Wading for Bugs: Exploring Streams with the Experts

Biological Assessment: Using Biological Indicators to Evaluate the Health of a Waterbody

138 Biological Assessment: Using Biological Indicators to Evaluate the Health of a Waterbody Why is biological assessment of a waterbody important? Because aquatic organisms are adapted to the environments in which they live, their long-term exposure to what is happening in their individual habitats can give telltale signs of waterbody health. For instance, sensitive organisms will not withstand slugs of chemicals or other human activities that alter their habitats. These organisms are considered indicators because they would be eliminated or forced to move if they were exposed to adverse conditions. Aquatic organisms are analogous to the canaries that miners took into the mines with them many years ago; the canaries’ negative reactions to lowered oxygen were warning signs to the miners. Similarly, we use biological indicators by comparing what we might expect in an ideal, healthy system to the absence or reduced numbers of aquatic organisms at sites we are studying; this information provides warning signs about the condition of a water resource. In contrast to sensitive organisms, tolerant species (or taxa) adapt readily to changes in habitat or water quality; sometimes the abundance of these organisms can increase dramatically while more sensitive ones disappear. The concept of biological assessment, known as bioassessment, is based upon the presence or absence of expected taxa, proportions of sensitive or tolerant organisms, diversity, and abundance. Invertebrates, like those in the stories of this book, are excellent indicators of water quality for many reasons: they don’t move around very much (relative to fish and larger organisms), they represent a broad range of sensitivity to the many things that may pollute or alter an aquatic environment, and they are easy to collect. Biological assessment can be a proactive or reactive exercise. In the former, we might ask questions such as, “What is the condition of my river?”, or Biological Assessment 139 “How do I know what should be in my stream before the planned housing development begins?” Questions that are reactive would be, “What is killing all of the fish in my stream?”, or “Why does my lake smell so bad?” For all of these questions, collecting invertebrates in a bioassessment can be helpful. Identifying these organisms and recording abundance will help assess the condition of a waterbody. By contributing to a long-term base of information, these data will help measure changes (good or bad) over time. How is a biological assessment designed? There are many different designs for a bioassessment, ranging from simply sampling a stream in your backyard to a monitoring program of an entire river basin, lake system, or extending to the waters within a county, state, or region. Broad monitoring programs are usually conducted by water quality agencies. Their mandates are to assess the conditions of their jurisdictional waters, protect the designated quality level, identify where pollution or other perturbation is occurring, and determine what to do about it. Other designs for bioassessment are more local and usually targeted to a specific location, area, or watershed. These designs usually incorporate a few sites that are spread out around a suspected source of pollution or an area of community interest such as a park, a neighborhood, etc. The various sites may be similar or show gradients of condition (for example, they can range from very good to pretty bad). In both of these comparisons, knowledge of a waterbody’s expected conditions is needed. This is determined by sampling “reference” sites—nearby locations that have not been affected by any major land development or sources of pollution. Once we know what occurs in these reference sites, we have a pretty good idea of what should be found in similar waterbodies within that region. In the United States, water quality agencies use bioassessments regularly and have developed a system of reference sites for waterbodies within their jurisdictions. 140 Biological Assessment Communicating with appropriate water quality agencies can help you locate reference sites, and will help in various ways as you conduct bioassessments. What kinds of methods are used for conducting bioassessments? Methods for conducting bioassessments of invertebrates are typically very simple. They normally consist of using nets in streams and along the shores of rivers or lakes. Generally, a bioassessment aims to look at all of a system’s invertebrates in order to capture the biodiversity, or the composition, of the community. Methods may focus on those habitats that are most likely to be inhabited by invertebrates, such as...