The effects of natural acidification processes and the acceleration by human land use and 'acid rain' are not the main focus of this article. However, the most serious consequences are summarized in the following sections.
With decreasing acid deposition since the 1980s in Europe and North America, the impacts of acidification and eutrophication are foreseen to show a decrease, as a consequence of biodiversity showing some recovery. However, a full return to pre-pollution conditions is not to be expected, because of changes in competition patterns and distribution of species. The introduction - whether voluntary or accidental - of species alien to European ecosystems or to other regions of Europe represents an increasing risk, favored by globalization of trade, exchange, and transports. Thus, even if chemical parameters (e.g., nutrient status, acidity) return to pre-pollution condition, indigenous species might not be successful in competing with well-established alien species.
The monitoring of soil acidification induced by 'acid rain' is difficult because of the long time frames involved and the parallel development of natural acidification processes. Both natural and anthropogenic-driven soil acidification can result in extremely low base saturation, low pH, and low ANCaqua. Anthropogenic soil acidification due to 'acid rain' is characterized by high soil solution concentrations of SO4— and NO—. In contrast, the anions responsible for H+ and aluminum leaching during natural acidification processes are organic anions and HCO—. High N and/or SO4— deposition have resulted in N and SOij— accumulation in forest soils, decreases in forest soil pH, and leaching of base cations.
Despite the lack of evidence for direct effects, there is no doubt that 'acid rain' has complex negative effects on forest ecosystems in central Europe and northeastern America. More recently, serious effects have also been recognized in China, particularly in the industrialized regions such as the Sichuan Province now exploiting nearby extensive coal deposits of high S content.
In many areas, leaching of base cations from soils has led to nutrient deficiency, especially of Mg2+ and K+. Today, many forest soils of central Europe have low base saturation and low pools of exchangeable nutrient cations. The Mg-deficient nutrition of trees (yellowing of needles) is a widespread phenomenon and has been related to decreased concentrations of Mg2+ in the soil solution.
Besides loss of nutrient cations, the buffering of H+ from deposition causes increased levels of Al in soil solutions, which might have detrimental effects on tree root growth and nutrient uptake. In water culture experiments, the ratio of Ca/Al and Mg/Al, rather than the Al concentration itself, largely determines the deleterious effect on roots.
Acidification is largely a problem in naturally nutrient-poor lakes and streams. Some organisms are directly sensitive to low pH values, for example, shell-bearing organisms such as mollusks, mussels, and many crustaceans, including crayfish (dissolution of CaCO3). Acidification and the elevated levels of dissolved Al ions in water can have direct harmful effects on the eggs and fry of many fish species (e.g., salmons, trout, and perches). Further, the number of phytoplankton species falls dramatically in acidified waters.
Al compounds can precipitate in the gills of adult fish and lead to suffocation (mainly 'mechanical' effect). The disappearance of fish species or other animals can have an influence on the food chains in a river or lake, and can thus have dramatic effects on the ecosystem structure as a whole. Fish-eating birds, such as divers, merganser, and osprey, are put under pressure, while insect-eaters, such as goldeneye, are favored.
Another effect can be shifts in plant communities. Certain mosses are favored by acidification. In nutrient-poor lakes, plant species such as shore weed or water lobelia, living in shallow waters close to lake shores, are overgrown by bog mosses.
Last but not the least, 'acid rain' can also have negative effects on human health, either directly (e.g., SO2) or indirectly through ground-level ozone formation followed by
NOX emissions. Further, the acidification of groundwater and drinking water supplies can be a direct human health hazard through high NO^ concentrations or indirectly through metals (Al, Cd) released from the soils and/or water pipes.
leading to slower growth rates and more fragile skeletal structures. How this will affect ecosystem community structure and the marine food web is unclear at present.
See also: Adsorption; Philosophy of Ecology: Overview.
Atmospheric N deposition very often leads to an excess of N in terrestrial and aquatic ecosystems, leading in general to increased growth. However, imbalances can be the consequence: tree crowns can grow faster than the root systems (increased risk of desiccation). Furthermore, depletion of other nutrients such as Mg2+ or phosphorous in acidified soils can counteract plant growth enhanced by excess N supply or can lead to nutrient imbalances and plant instability in spite of enhanced growth. Excess N input in forest ecosystems can change the occurrence of mycorrhizal fungi living in symbiosis with trees and support nutrient uptake by plant roots.
In some relatively nutrient-rich freshwaters atmospheric N deposition can contribute to eutrophication. However, usually phosphorus is the nutrient that limits growth in freshwater ecosystems.
Ocean acidification could have profound effects on ocean ecosystems' structure, food chains, population dynamics, and nutrient cycles. It is so far unknown how, for example, tropical and subtropical coral reefs and fisheries will respond to this man-made acidification. Corals, calcareous phytoplankton, mollusks, and other marine organisms use CaCO3 in seawater to construct their shells and skeletons. Some shallow-water animals, which play a vital role in releasing nutrients from sediments, also calcify. In a more acidic environment, it becomes difficult to secrete CaCO3,
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