Some particularly sensitive areas, such as the Adirondack Mountain region, have been more intensively sampled and the biota examined in detail for effects from acidity. To identify trends in aquatic ecosystems, present and historical survey data on water chemistry and associated biota are compared. In lakes, the chemical and biological history and pH trends may be inferred or reconstructed in some cases by examining assemblages of fossil diatoms and aquatic invertebrates in the sediment layers. In terrestrial ecosystems, vegetation damage is surveyed and effects of acidic deposition to plants and animals are determined from laboratory and field exposure experiments. Natural variation in populations and the complex interactions between acidity and other ecosystem components make it difficult to extend many of the research findings to populations or communities.
Acidity can also modify ecosystem processes such as decomposition and the flow of nutrients.
Therefore, models are often used to predict such effects by combining information on individual species' effects, population distributions, and the patterns and amounts of acidic deposition. Status and Trends Aquatic Species Research in the United States, Scandinavia, and Canada has demonstrated that acidity affects the physiology, reproduction, food resources, and habitat of aquatic species. Some reductions in sensitive species may be partially offset by increases in more acid-tolerant species, resulting in little change in the total number of organisms in the community even though the diversity of species may change. Effect of acid rain on some aquatic species. Melting snow, which accumulates the winter's deposition of acidic materials, and episodes of spring rainfall can be especially damaging to sensitive streams and lakes.