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Report on the Environment

Critical Physical and Chemical Attributes

What are the trends in the critical physical and chemical attributes of the Nation's ecological systems?

Physical and chemical attributes influence and sustain ecological systems. Critical physical attributes include temperature, light, and hydrology (rainfall, soil moisture, flow rates, and sea level), as well as infrequent physical events that reshape ecological systems, such as fires, floods, and storms. Examples of critical chemical attributes include oxygen, nutrients, pH, salinity, and the presence of other chemicals in the environment.35 Together, these attributes have driven the evolutionary history of species, and they continue to drive ecological processes, shape the conditions in which species live, and govern the very nature of ecological systems.

Species have evolved within particular physical and chemical environments. These are characterized by mean (i.e., long-term average) conditions as well as by fluctuations on time scales of a day (e.g., tidal and light/dark cycles), seasons (e.g., temperature and hydrological cycles), years (e.g., periodic climatic and fire events), and longer time scales. The occurrence of ice ages every 40,000 to 100,000 years reflects one of the longer time scales. Because critical physical and chemical attributes influence so many aspects of ecological systems, small changes in average conditions or changes in temporal variations can potentially have large effects on the extent and distribution of ecological systems and on the biodiversity of these systems.

Average conditions and the degree and periodicity of fluctuations in physical and chemical attributes vary over the surface of the globe, and species have evolved with specific niche requirements that reflect the physical and chemical states of the ecological systems in which they live. For this reason, a species that has evolved in tropical waters would have temperature requirements that are higher and narrower (the species is less able to tolerate fluctuations) than a species that has evolved in temperate waters where temperatures are lower and more variable. Reproduction and other activity patterns of species are often related to physical and chemical cues such as temperature, light, and salinity. Because species have evolved coincident with the presence (or absence) of physical disturbances, reproductive strategies may be linked with the occurrence of events that otherwise appear destructive. Thus, disturbances such as periodic fires or flooding may be essential for sustaining certain species and ecological systems where these disturbances have been present over evolutionary time scales.

Critical physical attributes reflect, in part, the influence of solar radiation. Solar radiation warms land and water masses and drives hydrologic cycles. The amount of light reaching the surface of the Earth and penetrating into its waters determines levels of photosynthesis, which is essential to the support of biological systems. Other examples of physical, chemical, and biological processes that are influenced by the amount and periodicity of light include temperature and weather conditions, photoactivation of chemicals, mutations, and the timing of reproductive cycles. Solar radiation can also have potentially harmful effects on some species. Light regimes can be influenced by changes in solar energy reaching the earth, changes in the transparency of water, and changes in sea level, which in turn can change the degree of light penetration reaching the sea floor, coral reefs, and kelp forests. The implication of climate change for changes in many aspects of ecological condition has received broad attention.36,37

EPA has been actively involved over its three decades in assessing and managing factors that alter the critical chemical and physical characteristics of ecological systems (e.g., temperature, pH, electrochemical [redox] potential, and the transparency of air and water). For example, the use of water for cooling purposes can result in temperature increases in receiving waters of a river, acid rain can lower the pH levels of lakes in sensitive regions, and wastewater and fertilizer can lead to low redox potentials, which affect biological communities and the cycling of both toxic and non-toxic materials. Although EPA is not directly involved in the control of hydrology—an important physical factor in the environment—hydrology greatly influences the fate and transport of pollutants in aquatic ecosystems. Changes in such factors as the amount of runoff or snowpack can affect ground water levels as well as flows into streams and rivers. Flood control efforts can alter flooding and sedimentation processes that sustain particular types of systems. Because ground water is a primary source to surface water bodies in many parts of the nation, changes in the quantity (water level) and quality of ground water influence ecological conditions not only in the hyporheic zone (below and adjacent to the stream bed) but also in surface waters. The potential impacts of climate change (whether natural or human-induced) have important consequences for virtually every aspect of ecological structure and function.

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