THE IMPORTANCE OF WATER

Water has the central role in mediating global-scale ecosystem processes, linking atmosphere, lithosphere and biosphere by moving substances between them and enabling chemical reactions to occur. Natural waters are never pure H₂O but a complex and ever-changing mixture of dissolved inorganic and organic molecules and suspended particles.

Water is by far the most abundant single substance in the biosphere. It is unique or extreme in most of its physical properties and these are the basis of its biological importance. Living cells are around 75% water; liquid water is essential for life processes and organisms must obtain water, in amounts broadly proportional to size, from their environment. Spatial and temporal differences in the availability of water and its solutes are important determinants of ecosystem richness

At the Earth's surface, freshwater forms the habitat of large numbers of species. These aquatic organisms and the ecosystems in which they participate represent a substantial sector of the Earth's biological diversity.

Water as a resource has two key features:

• it is absolutely essential for human survival,
• the amount of water in the world is constant, so that although it is used, the stock is not globally diminished but nor can it be increased.

At below global level, water is often not available where and when needed, nor in the appropriate amounts, nor with the necessary quality. The two last are particularly important to the maintenance of freshwater biodiversity.

The finite supply of freshwater on the Earth is now being used by a human population that has grown exponentially in the past few hundred years, and continues to grow, and which demands increasing volumes of water to service agricultural and industrial processes on which economic development depends. Freshwater systems are under growing pressure, as flow patterns are disrupted and the load of waste substances increases. Inevitably per capita shares of water for human use are decreasing and water stress is becoming more widespread (UN, 1997).

Agriculture consumes around 70% of all water withdrawn from the world's rivers, lakes and groundwater (FAO, 1996b). In places, more than half the water diverted or pumped for irrigation does not actually reach the crop, and problems of waterlogging and salinisation (deposition in soil of salts left by evaporation of pumped groundwater) are increasing. However, irrigated agriculture produces nearly 40% of world food and other agricultural commodities on only 17% of the total agricultural land area, and is thus disproportionately important to global food security (FAO, 1996b).

The total volume of water on Earth has been estimated at around 1,500,000,000 km³. Salt water in the world's oceans and seas accounts for almost all, perhaps 97%, of the total volume. Freshwaters make up most of the remaining 3%; this component consists largely of water in the form of polar ice (mostly Antarctica) and groundwater. See Table 1.

The world water cycle is the overall process by which water is redistributed between sources and sinks, with more or less transient residence in rivers, lakes and living organisms. Water is moved over the Earth by rivers, by ocean currents and (manifest in weather patterns) by circulation of the atmosphere.

Over the oceans, evaporation exceeds input from rivers and rainfall, and over land, precipitation exceeds evaporation, but at global level a broad balance exists between the volume of water entering the atmosphere as water vapour and the volume leaving it as precipitation. Sub-globally there is considerable variation in the distribution of water, eg. there is about twice as much atmospheric water in equatorial than in temperate latitudes

Groundwater, ie. water below the Earth's surface held within rocks or between rock strata constitutes perhaps 30% of global freshwater resources. The more superficial deposits are linked to the global water cycle, and are used for human consumption or agricultural purposes, whereas the deeper layers tend to be somewhat saline and do not (except over geological time scales) participate in exchanges with other parts of the system.

Water in lakes and rivers constitutes less than one-hundredth of one percent (<0.01) of the world's total water volume; lake water is the largest component in this vanishingly small subtotal.

Water passing through an area of land is available either in the form of soil moisture ('green water') where it is used in production of natural or agricultural plant biomass, or in aquifers or surface systems ('blue water') where it is a component of aquatic ecosystems and is used for human social and economic production.

Source: Anon. (USSR Committee for the International Hydrological Decade) 1978.

There are very large regional differences in the distribution of freshwater in all its forms, depending on the volume of precipitation and the area and geomorphology of continental land surfaces, and deep geology, in the case of groundwater reserves (see Table 2). For example, South America has few lakes - only about one twentieth of the lake volume average for other continents, but around four times as much water flowing in rivers, and a very large total wetland area (this high discharge volume may underlie the extreme diversity shown by Amazon fishes).

In contrast to the World Ocean, which is relatively uniform in composition over very large distances, waters on the Earth's surface vary widely over short distances, according to catchment geology, land cover and climate, and the materials of anthropogenic origin introduced to them.

Important variables affecting water composition include: solubility and weather resistance of basin rocks, distance from the marine environment (aerosol source), aridity, nutrient flow through basin vegetation, temperature, and uplift rates. Some watercourses are naturally unfit or poorly suited for some human uses, including drinking (Meybeck and Helmer, 1989).

Anthropogenic changes in water quality are superimposed on the natural background variations. A similar sequence of water quality issues became apparent in both Europe and North America during rapid socio-economic development over the past 150 years. Problems of faecal and other organic pollution were evident in the mid-19^th century, followed by salinisation, metal pollution, and eutrophication in the first half of the 20^th century, with radioactive waste, nitrates and other organic micropollutants, and acid rain most prominent in recent decades (Meybeck and Helmer, 1989). At the same time, the scale of water quality problems tends to increase from local to regional and global. Newly-industrialising countries are likely to face these problems over a much more compressed period, and typically without the capacity to monitor and analyse water quality, or manage water use appropriately.

Source: modified after Chapman (1992).

Levels of organic micropollutants (organochlorine pesticides, polychlorinated biphenyls, inductrial solvents) and of trace elements (mercury, arsenic, cadmium, copper, etc) give rise to water quality problems worldwide in the four main classes of water system (Table 3) (Chapman, 1992; UNEP, 1991, 1995). With regard to quality for human use, contamination by pathogens of faecal origin is the major problem in river systems, and eutrophication probably the most widespread problem affecting lake and reservoir waters.

A river system is a complex but essentially linear body of water draining under the influence of gravity from elevated areas of land toward sea level. The typical drainage system consists of a large number of smaller channels at higher elevation merging as altitude falls into progressively fewer but larger channels, which in simplest form discharge by a single large watercourse. Most such systems discharge into the coastal marine environment; some discharge into lakes within enclosed inland basins; a few watercourses in arid regions enter inland basins where no permanent lake exists.

The source area of all the water passing through any given point in the drainage system is the catchment area for that part of the system. In parallel with the hierarchical aggregation of tributaries of the major river system, sub-catchments aggregate into a single major catchment basin; this is the entire area from which all water at the final discharge point of the system - ie. usually the sea - is derived. Strictly, the watershed is the line of higher elevation dividing one catchment basin from another, but this term is increasingly used as a synonym of catchment.

The speed and internal motion of river water depends largely on water volume and the shape of its channel. These factors typically differ greatly through the river system, from narrow, steep and fast upland feeder streams, to broad level and slow downstream reaches. Combined with differences in depth, riparian vegetation, seasonal variation in flow, and other factors, there is a great variety of potential habitats. Different organisms within the system tend to be adapted to different sectors of it, with consequent differences in form and function.

Two features are of primary importance with respect to the habitat quality of a river:

• materials, such as nutrients, pollutants or sediment particles are capable of entering the drainage system from any point within the watershed boundary of that system, and
• dissolved substances or particles are transported in a downstream direction and so can have an effect far distant from the point of entry (eg. sediment particles derived from watershed slopes can be carried into the coastal and marine environment where they frequently have an adverse effect on local habitats).

A large river and its drainage basin make up a large-scale ecosystem, with both terrestrial and aquatic components, which must be addressed in an integrated manner for management interventions to have a chance of success; ie. the catchment is the basic unit of management at the landscape scale.

Freshwater habitats are widely considered to be transient in time and space in comparison with both terrestrial and marine habitats. This is broadly true, certainly for very small or very shallow freshwater habitats. However, although individual water bodies vary in extent and persistence, the main types of freshwater habitat have probably existed since precipitation first fell on the Earth, and large rivers are probably much longer-lived as a class of systems than lakes (Gray, 1988).

River systems can change course radically as a result of deposition and erosion of their channel, and the uplift and erosion of watershed uplands. Despite the dynamic physical state of these systems, large rivers rarely disappear, and although direct evidence is scarce, indications are that some have been in continuous existence for tens of millions of years. This is consistent with the fact that running waters include representatives of almost all taxonomic groups found in freshwaters, and that several invertebrate taxa occur only in running waters or attain greatest diversity there.

The great majority of existing lakes, of which around 10,000 exceed 1 km² in extent, are geologically very young, and occupy basins formed by ice masses or glacial erosion during recent ice ages (Gorthner, 1994). These lakes date from the retreat of continental ice-sheets some 10,000 years before present. All such lakes are expected to fill slowly with sediment and plant biomass, and to disappear within perhaps the next 100,000 years along with any isolated biota.

Only about 10 existing lakes are known to be much older (Gorthner, 1994; and see Table 15), and most of these occupy basins formed by large scale subsidence of the Earth's crust, dating back to at most 20 million (Lake Tanganyika) or 30 million (Lake Baikal) years before present.

There is good evidence that some extinct lake systems in the geologic past were very large and very long-lived under different climatic and tectonic conditions. In general, the long-lived lakes are of particular interest in terms of biodiversity because these systems tend to be rich in species of several major groups of animals and many of these species are restricted to a single lake basin.

Humans rely heavily on biological resources in freshwaters, and use freshwater systems for a wide range of purposes. A large river provides a moving and apparently endlessly renewable stream of water, for transport, water supply, waste disposal, and from which food and hydroelectric energy can be extracted.

In addition to utilitarian benefits derived from freshwaters and freshwater biodiversity, humans also derive many benefits from freshwater systems as elements in the landscape, particularly so in wildlands, but also in highly modified agricultural or urban settings. The aesthetic and cultural benefits are derived in part from the visual appearance of the system in the landscape setting, and may not depend directly on the health of freshwaters, or the levels of biodiversity therein.

The principal use of freshwater species, not considering properties of aquatic systems themselves, is as food. Subsidiary uses include the aquarium trade, materials for medicinal or ornamental use, and as fertilizer. For many human communities, particularly in countries less-developed industrially, capture fisheries provide a major portion of the diet. Finfishes aside (see below), other exploited animal groups in inland waters are far less important globally but may still be highly significant (see Table 4).

Relatively few plants associated with inland waters are heavily exploited in the wild state; most are marginal or wetland species. Some (eg. Aponogeton spp. in Madagascar) are collected for use as ornamentals; reeds are used as building materials (eg. thatch); and some are collected for food or as medicines (eg. Spirulina algae). Rhizomes, tubers and seeds (rarely leaves) of aquatic and wetland plants are used as a food source, mainly in less developed regions where they can be important to food security in times of shortage, but globally they make a relatively minor contribution to human nutrition. Most important are some forms of edible aroid (Araceae), notably some cultivars of Colocasia (taro) and the giant swamp taro Cyrtosperma chamissonis which grow in flooded conditions and are important food crops in the Caribbean, West Africa and the Pacific islands. Conservation and collection of wild forms of these is considered a high priority. Sago Palms Metroxylon spp. in southeast Asia and the Pacific and Watercress Rorippa nasturtium-aquaticum in Europe are other examples of cultivated aquatic plants whose wild relatives merit conservation.

Rice is the major cultivated wetland plant, and provides the staple food of around half the world's people. Most current strains are based on Asian Rice Oryza sativa and African Rice O. glaberrima. Worldwide, more than 500 million metric tonnes of rice are produced each year, from around 150 million hectares; most production is based on rice paddies, which form an important artificial wetland ecosystem in the tropics, especially in Asia. There are about nineteen species in the genus Oryza; wild populations of some are in decline but varieties of O. sativa are well represented in germplasm collections, notably at the International Rice Research Centre in the Philippines.

Aquatic plants have been widely used for medicinal purposes, documented for at least two millennia, but such use appears at present to be minor and probably of real significance in few areas. However, interest in ornamental or aquarium water plants is very widespread and of some economic importance.

The true value to humans of different inland water ecosystems can only be estimated by seeking more comprehensive means of evaluating these systems in economic, social and cultural terms, so as to take account of the less tangible values of ecosystem goods and services, including those provided by biological diversity. A recent attempt to ascribe a global value to ecosystems (Costanza et al., 1997) estimated mean values per hectare of major ecosystem types, taking as many of these less tangible factors as possible into account. Of non-marine ecosystems, wetlands (average value US$ 14,785 per hectare) and lakes and rivers ($8,498 per ha) were several times more valuable per unit area than terrestrial ecosystems such as forests ($969) and grasslands or rangelands ($232). Taken together, inland water ecosystems were estimated to contribute more to total global flow value (US$ 6579 x 10⁹ per year) than all other non-marine ecosystems combined (US$ 5740 x 10⁹ per year) despite their far lesser extent. This provides a strong case for effective conservation management of extant inland water ecosystems.

Where countries have access to both marine and inland aquatic resources, reported yield from inland waters is a small fraction of marine yield, a pattern reflecting the higher productivity of marine shelf waters and reinforced by weak marketing and distribution infrastructure for freshwater catch. Even in land-locked countries, the recorded inland harvest is often, but not always, low both in absolute size and in relation to consumption of meat and other agricultural produce.

The total inland water fishery production has two components: capture fisheries and aquaculture. National statistics do not adequately reflect the actual magnitude or location of inland fisheries. The reported inland capture production is certainly an under-estimate because much of the catch is made far from recognised landing places where catches are monitored, and is consumed directly by fishers or marketed locally without ever being reported. The evidence suggests that actual capture fisheries catch may be twice the reported total, ie. around 12 million mt per year (Coates, 1995), and inland waters are suspected by some to provide food (as opposed to oils and meals) in amounts not much less than the recorded marine catch (Borgström, 1994).

Inland water capture fisheries, particularly in countries less-developed industrially, do provide a staple part of the diet for many human communities. This is the case in West Africa generally, locally in East Africa, and in parts of Asia and Amazonia. In some land-locked countries inland fisheries are of crucial importance, providing more than 50% of animal protein consumed by humans in Zambia (Scudder and Conelly, 1985), and nearly 75% in Malawi (Munthali, 1997). Fish protein may be critical in times of food stress. It is impossible at present to develop a global view of the rôle of inland fisheries because these operate mainly at artisanal and local level in rural areas, in general far outside the scope of available statistical data.

Table 5 shows reported inland fishery data for a selection of countries. It should be noted that two-thirds of the 30 countries showing the highest apparent per capita consumption of inland fish are classed as LIFDs (low income food-deficit countries), and around half of those with the highest catch. Bangladesh is foremost among countries appearing in both lists; others include China, Indonesia, Viet Nam.

Overall inland capture production has risen moderately during the decade from 1984, around 1.7% annually, but there are significant regional differences. Inland production has declined in Europe and the former USSR, mainly because of deteriorating habitat quality and excess exploitation. In Asia, excluding new states of the former USSR, and in Africa, production has risen. However, this cannot be attributed to any improvement in the health of inland waters. Increase in Asia is attributed mainly to fisheries based on stocking of large artificial reservoirs created during recent rapid economic development, and increase in Africa is mainly due to capture fisheries in the Rift Lakes, where the introduced Nile Perch is the basis of a significant export trade system.

It is difficult rigorously to assess the condition of inland fish stocks because they appear able to respond rapidly to changing environmental conditions. However, there is a consensus that, regionally, most stocks are fully exploited and in some cases over-exploited.

Exploitation has become more efficient because of new technologies, and developing infrastructure has allowed easier access to freshwater resources. Some stocks, especially in river fisheries, appear to be in decline, but this is seemingly a result mainly of anthropogenic changes to the freshwater environment.

Inland fisheries, and freshwater biodiversity generally, do not receive sufficient attention in local, catchment-wide or national planning decisions. These sectors are often strongly impacted by decisions made in eg. the hydroelectric, navigational, flood control or agricultural development sectors, without reference to the need to maintain biodiversity or fishery production.

Salient features of inland water fisheries are summarised in Table 6.

In recent years reported inland production has made up around 7% of the world total capture production. Despite this relatively low figure, and without taking account of under-reporting of inland capture, inland production has special significance because (Coates, 1995):

• many more people have access to inland waters than to coastal marine waters; gears for subsistence fishing do not have to be technologically advanced (although such equipment is increasingly available) and costly to purchase, and often many sectors of the community are involved in inland capture fisheries;

• a greater proportion of the inland catch appears to be used for direct human consumption, close to point of origin;

• some countries are land-locked and have no internal source of fish other than freshwaters;

• most of the marine catch is landed by highly industrialised fleets from a small number of countries but inland production exceeds marine landings in about 25% of reporting countries, including a large number of Low Income Food Deficit Countries;

• waste through discarded bycatch is large in marine fisheries but negligible inland.

Inland waters typically intersect several subnational administration units (counties, provinces, etc) and are subject to management and use decisions made within several different sectors (forestry, navigation, fishery, waste disposal, recreation, etc). Although it has been recognised for some time that the catchment basin is the fundamental unit within which management must be formulated, reconciling the many different interests concerned and coordinating actions have proved difficult.

Waters that delineate or cross international boundaries present a special class of management issues. Such waters and the living resources they contain are shared by one or more countries, and require positive international collaboration for effective use and management.

Available water in any given country within an international basin (or other administrative unit within a basin more generally) can be divided into endogenous, ie. locally generated runoff available in national aquifers and surface water systems, and exogenous, ie. remotely generated runoff imported in flow from upstream. Some countries (eg. Canada, Norway) have an abundance of water from endogenous sources, others (eg. Egypt, Iraq) have a small endogenous supply but large exogenous volumes (others have small supplies from both sources). Use of exogenous water carries an increasing risk because of dependence on sufficient supply from upstream countries.

The United Nations Register of International Rivers (Anon., 1978) recognised 214 major international river basins in 1978. Since that time, fragmentation of some previous country units, eg. the USSR, has compounded problems of international cooperation by increasing the number of countries having a share of international inland waters. A preliminary revised listing of countries within basins included in the present assessment is incorporated in Annex 1.

Notes: this table outlines key features of inland water fisheries and is a highly selective and simplified overview of a complex subject with a vast technical literature.

Sources: information derived mainly from FAO (1996) and Coates (1995).