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Report on the Environment

Physical and Chemical Attributes

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

Importance of Physical and Chemical Attributes

Physical and chemical attributes influence and sustain ecological systems. They 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.

  • Critical chemical attributes include levels of oxygen, nutrients, pH, salinity, and other chemicals in the environment.1
  • Critical physical attributes include temperature, light, and hydrology (such as rainfall, soil moisture, flow rates, and sea level), as well as infrequent events that reshape ecological systems, such as fires, floods, and storms.

Physical attributes reflect, in part, the influence of solar radiation. Solar radiation warms land and water masses, drives hydrologic cycles, and supports photosynthesis (which is essential to support biological systems).

Physical, chemical, and biological processes that are influenced by the amount and timing of light include temperature and weather conditions, photoactivation of chemicals, mutations, and the timing of reproductive cycles. Solar radiation can have potentially harmful effects on some species.

Physical and chemical attributes vary across the nation, and species have evolved with specific physical and chemical requirements that reflect the particular physical and chemical states of the ecological systems in which they live. For this reason, a species that has evolved in tropical waters where the annual temperature range is relatively small is less able to tolerate temperature fluctuations than a species that has evolved in temperate waters where the temperature range is relatively large and more variable.

Reproduction and other activity patterns of species are often related to physical and chemical cues such as temperature, light, and salinity. They may also be linked to physical disturbances (such as periodic fires or floods).

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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 of, distribution of, and biological diversity within ecological systems. Factors that alter the critical chemical and physical characteristics of ecological systems include temperature, pH electrochemical (redox) potential, and the transparency of air and water. Activities and events that can alter physical and chemical characteristics include climate change, water contamination, and changes in water runoff, snowpack, ground water, and light regimes.

  • Climate change. Whether natural or human-induced, changes in climate affect virtually every aspect of ecological structure and function.
  • Water contamination. Water used for cooling purposes can result in temperature increases in river waters. Acid rain can lower the pH levels of lakes in sensitive regions and make them so acidic that certain fish species cannot survive. Wastewater and fertilizer can lead to low dissolved oxygen concentrations in water, which affect biological communities and the cycling of both toxic and non-toxic materials.
  • Changes in the amount of water runoff or snowpack. These can affect ground water levels, as well as water flows in streams and rivers. These changes can lead to flooding and drought.
  • Changes to ground water. Because ground water is a primary source of water in 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 streams, but also below and beside the stream bed, where many aquatic organisms live.
  • Changes to light regimes. For example:
    • Changes in sunlight reaching the earth's surface because of haze can affect the growth of forest canopy and understory.
    • Changes in the transparency of water caused by sediment and turbidity can affect submerged aquatic plants in lakes.
    • Changes in sea level can affect coral reefs in the ocean.

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ROE Indicators

Ten ROE indicators are available for this question, providing insight into trends for some of the critical physical and chemical attributes of ecological systems. Nine are national-scale indicators (Lake and Stream Acidity, Nitrogen and Phosphorus in Agricultural Streams, Nitrogen and Phosphorus in Large Rivers, Nitrogen and Phosphorus in Wadeable Streams, Sea Level, Sea Surface Temperature, Stream Flows, Streambed Stability, and Temperature and Precipitation) and one is a regional indicator (Hypoxia in Gulf of Mexico and Long Island Sound).

Attributes not covered by the current indicators include solar radiation over land and water as well as penetration into the nation's waters, disturbance regimes associated with flooding and fire, water levels in lakes, the amount of snowpack or ground water available to support base flow in rivers and streams, and soil quality such as salinity or base cation saturation.

Information in the indicators represents baseline, decadal, and even century-level trends. However, for hydrologic and temperature patterns, these time periods may be too short to assess long-term changes. The field of paleoclimatology offers some promise for extending information to longer time frames.2

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[1] Information on nutrients and potentially toxic chemicals is presented in the Air, Water, and Land sections.

[2] National Oceanic and Atmospheric Administration. 2003. North American drought: A paleo perspective.

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