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CADDIS Volume 1: Stressor Identification

Step 1: Define the Case

Step 1: Define the Case.

Figure 1-1. Illustration of where Step 1: Define the Case fits into the Stressor Identification process.

The first step of the Stressor Identification (SI) process is to define the subject of the analysis (i.e., the case), by determining the geographic scope of the investigation and the effects that will be analyzed. The case definition sets the foundation for the rest of the causal analysis: it influences the information that will be assembled, the causes that will be considered, and the way in which conclusions will be presented. For this reason, it is important to get input from managers and stakeholders at this early stage of the process.

Causal analysis is triggered by the observation of a biological effect, including:

  • Kills of fish, invertebrates, plants, domestic animals, or wildlife;
  • Anomalies in any life form, such as tumors, lesions, parasites, or disease;
  • Changes in community structure, such as loss of species or shifts in species abundance (e.g., increased algal blooms, loss of mussel species, increases in tolerant species);
  • Response of indicators designed to monitor or detect biological condition, such as the Index of Biotic Integrity (IBI) or the Invertebrate Community Index (ICI);
  • Changes in organism behavior;
  • Changes in population structure, such as population age or size distribution;
  • Changes in ecosystem function, such as nutrient cycles, respiration, or photosynthetic rates;
  • Changes in the area or pattern of different ecosystems, such as shrinking wetlands or increased sandbar habitats.

In many cases, the observed biological effects are linked to a biological impairment as defined by a regulatory authority. For example, a stream reach may not meet standards for a designated use (e.g., fishing) or it may not make its specific aquatic life-use class designation assigned by the state.

The first part of defining the case involves defining both the biological impairment and the specific biological effects that led to that impairment. For example, as mentioned above, the biological impairment triggering causal analysis may be failure to meet a state's "Class C" water quality standards. The specific biological effects leading to that impairment may be decreases in brook trout or decreases in the abundance of larval mayflies, stoneflies, and caddisflies (EPT taxa). Describing the impairment in terms of what is actually happening biologically makes it easier for you to consider the mechanisms behind the impairment, and allows you to draw on supporting evidence from other areas (e.g., similar situations in other locales, scientific literature, etc.).

The second part of defining the case is determining its geographic scope. The geographic scope of the case has two parts: (1) the impaired stream reach (or similar stream part), and (2) other sites within the same aquatic system (e.g., the same stream, watershed, bay, or reservoir) that are either unimpaired or impaired in a different way, and thus may be used for comparison. Whenever possible, all locations within a case should be part of the same system. They also should be located relatively close together, and aside from anthropogenic effects, should be as similar as possible physically, chemically, and biologically. Developing a map of the areas making up the case may make it easier for you to clearly outline the case's geographic scope.

The third part of defining the case is describing the assessment's objectives. Objectives are influenced by the management context of the investigation, and often focus on identifying which of many candidate causes are limiting biological condition.

Once the case has been defined, you move on to Step 2: List Candidate Causes.

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Next: In-Depth Look | Continue to Step 2 | Step-by-Step Guide Introduction

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