Freshwater acidification due to acid rain can lead to a decline in, and loss of, fish populations. Below pH 4.5 no fish are likely to survive. Fish loss is occurring in many countries, including Scandinavia, Scotland, Wales and North America. A decrease in pH is often associated with an increase in toxic metal availability, being particularly true for aluminium and mercury. Decreased pH and elevated aluminium have been shown to increase fish mortality, decrease fish growth, decrease egg production and embryo survival, and result in physiological impairment of adult fish. In general, embryos, fry, and juveniles are less acid-tolerant than adult fish. Aluminium can precipitate onto fish gills, inhibiting diffusion and resulting in respiratory stress.
Acid deposition is a possible cause of declines in amphibian populations. The larval stages of aquatic amphibian species are most affected by acidic water. Many frog species use temporary ponds, but these tend to be small and shallow, and are easily affected by precipitation chemistry because their only sources of water are rainfall and snowmelt. Frogs that use large, permanent bodies of water for breeding generally lay their eggs in the summer, so they do not experience the acid pulses from snowmelt. However, the eggs and larvae of these species are even more sensitive to subtle changes in pH levels than those of species that breed in the temporary ponds. As for fish, the toxic effect of decreased pH levels on amphibians is complicated when concentrations of metals, such as aluminium, in the water increase, but as a general rule, embryos of sensitive amphibian species are killed by water with a pH of 4.5 or lower, while embryos of tolerant species can survive down to a pH of 3.7.
Fluoride and heavy metals can accumulate in acidified soil to levels that are toxic to soil invertebrates. Species sensitive to metals are replaced by ones that are more metal-tolerant. For example, soft-bodied species such as earthworms and nematodes seem to be more readily affected by elevated metal concentrations. Invertebrates play an important role in forest floor litter decomposition. As forest floor litter builds up, mineral release is delayed, and the availability of nutrients to plants is reduced. Herbivores are ultimately affected when the quantity or quality of their food supply decreases.
Although birds and mammals are not directly affected by water acidification, they are indirectly affected by changes in the quantity and quality of their food resources. Some birds such as the osprey find difficulty in living around an acidified lake because there are far fewer fish to be found. However, the diver finds hunting easier in an acid lake because the water of an acid lake is clearer than that of a normal lake. In Scotland, Otters are quite rare around acidic streams and rivers, as their main food supply, fish, is reduced.
Calcium is an essential element for both mammals and birds. An adequate dietary supply is crucial during reproduction. Birds need calcium for the proper formation of eggshells and for skeletal growth of hatchlings, and mammals need calcium for skeletal development of foetuses. Many invertebrate species that contain high concentrations of calcium, such as molluscs and crustaceans, are very sensitive to pH levels and are among the first to disappear during the acidification of wetlands.