Pressure indicators

Agriculture

Agriculture impacts freshwater ecosystems because of water extraction and discharge of pollutants. Two of the major pollutants derived from agriculture in freshwater bodies are nitrogen and phosphorus (Vitousek , 1997,et al., 1997; Shortle et al., 2001; PNUMA, 2003; Silk and Ciruna, 2004; MEA, 2005) which come primarily from synthetic chemical fertilizers. Both are carried to water bodies by surface runoff either dissolved or adsorbed to soil particles and, in the case of nitrogen, also by atmospheric deposition (Shortle et al., 2001). N and P buildup in water bodies leads to eutrophication, i. g. booms of algal and bacterial populations, which consume dissolved oxygen and reduce its availability for the rest of the flora and fauna. This, coupled with the reduction of photosynthesis of submerged vegetation (resulting from increased turbidity), affects all levels of the food chain and, in the short or mid term, reduce biodiversity (EPA, 1992; Ongley, 1996; Vitousek et al., 1997; Carpenter et al., 1998; Petr, 2000; UNDP et al., 2000; Baron et al., 2003; EEA, 2003; PNUMA, 2003; Silk and Ciruna, 2004; MEA, 2005). Furthermore, runoff from agricultural fields can carry sediment to water bodies, which degrade habitat quality by increasing turbidity and siltation (Revenga et al., 2000). The Apparent fertilizer consumption is an indirect indicator of the pressure of agricultural practices on freshwater ecosystems in Mexico. This indicator is widely used by international organizations like the United Nations and environmental ministries or agencies of several countries, including Mexico. This indicator is included in the chapter on Water under the section Quality.

The use of synthetic pesticides in agriculture increased significantly worldwide in the last fifty years. Although agriculture is the activity with the highest pesticide consumption, these are also used in industry, forestry and urban areas. Pesticides enter surface water bodies either dissolved or suspended in runoff, adsorbed to soil particles, by atmospheric deposition, or by direct application to water bodies (Shortle et al., 2001; Schiermeier, 2003). Although their effects depend on the organism and pesticide used, in general the following effects have been documented: cellular and DNA damage, cancer, tumors and lesions in fish and other animals, immunosuppression, endocrine disruption, teratogenic effects (physical deformities, such as hook-shaped beaks in birds), inhibition or disruption in reproduction and general deterioration of the health status of fish (EPA, 1992; Ongley, 1996; Shortle et al., 2001; Gevao and Jones, 2002; Silk and Ciruna, 2004). The effects of pesticides, however, go beyond the individual level, affecting the entire food chain of an ecosystem (MEA, 2005). Also, cases of significant loss of aquatic biodiversity due to improper pesticide use  have been reported (Ongley, 1996). The Apparent pesticide consumption is an indirect indicator of the pressure of agricultural practices on freshwater ecosystems in Mexico. This indicator is used by international organizations like the United Nations and environmental ministries or agencies of several countries, including Mexico. This indicator is included in the chapter on Water, specifically under the section Quality.

Livestock activities

Livestock activities may affect freshwater ecosystems through habitat modification and water pollution. The main wastes discharged may contain pathogens (bacteria and protozoa), organic matter (urine and manure) and sediments resulting from the erosion of soil caused by cattle (EPA, 1992; Allen-Diaz et al., 1999; Belsky et al., 1999; Baron et al., 2003). Wastes from cattle-raising reaches water bodies either through direct discharges or by surface runoff; the primary effect is eutrophication, which promotes booms of algal and bacterial populations that consume dissolved oxygen and reduce its availability for the rest of the flora and fauna. It also reduces photosynthesis in submerged vegetation (by increasing turbidity), which affects all levels of the food chain. The Livestock population serves as an indirect indicator of the pressure of cattle-raising on freshwater ecosystems in Mexico. This indicator is included in the chapter on Water, under the section Quality.

Municipal and industrial wastewater

Wastewater from urban centers that pollute water bodies come from households, buildings and urban runoff collected in sewerage. The primary pollutants in wastewater include nutrients (mainly nitrogen and phosphorus), pathogens (bacteria and viruses), heavy metals, biodegradable organic matter, synthetic organic chemicals, hormones and pharmaceuticals (Silk and Ciruna, 2004). Nutrients from fertilizers and detergents, animal wastes and decaying organic matter cause eutrophication in receiving water bodies (Revenga et al., 2000; EEA, 2003; Silk and Ciruna, 2004). Heavy metals, mostly from the operation and maintenance of motor vehicles, can accumulate in aquatic organisms, affecting food webs, species abundance and community structure (Revenga et al., 2000; Tarras-Wahlberg et al., 2001). Organic matter consumes dissolved oxygen in water and reduces its supply for flora and fauna. In addition, it releases ammonium, which is converted into ammonia after natural chemical reactions and poisons fish and other organisms (Revenga et al., 2000). Hormones and other drugs accumulate in the food chain, causing carcinogenic effects and disrupting the endocrine system of aquatic organisms (Ludwig in Silk and Ciruna, 2004). Last, synthetic organic chemicals from detergents, cleaning products, fertilizers, pesticides, paints and oils from household wastewater can also cause morphological, physiological and behavioral alterations in many aquatic organisms (Silk and Ciruna, 2004).

Industrial discharges that pollute freshwater bodies come from the chemical, sugar, mining, petroleum, iron and steel, pulp, paper and textile industries, among others. Pollutants from these sources reach water bodies by direct discharge or from atmospheric deposition, as is the case of some heavy metals and synthetic organic compounds (Silk and Ciruna, 2004; UNEP, 2007). These include heavy metals (particularly lead, cadmium, chromium, mercury and zinc), organic compounds (benzene, toluene, xylene and other synthetic chemicals such as dioxins, furans, polychlorinated phenols and polynuclear aromatic hydrocarbons), pesticides, petroleum, fats and oils (Folke, 1996; Culp et al., 2000; EEA, 2003; Silk and Ciruna, 2004). Industrial wastewater can also cause thermal pollution of water bodies when cooling water is discharged into them (Revenga et al., 2000; Silk and Ciruna, 2004). The effects on aquatic biota may be indirect, mainly due to habitat degradation (thermal and chemical pollution of both water and sediments), or directly in organisms by alterations in metabolism, growth, reproduction and the endocrine system, as well as carcinogenic side effects. Since some of these compounds accumulate in the tissues of organisms, their effects affect the entire food chain, eventually impacting the dominance, abundance and natural diversity of freshwater ecosystems (EPA, 1992; Folke 1996; Revenga et al., 2000; Culp et al., 2003; EEA, 2003; Silk and Ciruna, 2004). The indicators Municipal wastewater discharges and Industrial wastewater discharges show the pressure of urban centers and industries on freshwater communities living in the country’s water bodies where wastewater is discharged. Both indicators are treated separately in the chapter on Water under the section Quality.

Dams and reservoirs

Dams and reservoirs store water for agricultural, domestic and industrial uses, control of water flows that could cause flooding, and are indispensable for power generation (Revenga et al., 2000). However, these have affected the biodiversity of freshwater ecosystems worldwide (WCMC, 1998; Baron et al., 2003; Revenga and Kura, 2003; Baillie et al., 2004; MEA, 2005, UNEP, 2007). The construction of dams and reservoirs have effects both upstream and downstream. These indirectly affect biodiversity of freshwater systems by alterations in water quality, changes in temperature, transportation of chemicals, nutrients and sediments, accumulation of heavy metals and eutrophication, in addition to the lower availability of nutrients for communities living downstream (Arriaga et al., 2000; Baron et al., 2003; Revenga and Kura, 2003; MEA, 2005; UNEP, 2007). Dams and reservoirs directly reduce the volume, duration and frequency of water flows in rivers, leading to the rapid disappearance of waterfalls, riparian vegetation and wetlands (Berkamp et al., 2000; Dynesius and Nilsson in Revenga et al., 2000; MEA 2005). In addition to the fragmentation of these ecosystems, the migration patterns of some fish species are also affected, ideal habitats for invasive species are created and coastal ecosystems may be affected, with repercussions on fisheries that depend on rivers for nutrient supply (WCMC, 1998; Revenga et al., 2000; UNDP et al., 2000; Groombridge and Jenkins, 2002; Revenga and Kura, 2003). Many groups of plants and animals are affected by dams and reservoirs, particularly microorganisms living in sediments, plankton, invertebrates, algae, bryophytes, vascular plants, insects, fish, birds and mammals (Arriaga et al., 2000; Berkamp et al., 2000; Groombridge and Jenkins, 2002, Baron et al., 2003; MEA, 2005). The Fragmentation of rivers by large dams is an indicator of the loss of continuity of freshwater ecosystems as a result of the construction of dams. However, this indicator is not included because there is no information to calculate it; instead, only the indicator Water storage capacity of major dams is shown as a measure of the pressure exerted by this factor on the country's freshwater ecosystems. This indicator is included in the chapter on Water under the section Availability.

Water extraction for consumptive uses

The extraction of water from rivers, lakes and ponds for agricultural, industrial and urban uses may seriously harm freshwater ecosystems (Poff et al., 1997; WCMC, 1998; UNDP et al., 2000; Groombridge and Jenkins, 2002; Revenga and Kura, 2003; MEA, 2005; UNEP 2007). In lakes and ponds, water extraction can reduce the volume stored, whereas in the case of rivers and streams it may change the magnitude, frequency and duration of water flow. Changes in the level of water bodies lead to the loss of wetlands and riparian communities (Poff et al., 1997; Revenga and Kura, 2003), disruption of the flow and movement of nutrients and sediments, changes in the abundance and diversity of many species of aquatic plants, invertebrates and fish associated with riparian environments (Welcomme, 1992; CSD, 1997; Poff et al., 1997; UNDP et al., 2000). The establishment of invasive species and their effect on the displacement or extinction of native species documented is another documented result of the changes in the magnitude and frequency of water flows and volumes (Busch and Smith, 1995). In the case of rivers, water extraction can disrupt the life cycle of many species, because flow cycles serve as signals for spawning, hatching, nursing, feeding, reproduction and migration of different species (Poff et al., 1997). Total water extraction for consumptive uses is an indicator of the pressure exerted by of water demands for agricultural, industrial and urban development on the country's freshwater ecosystems. This indicator is part of the chapter on Water under the section Availability.

Fishing

Fisheries in inland waters are an important food source for many communities in Mexico and elsewhere, especially for those in poor areas. Besides fish, other animals caught include mollusks and crustaceans (Kura et al., 2004; FAO, 2004). The primary effect of inland fisheries is the removal of specimens from natural populations of target species, which affects the population size, sex ratio, reproductive characteristics and potential, and genetic composition of exploited populations (Harvey, 2001; Garcia et al., 2003; Bjorkland and Pringle, 2004; FAO, 2004; UNEP, 2007). When extraction is excessive, populations may recover slowly and may become genetically impoverished or even extinct, either locally and globally. Indirectly, fishing also affects ecosystems where species of commercial interest live, mainly by altering the food-chain flow and dynamics, and by the removal of species important for the ecosystem (Harvey, 2001; García et al., 2003; Bjorkland and Pringle, 2004; UNEP 2007). Examples of changes in the abundance and species composition associated with inland fisheries have been documented worldwide (see examples in Bjorkland and Pringle, 2004). The National inland fish catch is an indicator of the pressure of inland fisheries on natural populations of commercial species and on their ecosystems. OECD considers this a key environmental indicator, and it is also used by the environmental ministries or agencies of many countries, including Mexico. However, since there are no reliable data to calculate this indicator, it was not included in the publication.

Invasive species

The introduction of species is, after habitat degradation, one of the major causes of extinction of species in inland aquatic ecosystems (Richter et al., 1997; Groombridge and Jenkins, 2002; Revenga and Kura, 2003; Mendoza et al., 2007). These species have been introduced either intentionally for aquaculture, sport fishing or biological control, or released inadvertently from aquaculture activities (from dams or reservoirs and natural water bodies) or from ballast water of ships or other vessels (WCMC, 1998; Revenga et al., 2000; UNDP et al., 2000; Ciruna, 2004). The most frequently invasive species include fish, invertebrates (mainly mollusks and crustaceans) and macrophytes. Invasive species affect the native flora and fauna through habitat degradation, predation, competition, parasitism, introduction of diseases and pollution, reduction and the gene pool, and result in the short-, mid- or long-term reduction in the biological diversity of freshwater ecosystems (UNDP et al., 2000; Baron et al., 2003; Revenga and Kura 2003; Bjorkland and Pringle, 2004; Ciruna, 2004; Aguirre et al., 2009). Examples of ecological impacts in these ecosystems by invasive species have been widely documented (see examples in Revenga and Kura, 2003; Ciruna, 2004; Aguirre et al., 2009). The indicator Invasive species in national freshwater ecosystems shows the pressure of this issue on ecosystems in these water bodies.

Oil activities

Oil and natural gas activities in continental areas affect freshwater ecosystems mainly through the deterioration of water quality (E&P Forum-UNEP, 1997; EPA, 1999; Carabias and Landa, 2006). Water pollution derives primarily from leaks in the pipeline network or by regular discharges during oil operations. Wastewater discharged carries hydrocarbons, dissolved salts, drilling sludge, organic compounds and heavy metals, including toxic substances (E&P Forum-UNEP, 1997). Most of the aquatic biological communities and species inhabiting these water bodies are very susceptible to direct contact with the hydrocarbons and their water-dissolved derivatives (E&P Forum-UNEP, 1997; EPA, 1999; NOAA, 2002). The indicator Hydrocarbon leaks and spills and pollutant discharges in inland waters reveals the pressure exerted by the wastes and products associated with oil activities on these ecosystems. This indicator is in the UN list of Sustainable Development Indicators.

Global Climate Change

Global climate change, resulting from the intrinsic variability of the climate system and of external factors (either natural or derived from human activities), has had important effects on the hydrological cycle, which in combination with other anthropogenic disturbances has led to significant impacts on the biodiversity of various ecosystems, including freshwater systems (IPCC, 2001; 2002, MEA, 2005). The effects reported include the rise in water temperature, reduction of the extension and thickness of ice masses; decreased dissolved oxygen in deep waters of lakes and ponds; changes in the interaction between rivers and their watersheds, and in biogeochemical cycles; and increased frequency of extreme events, including floods and droughts (Gitay et al., 2001; SCBD, 2003; Groisman et al., 2004). This has also affected the species living in freshwater environments, either through changes in the dates of onset or end of the breeding season, in the areas of distribution or in migration patterns, among others (IPCC, 2002). To date, no indicator or set of indicators have been proposed for an accurate assessment of the effect of this factor on freshwater ecosystems.