Espariola de Llmnologia, Madrid, Spain ter flowing over the trunks of the trees, which at pre- sent attracts a considerable amount of attention, also elsewhere. In Europe most of them had dissapeared a because it is enriched in the leaves by particular ions long time ago. From the point of view of ecology this -specially potassium-, which are then transported is regrettable, and it also means the disappearance of down. aquatic ecosystems characterized by supply of nutrients Runoff is an important part of the cycle. In deep, ma- in the surface, encouraging development of surface ture soils, runoff water tends to be poor in ions and with communities -neuston, pleuston-, as well as the dis- a rather uniform composition over large areas -even- mise of an important mechanism of interplay between tually a characteristic of climax-. There are other re- water ecosystems and terrestrial ecosystems, linked by gularities in runoff, related to successional or otherwise processes of succession that maintain, from the point of time-ordered microbial activities in soils, being shown. view of the evolution, the interaction between the river for instance, in the initial peak of nitrogen enrichment and the forest. of water. In areas or situations of less regular rain, poor Through the construction of dams, man slows down vegetal cover. or with heterogeneous soils or geological tluxes and, in a certain way, the result of this operation substrata, runoff is less uniform over large surfaces and. may be considered as opposed to the pumping out of in general. comes out more mineralized. This is impor- water from aquifers. One question to be asked is to tant and would justify closer examination, because it is what point the energy obtained through dams (hydroe- one of the main effects of man on water quality. On hu- lectric power) compensates the energy used in pumping manized landscape, runoff and flowing water are, on waters from the aquifers. average, more concentrated in all sorts of dissolved -and suspended- materials, and locally more hetero- geneous. Particular effects, coming from concrete, asp- Morphology of drainage systems halt, or salt used in fighting ice accumulation on the roads, contribute to the same properties in ways that The evacuation system develops in association with can be easily traced. Considering other anthropic ef- the morphology of the earth's surface and in relation fects, perhaps not so generally negative, that might be with the erosional activity of the water courses. Some important in the future, we might remember the possi- regularities in the resulting structures are expected and ble injection into the ground of solutions contributed are indeed common, like the distribution of altitudes through desalinized seawater. In the same chapter we along the water courses as a function of erosion and must consider the dangers of water transfers between transport and in relation with the materials of the sur- different watersheds and among them the effects of di- face drained. The old idea about the equilibrium profi- lution, washing, or, on the contrary, salinization. as a le of a river is an example of the (dynamic) regularities result of changing the correspondence between water involved and is still worthy of further study. In the up- amount and quality, quality of the soils and response of per stretches of the river, the fluvial system is made by vegetation. a succession of convergences of separate segments; be- Underground waters are important reservoirs and low, on the aluvial plain, the work of the river is ex- parts of a circulation system, but the total amount of pressed in the formation of meanders. In this active pro- ground and deep water is not well known and informa- cess the river erodes along one shore, intercepting tion about their persistence and flow is scarce. Piezo- terrestrial communities in different stages of ecological metric readings, inferences based on chemical analysis, succession, and, on the opposite shore the river opens including isotopes, and general geophysical surveys an area to terrestrial invasion and colonization, and provide only partial information. It is an extremely im- over this area successive stages develop in the form of portant subject, in relation with the increasing intensity bands parallel to the receding shore. The contrast bet- with which such reserves of water are tapped. The con- ween cutting a banded pattern of communities at a right sideration of such reserves and any forecast about them angle, and allowing the addition of a paralel new area has to take into account the circumstances of a world to the preceding complex is a paradigm of succession, in which human action is increasing. Man pumps out and can be appropriately described by an asymmetrical deep water and accelerates its flow towards the oceans; matrix. this acceleration often means replacement of part of the Close to the sea, the interaction between dynamics of water by other water of dubious quality, that can have the river and tidal dynamics may generate wetlands with a considerable charge of pollutants derived from human very complicated drainage systems. The trend observed action. The acceleration is paid for by the energy inves- in the upper reaches of the river appears reversed, now ted in pumping water -and in planning and construc- the river tends to be divided in different outlets. Other- ting such exosomatic expansions of our civilization-. wise, the main stream becomes a large estuary. All this The action of mankind on rivers is quite complex. Ri- is strongly dependent on the geological characteristics vers become simplified, converted into regular channels of the coast and on the local tidal regime. and flow is made more direct and uniform; this is The structure, profile and expansion through conver- accompanied by a loss of biotic and biospheric diver- gence, meandering and divergence of the main draining sity. Think only of the destruction of most of the very veins shows many regularities, appropriate for quanti- diverse water bodies in the alluvial plains that were as- tative or numerical expression, a number of laws or re- sociated with meandering courses of water, in America gularities have been suggested, in relation with conver- (madrejones, pantanales), Australia (billaboongs) and gence of affluents, morphological regularities of mean- ders that accept a simple geometrical explanation, and zation. and the response to unpredictable disturbances also development and characteristics of deltas. It is easy that, more often than not, destroy a proportion of the to understand that relations between flows, develop- organization achieved. The expected or more general ment of water courses and drained surfaces often allow result is that the system, as a result of the disturbance. to allometric or fractal expressions. Today these rela- is changed or reconstructed acquiring some characteris- tions provide a testing ground for the now fashionable tics that approaches it to some former state that was al- approaches. Replication of definite trends and n~otives ready passed through and that was considered as a at different scales lend themselves to :I comparison with younger stage in ecological succession. This is only an fractals. This area of study is open to fascinating deve- example of very general rules of change that also inclu- lopments, and from the beginning we kice problems that de growth and death, and, in ecological terms, it is ma- go very deep and that might bc worth following. This nifested in inevitable succession and in the repeated re- all adds to the attractiveness of the subject. setting or reinitializing the process at a given stage. As As an example I refer only to the difficulties in com- a very important corollary, once ecological succession bining two complementary, hut independent, fractal ap- is accepted it cannot be accepted that ecosystems exist proaches to the description of the landscape. Television without being coinvolved in successional processes (It has made us familiar with artificial fractal I~indscapes. is like in evolution, where, once accepted, no biologist Their construction is now of no direct concern: In ge- can imagine that a string of generations can exist wit- neral we start with certain hypsographic curve (distri- hout evolution, external or internal). bution of surfaces according to their altitude) and some Ecology is full of appropriate examples, but if we randomness is injected in the distribution of the altitu- were in need of more, we could be served with largesse de of the different points, with some restrictions impo- considering the distribution of flow in the rivers: here sed for neighborhood, etc. the disturbances are associated with the distribution of Assume that rain falls on such a landscape and also rainfall in the respective hydrographic basins. Much that each drop of rain flows down through the steepest worry and ingenuity has gone into the examination of slope locally available and in doing so produces a cer- temporal distribution of rainfall and river flow, over dif- tain amount of erosion, taking away some solid mate- ferent scales of time. Randomness is tantalizing, and ef- rial. Making some assumptions about the rain fects like the distribution of extreme values, the aadhe- -amount, degree of periodicity, uniformity or regula- renceh between neighboring values, suggesting short rity of the differences between neighboring places, cycles or particular interactions over time, never find a etc.- we may ask the following question: If the first satisfactory explanation. but remain a source of stimu- landscape could be generated by some fractal expres- lus for research. sion, can we expect that the second or final landscape The following model may be of more general appli- can also be fitted to some fractal expression, or subjec- cation in ecology: Assume a standard system of deve- ted to fractal interpretation? The problem also concerns lopment, selforganizative or successional, subjected to the relation between surface morphology and the pro- a spectrum of disturbances, from which it can be said perties of the drainage system. only that the most strongest -measured, if convenient, From an ecological point of view, the system of con- in terms of external energy involved- are less frequent verging affluents is also interesting because the conver- than the less strong. This general view of a spectrum of gence and union of different flows can make the resul- disturbance is not pure. but contains some difficulty ting common flow more uniform (or else exaggerate the born in biology and evolution, because very common peaks in cases of strong rainfall hitting neighboring valleys disturbances become conditions for life and must be in- simultaneously). This system also favours chemical ho- ternalized by the organisms, like daylnight {{disturban- mogenization, through mixing, of waters that might have ce., seasonal change, no matter how important they are slightly different composition. Finally, the mixing of po- in terms of energy. In fact the daylnight cycle is also pulation~drifting through by the respective affluents, very important in terms of the energy involved. To as- combines different genotypes into a new environmental sign a given frequency, a posteriori, to a given class of system in which strong selection may be expected. disturbances does not mean that we have to expect a re- We often imagine that the underwater systems and gular distribution over time of the events to which it re- aquifers in general are a projection and continuation of fers. The events may cluster, like glacial periods at their the same river below the surface, with the same, if slo- scale, or rainy and dry years, which may be associated wer, role in transportation, all related to the permea- in a row, from pairs to groups of seven. An interesting bility and texture of the bed materials. The domain of un- question that can be approached through simulation is: derground waters is, in fact, much larger and may be asso- Given such circumstances, what is the probability of ciated with deeper geological features of the country. being right in attempting some prediction, which we can simplify asking only for the probable lenght of time in which we can expect disturbances of an even sign or of Speeding up the features of succession a decreasing sign, or at less, not interrupted by major and change disturbances. Some exercises of simulation show how difficult it is to find encouraging answers, that is, ans- Life combines relatively slow (sometimes program- wers that make some success at prediction likely. This med, sometimes just allowed) processes of selforgani- is very discouraging if one tends to believe or to expect that one of the roles of ecology in society is prediction, negatively correlated with the power k that characteri- even if understanding is not complete. zes the random walk regime. The alternative interplay between .regular>, develop- We should proceed through a number of steps: ment and disturbance informs change in ecology, but is 1) Flow cultures. Difficulties in adopting an exclusi- not exclusive to life. It can also be recognized in the se- vely Eulerian or Lagrangian approach. diment~deposited in river beds and elsewhere. Regular 2) A more flexible approach and a guide to the evo- cycles (cycles of Bouma, etc.) begin suddenly with coar- lution of riverine life. se materials, as a result of a rapid increase in the trans- 3) Turbulence and its equivalents. port capacity of the system, in our case of a river, and then follow with materials of decreasing size, eventual- The simplest model of a river is a chemostat or flow ly reaching sands and muds. What is important, as a culture. In it natural growth of a population tends to paradigm of change that compares favorably with move to a balance with the amount of the population ecological succession, is the gradual change from driven away by the flow. If r is the rate of increase of coarse to fine materials, as opposed to the suddeness of populations -no deaths considered now-, N the the change in the contrary sense. From the point of quantitative expression of the population, X a length view of river ecology this is important for the distribu- measured along the axis of the <(river>>,positively in the tion of boulders and other materials on the bottom downstream direction, and V the speed of the water or of a river. River biologists usually used to relate culture medium, and t is time, in one point of the river the motility of boulders, according to their size, with we should have definite speeds of water. In this connection also, there is relation between populations and the past history of flows. an expression that can be compared to that one used in plankton biology, with reference to a vertical dimension z, and to which a term referring to turbulence (A) is ad- Approaches to modeling river populations and ded, in the form: ecosystems
It is difficult to study in detail biotic populations in flowing waters and to conceptualize in their ensemble. If organisms were to develop suspended as plankton This probably contributes to explaining the traditional in water moving in a laminar flow, it is obvious that unw~llingness to include rivers in general ecological they would be rapidly washed out of the system. There approaches to limnology. Many approaches have been is an aphorism by Ambiihl that goes as follows: ((No life attempted in the line of description of communities, is possible without water, and no life in water is possi- developed as a function of properties of local substrata, ble without turbulence>>.This is easy to understand, and and organized in mosaic-like fashion. When limnolo- the practical realization of a persistent flow culture con- gists have been more aware of the strong relations sists in joining a number of small reservoirs in each of between local communities. as associated with other flo- which a separate mixing is achieved. This is properly wing with the water, they have felt that this should be what happens in a river, with the difference that the in- hailed as progress, under the name of the continuum tensity and the length of mixing are not constant in any concept. The real position of the epicontinental aquatic given section. Indeed, mixing intensity and modality part of the biosphere as a transport system, enhances changes continuously according to the place. For this its unique position and perhaps opens the way to a more reason it does not make much sense to dwell on the tra- realistic approach to its biology. As a preamble we ditional discussion between the Eulerian or the Lagran- might recognize that downstream there is a steady co- gian approach, if one should consider what happens in lonization pressure from populations living upstream, one point of the environment -not including the im- which must be countered by selection going another portant question of how big the point is- when it is a way. Populations living in the upper stretches can in- fixed point with reference to an exterior space, or when fluence populations living below, through different sorts it is considered that the point drifts with the water. It of chemical influence, whereas the converse action becomes a purely %academic,>question. upwards does not exist. Simply, populations in upper Actually there is continuous change in speeds and tra- stretches exist and are maintained through steady jectories, all combined together, that forces us to a sta- adaptation to local conditions and brute force in multi- tistical approach, which would be difficult to qualify as plication. But in the lower stretches very subtle rela- either Eulerian or Lagrangian, Biological factors make tions become more important. It is possible to say that the model more complicated. In simple flow systems. lower stretches exploit upper stretches of the river. And in the form of reactors for the production of bacteria or this would be reason enough to explain the fact that bio- algae, or of their metabolites, some sort of turbulence tic diversity usually increases downstream. In general, is striven for, but organisms that attach to the walls, if systems exploited from outside have their biotic diver- genetically different, often gain preeminence. The ca- sity depressed. In relation with the model of .random pacity for holding the position is an asset in evolution. walk,, to be commented below, biotic diversity is usually In other words, not only a wide range in the conditions of flow. but also in the characteristics of the organisms, would be proportional to the elapsed time -from the is important in associating them with given flow situa- starting line- multiplied by a coefficient of diffusion tions, or linking their probabilities of survival to the size (A). of the solid materials, from grains of sand to boulders, Whereas in modelling plankton systems over the z to which they attach. axis, turbulence (A) and sinking speed (V,) can be con- This is extremely important in discussing life in ri- sidered separately, in general, the speed of water (V,) vers. In the preceding expression, a sort of trade-off bet- is seen as a main factor in flowing water, and the tur- ween r and V is implicit, in the sense that species that bulence begins to be considered rather as a modifying reduce the effective speed of drift can slow down rate factor. This is not completely appropriate, however, be- of reproduction and this is the most obvious advantage cause of the existence of environments in proximity, in selection. This has represented a constant pressure with quite different regimes, and the organisms can in the evolution of what have been considered as adap- move from one to the other easily, as in sediment, or tations to life in streams: attachement to stones, life in the margins of rivers. Of course small eddies can be among the sediments, and other forms to oppose to una- observed in the sluggish large streams where the gene- voidable drift of at least a fraction of the populations, rally downwards motion of water is locally reversed. like the tendency to swimm against the current through There is another aspect which shows the complexity of mechanisms that allow optical clues in relation to the the problems involved and the interest of a certain uni- environment, or forms of behaviour like the tendency formity in their modeling. The rivers of a water-course of aquatic insects in their adult and aerial life to crawl are a true bank of germs of potential colonizers of the or fly in a direction opposite to the flow of the river in water. Diaspores are left on the banks under quite dif- whose waters they were born. Of course. the simplest ferent situations. It is clear that colonization by species distinction is between potamoplankton, moving with the of algae -Hydrurus and many others- at the time of water. and potamobenthos, fixed to the bottom, but high water comes from germs left over the banks. As the distinction is never neat, as can be deduced from water level falls and other species develop, most of them the discussion about the appropiateness of Eulerian and leave some resting stages on the banks. It is amazing Lagrangian approaches to the description. how many alive organisms. in suspended animation, or The drift of one part of the benthic populations can as cysts and various resilient forms (coccons of Aeolo- be quantified, supported by the catches of nets exten- soma, rotifers, nematodes, tardigrada) are found in the ded normally to the current and a net speed of move- soils in the vicinity of water, in winter frozen but alive, ment downwater assigned to each species. Such speed apt to recolonize the river. This bank of germs at seve- is not constant, being subjected to circadian and seaso- ral different stages offers ample possibilities for the re- nal cycles, and to other factors. Persistence of popula- colonization of the river. but scarce attention has been tions requires that the net drift has to be compensated given to it. The point here is that its persistence is equi- by multiplication of the populations and other eventual valent to a form of turbulence for the whole popula- inputs. As the speed of water changes, often catastrop- tion, and to this persistence should be added the pre- hically, it is obvious that the aerial life stage in insects servation of population in the interstitial water at the is very important for the persistence of their popula- different levels. tion~,and this form of life -which we could call schi- We end with a concept of turbulence that surpasses zophrenic- is expectably very common among the the consideration of a criss-cross of trajectories at ran- aquatic fauna. dom, and directed to draw a statistical spectrum. The physical or more random part of it becomes mixed with the results of the behavior of a number of organisms Different approaches to an enlarged concept of that impose arbitrarieness and biaised trajectories, in di- turbulence rections and speeds, and in the interaction between ve- locity and trajectories, as well as changes subjected to One simple approach that may have some advanta- circadian rhythms. Natural selection works against a ges is based on the concept of (