Fact Sheet for Water Quality Managers EPA/822F-16/002 February 2016
A Practitioner's Guide to the Biological Condition Gradient: A
Framework to Describe Incremental Change in Aquatic Ecosystems1
Background
The Biological Condition Gradient (BCG) is a scientific framework that can be used by states, tribes,
territories and counties (hereafter referred to as states) to interpret biological responses from the
cumulative effects of stressors for all types of water bodies. The BCG was proposed in 2006 as a
conceptual framework developed by EPA in partnership with scientists from states, USGS and the
academic community. This technical support document describes the steps for developing a quantitative
BCG model based on over 10 years of piloting the tool with states in a scientifically defensible manner.
This documentation will assist other states in developing a reliable and robust BCG to support their
Clean Water Act programs.
Who will use the BCG and for what purpose?
Currently most states are using biological assessment information to support their water quality
management programs. The BCG contributes to the EPA biological assessment and criteria toolbox,
which includes biological indices, models, and statistical approaches and guidance. The BCG builds upon
and complements these approaches to provide a more refined and detailed measure of biological
condition and will help states to:
• More precisely define and measure biological condition for specific waters
• Identify and protect high quality waters
• Evaluate potential for improvement in degraded waters and track improvements
• Develop biological criteria
• Clearly communicate the likely impact of water quality management decisions to the public
These applications support CWA programs such as 305(b) assessments and reports, 303(d) listing of
impaired waters, and TMDL program implementation. The document describes how to develop a BCG
and includes examples of how states are using, or considering using, it to support their programs.
How are BCGs developed?
• There are a series of steps described in the document that utilize available data and local
expertise to develop narrative and numeric decision rules to assign different sites with varying
biological condition to the 6 levels of the BCG.
• In this process, States convene an expert panel to evaluate biological assessment information
and determine how 10 characteristics of an aquatic ecosystem, defined as attributes, change in
response to increasing levels of stress, from a natural or undisturbed/minimally disturbed
condition to a severely altered condition.
• The 10 attributes address the presence of pollution sensitive or tolerant species, historical or
non-native species, as well as organism condition, ecosystem function and connectance.
• Statistical and modeling approaches are then used to establish numeric rules. These rules are
tested, refined, and the underlying logic documented so the rules can be applied in the future in
a transparent manner without needing to reconvene the expert panel.
1 EPA document number: EPA 842R1600, can be accessed at: http://www.epa.gov/wqc/biological-assessment-technical-
assistance-documents-states-tribes-and-territories
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Fact Sheet for Water Quality Managers
EPA/822F-16/002 February 2016
The Biological Condition Gradient:
Biological Response to Increasing Levels of Stress
Levels of Biological Condition
Level 1. Natural structural, functional,
and taxonomic integrity is preserved.
Level 2. Structure & function similar
to natural community with some
additional taxa & biomass; ecosystem
level functions are fully maintained.
Level 3. Evident changes in structure
due to loss of some rare native taxa;
shifts in relative abundance; ecosystem
level functions fully maintained.
Level 4. Moderate changes in structure
due to replacement of some sensitive
ubiquitous taxa by more tolerant
taxa; ecosystem functions largely
maintained.
Level 5. Sensitive taxa markedly
diminished; conspicuously unbalanced
distribution of major taxonomic groups;
ecosystem function shows reduced
complexity & redundancy.
Level 6. Extreme changes in structure
and ecosystem function; wholesale
changes in taxonomic composition;
extreme alterations from normal
densities.
Watershed, habitat, flow regime
and water chemistry as naturally
Chemistry, habitat, and/or flow
regime severely altered from
natural conditions.
How is this document organized?
• Chapters 1 and 2: Purpose and scientific underpinnings of the BCG
• Chapters 3 and 4: How to quantify the BCG Y axis (the biological levels of condition that span
undisturbed to severely altered conditions)
• Chapter 5: How to define the BCG X axis (Generalized Stress Axis)
• Chapter 6: Case studies in streams
• Appendix A: Supporting information on the Generalized Stress Axis
• Appendix B: Ongoing work in large rivers, estuaries, and coral reefs
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