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P R O T E C T I N G E C O S Y S T E M S F O R P E O P L E A N D P L A N E T / 1 3 1Significance of FreshwaterEcosystemsWater has the pivotal role in mediating global ecosystem processes,linking together the atmosphere, lithosphere and biosphere bymoving substances between them and enabling chemical reactionsto take place. Not only is it essential for maintaining livingorganisms, but its physical properties allow it to be used by humansfor energy generation, transport and waste disposal, and in avariety of industrial processes. Access to a sufficient amount ofgood-quality water is essential to human health; productivefreshwater ecosystems are crucial to the livelihood of fishers andother littoral communities; and healthy freshwater systems provide arange of services to people worldwide.Characteristics of freshwater ecosystemsThe term ‘ecosystem’ refers to the communities of plants, animals andother organisms, and the physical environment of any given place,these elements being linked by the flow of energy and the circulationof materials from producers to consumers and decomposers. Becausethe term refers primarily to system processes rather than a definedplace, it can be used at a range of spatial scales, so that one mayrefer to aquatic – as opposed to terrestrial – ecosystems, or generic‘lake ecosystems’ or to an individual lake as ‘an ecosystem’.The presence of liquid water is one of the defining features ofplanet Earth, and water is virtually ubiquitous in the globalenvironment. This chapter is concerned only with freshwaterecosystems in the traditional sense, that is surface water includingrivers, streams, lakes, ponds, marshes and other wetlands, togetherwith the ground or soil water to which they are linked and deeperaquifers. 1 These include intrinsically valuable natural or semi-naturalsystems, and also increasingly a number of man-made or artificialaquatic environments. Artificial ecosystems may be very extensiveand retain their own biological value, for example the flooded riceagro-ecosystems in Asia, or also be important in terms of thefunction they provide. In a broader sense, however, almost the entireterrestrial environment can be considered a ‘freshwater ecosystem’1. In practice, the terms ‘freshwater ecosystem’ and ‘inland water ecosystem’ (orderivatives) tend to be used interchangeably, although the latter is more inclusive incovering enclosed saline waters and estuaries. In traditional usage, ‘wetland’ refersto areas of waterlogged soil and of land, such as river floodplain, that is seasonallyor permanently covered by relatively shallow water. In the broader usage of theRamsar Convention, wetlands are ‘areas of marsh, fen, peatland or water, whethernatural or artificial, permanent or temporary, with water that is static or flowing,fresh, brackish or salt, including areas of marine water the depth of which at lowtide does not exceed six metres’.because all known life forms depend on water. Almost alllandscapes and microhabitats interact with water during thehydrological cycle and in fact, the evaporative part of thehydrological cycle would constitute by far the greatest ‘use’ ofwater.While surface waters can include biological communities ofconsiderable complexity, with representatives from many differentphyla, or categories of organisms, species in soil water mainlycomprise fungi and other micro-organisms, and deep aquifers aresterile or inhabited by a very few kinds of archaea and bacteria.Cave waters, notably in karst regions, often include specializedrestricted-range species of invertebrates, fish and amphibians. Athigher taxonomic levels, global diversity in freshwaters is lower thanon land or at sea (with only one phylum, Gamophyta, entirelyconfined to freshwaters). At the species level, diversity appears tobe high in relation to habitat extent; for example, globally thenumber of fish species per unit volume of water is more than 5,000times greater in freshwaters than in the sea. Terrestrial orhydrological barriers to dispersal promote endemism in freshwaterspecies, and many fish and other inland water species are restrictedto individual water systems, such as river basins, lakes and caves, orto particular stretches of river or lake edge.For many purposes, the catchment (watershed, basin), defined asthe entire land surface from which water flows downhill to a givenpoint, is the principal unit of management. While the catchmentapproach is desirable, it is not alone sufficient to address all issuesrelated to inland water ecosystems. The quantity and quality of waterreaching the coast affects estuarine and coastal waters; catchmentscan be interconnected by infrastructure; aquifers in which groundwateroccurs may bear little relationship to the surface topography thatdetermines the geometry of river catchments; and economic and socialfactors external to a single catchment can exert significant impacts.Uses and benefits of freshwater ecosystemsThe living and the abiotic components of an ecosystem (organisms,sediment, water) interact in many ways, engaging in processes thatmay be biological, physical, chemical or hydrological in nature(organic production, nutrient cycling, carbon storage, waterretention, habitat maintenance). Some ecosystem components maybe regarded in economic terms as ‘goods’ and the functional resultsof ecosystem processes as ‘services’. Humans derive direct andindirect benefits from both aspects of ecosystems, and fromproperties such as ‘biodiversity’ that can be attributed toecosystems. Several different classifications of these benefits havebeen produced as aids to discussion and analysis; one example isgiven in table 6.1. Some benefits depend on the presence ofparticular individual species. Elements of biodiversity thus underliemost of the benefits and functions cited.