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D. Global decline of water resources

Global water availability has declined, on average, by approximately two-fold during last 25 years, although this decline varies greatly depending upon the particular region. In Africa, it has declined by about three times; in Asia, by about two times; and, in Europe, by about one-eighth (Shiklomanov 1998). The main reasons for this decline in availability has been population increase, increased water consumption (e.g., in Africa), and drier trends in climate. As consequence of global climatic processes during the last decade, river runoff in Africa has declined by about one-third, while, at the same time, there has been a significant increase in runoff in South America. A significant decrease in precipitation has been observed since the mid-20^th century in North Africa, Southeast Asia, Indonesia, and Australia (Georgeyewsky 1998). These trends are especially important considering that, in the highly populated regions, two-thirds of the water has already been allocated and utilised for agriculture. In future, even more intensive water withdrawals are to be expected in Africa and South America (Shiklomanov 1998). There is increasing evidence that one of the factors amplifying this deficit is the long-term, progressive degradation of natural plant cover (IGBP 2000).

These trends are especially alarming when considering the expert opinion that the minimum amount of water needed for human consumption is 2000 m³ per capita per year. Accepting this volume of consumption, more than one-third of the world’s population already has catastrophically low levels of water supply. It is also highly probable that the deficits, recently observed in the water balance of the Earth, will increase due to population growth, increased economic activity, degradation of water mesocycles, and global climate change. Such forecasts may be optimistic, especially if emissions of greenhouse gases and the reduction in plant biomass within landscapes is not to be reversed. Deforestation, which leads to the loss of water retention, is especially dangerous in the face of global climate change, since various climate change scenarios suggest that a doubling of CO₂ will increase evaporation and precipitation globally from 7% to 15%. This, in turn, will increase the amount of moisture in atmosphere by 6% per degree Celsius of temperature change (IPCC 1996), thereby increasing cloud formation, and the likelihood of heavy rains and flash floods. Such intensification of the water cycle will have serious consequences for agriculture (i.e., erosion), and for drinking water supply (i.e., eutrophication). Increased pollution and nutrient loads will increase the potential for increased numbers of toxic algal blooms and occurrences of water borne diseases (Epstein 1998).

On the other hand, these changes may indeed have some potentially positive effects; e.g., milder winters with reduced freezing of soils. This could increase the active soil layer moisture and, in turn, increase ground water recharge and river base flows (especially in Europe and North America). Following from this, one might expect that a deficit in surface water supply might be compensated for by increased groundwater reserves. In Europe, groundwater accounts for 60% of the domestic and drinking water supply, and further intensification of ground water withdrawals in some areas may cause serious problems, influencing riverine run off, contributing to earth surface subsidence and the elimination of vegetation due to, inter alia, salt water intrusions. Moreover, the accelerated infiltration of surface water into aquifers as a consequence of groundwater withdrawals may enable the transfer of pollutants from the land surface to the groundwater system. In fact, only confined aquifers capped with impermeable clay layers are protected from such infiltration by land-based, anthropogenic pollutants.

E. Plant cover as a moderating factor in energy and water flows, nutrient circulation, and ecosystems

By integrating data from different types of ecosystems, Varlygin and Bazilevich (1992) have demonstrated that energy and water are the two key factors driving primary productivity, and, thus, driving the pattern of succession in the development and type of climax ecosystem. They stated that, in boreal, subpolar and polar regions (with exception of arid areas), primary production increases proportionally with heat inflow. In subboreal, subtropical, and tropical regions, primary production is correlated with water availability (Figure 4.1). This trend is, to a significant extent, susceptible to modification by anthropogenic actions and the elimination of plant cover that affects water yield and seasonal distributions of flow. There is some experimental evidence that suggests that artificial plantations of fast growing trees (e.g., eucalyptus and pine), while restoring plant cover, does not restore water (out) flows to the natural, forested community state. Coupled to this is the observation that "smaller basins (with similar pedological material) release less water than larger ones" (Adokpo Migan 2000). Ryszkowski (1998) has demonstrated that, on agricultural lands with high levels of plant biomass, the heat budget has a lower variability and a reduced range of extreme temperatures. Furthermore, buffer strips in the landscape can help to reduce nutrient loss to aquatic systems and stabilise water dynamics.

The shaping an optimal plant cover structure, from the point of view of the sustainability freshwater resources and other socio-economic priorities within a given region, required the use of forecasting or prognostic models.

F. The role of aquatic macrophytes in the restoration and management of freshwater ecosystems

Aquatic macrophytes, in most of freshwater ecosystems, play a central role in the regulation of the nutrient dynamics, and, in consequence, can act to buffer these systems from eutrophication (Jeppesen et al. 1998). Similarly, such plants can help to enhance fish production (Petr 2000).

Macrophytes, as a tool for the sustainable management of freshwater resources, should be considered in two complementary dimensions:

• First as low cost technologies for low and medium levels of sewage treatment, especially easy to apply in tropical and subtropical countries where there is a year-round growing season. In temperate climates, due to the limited growing season, there is a need for more precise adjustment and control of sewerage flows, or even for compensating technologies during the winter period (UNEP 1999).
• Second, according the ecohydrology concept (Zalewski et al. 1997, Zalewski 2000), as a component of an integrated strategy of basin management. Macrophytes can be a tool for enhancing the capacity of the natural landscape to absorb human impacts on freshwater ecosystems.

However, it is necessary to note that the relationships between macrophytes, the abiotic components of ecosystem, and the biota are complex, and that non-reproducible patterns of interaction occur in different climatic zones and within typologically different ecosystems. In addition, these interactions can be modified to a serious extent and in many situations influenced by introductions of exotic species (e.g., Harper 1992). Consequently, more field and experimental data are needed. According to a review by Petr (2000), at ecosystem scales, the excessive, uncontrolled growth of macrophytes may diminish the potential for freshwater and fisheries resource use. At moderate levels of macrophyte growth, however, such growth may have positive effects on water quality and fish production.

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