The impact of acid deposition on an ecosystem depends on the ability of the system to assimilate the excess acid (usually nitrogen or sulfur), which, in turn, depends on the components of the ecosystem. Sensitivity is defined as ( Kuylenstierna et al. 1989) "the responsiveness of an important variable to an independent forcing variable". Biotic communities are usually used to indicate the level of sensitivity of an area to acid deposition. Factors used to determine the sensitivity include bedrock lithology, soil type, land use, and rainfall. These factors in combination indicate, at least crudely, the flow rate through the system and buffering capacity within the system. The critical deposition load for an ecosystem is defined as (Kamari, 1989) "the highest deposition of anions corresponding to strong acids that will not cause chemical changes leading to long-term harmful effects on essential ecosystem properties."
A given region may have some very fragile ecosystems for which the critical load is very low. As a practical matter in establishing feasible goals in reducing acid deposition, a second level of ecosystem loading is defined. The "target load" recognizes that it is impossible to not exceed background levels in order to protect every stream and lake in a particular region. A target load is a value that permits some acceptable damage but protects the majority of ecosystems in the region.
It might be said that critical loads are scientifically determined for individual ecosystems, whereas target loads are levels determined by the society's collective judgment on the level of ecosystem degradation it is willing to accept. Critical loads are determined by scientists, and target loads are value-laden terms that are determined by political-economic-social processes.
The Regional Acidification Information and Simulation (RAINS) model (Kaman et al, 1989) is frequently used as a basis for evaluating regional acid deposition and establishing target loads.
References
Kaman, J., D. F. Brakke, A. Jenkins, S. A. Norton, and R. F. Wright, Eds, 1989: Regional Acidification Models. Springer-Verlag. 306 pp.
Kamari, J, 1989: Sensitivity, load and time: Aspects of determining a target load for lake regions. In Kaman, J., D. F. Brakke, A. Jenkins, S. A. Norton, and R. F. Wright, Eds, 1989: Regional Acidification Models. Springer-Verlag. 306 pp.
Kuylenstierna, J. C. I., and M. J. Chadwick, 1989: The relative sensitivity of ecosystems in Europe to the indirect effects of acidic depositions. In Kaman, J., D. F. Brakke, A. Jenkins, S. A. Norton, and R. F. Wright, Eds, 1989: Regional Acidification Models. Springer-Verlag. 306 pp.