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stonefly larvae
What are Benthic Macro-Invertebrates?
Benthic macro-invertebrates are invertebrates (or animals without a backbone) that live on the bottom of streams during all or part of their life cycle. Their name sounds complicated, but it literally describes these creatures. "Benthic" means bottom dwelling, and "macro" indicates that benthic macro-invertebrates can be seen with the naked eye. Although benthic macro-invertebrates (BMI´s) often go unnoticed because of their size and habitat, they are an extremely important part of river ecosystems, and serve as a link in the food web between decomposing leaves and algae, and fish and other vertebrates.
Many benthic macro-invertebrates are the larval forms of flying insects, such as mayflies, stoneflies and caddisflies. Others are small animals that spend their entire lives in the stream, such as water mites and planarians. Some can travel between water and moist terrestrial environments, such as fresh water snails. This is a diverse group of creatures characterized not by scientific classes, but by the habitat they occupy when in the stream, and the fact that they all lack a backbone.
Fish, birds, turtles, salamanders, newts, and frogs all feed on BMI´s at different stages of their life. In fact 80% of the diet of riparian birds consists of adults insects that spend most of their life in the water as benthic macro-invertebrates. In the Napa watershed, steelhead trout are now listed as a federally threatened species, and BMI´s are a critical food source for these fish. In addition, these small creatures occupy the same habitat as developing steelhead eggs. Therefore healthy populations of BMI´s likely mean that healthy habitat is available for young steelhead.
Several characteristics of benthic organisms make them useful indicators species.
- their populations fluctuate depending on physical and chemical changes in their habitat
- they include a great diversity of species
- they do not move far during their time in the stream, and can not move out of degraded areas
- they are easily collected in streams and rivers
- there are large numbers of species that occur in every conceivable aquatic habitat
Benthic Macro-Invertebrate Data Collection
The relationship between the composition of the aquatic community, water quality, and habitat has long been recognized. For the past year the RCD biologist has been working with Friends of the Napa River on a project designed to determine the health of streams in the Napa River watershed by assessing populations of benthic macro-invertebrates (BMI´s).
Goals and Objectives:
1. To establish a macro-invertebrate monitoring program for assessing the ecological condition and trends within the Napa River basin. This program is specifically aimed at tracking water quality for supporting native fish populations.
2. To determine if particular land use activities have a detectable biological response.
3. To develop and disseminate materials for educational use.
Populations of BMI´s are monitored by collecting and identifying samples from a stream riffle. By looking at how many and what types of BMI´s are found in the sample, scientists gain an understanding of the water quality and habitat at that site. Some BMI´s are extremely sensitive to disturbances, while others are relatively insensitive. A healthy stream system will have a diverse population of BMI´s with both sensitive and insensitive species represented. If a sample is dominated by insensitive species of BMI´s, then the stream likely has a disruption to healthy functioning.
Through the Friends of the Napa River´s Benthic Macro-Invertebrate Project, a total of 35 sites throughout the Napa watershed were sampled in the fall of 1999 and spring of 2000. All samples have been sent to a laboratory in Oregon. The report will be released in late fall after review by the Scientific Advisory Committee, and will be available to all interested parties. The project has secured funding for continuation next year, and is intended to cover a five year period.
Educational Program
In addition to data collection, the Benthic Macro-Invertebrate Project has established an educational program for elementary through high school students. An AmeriCorps member developed a worksheet packet for distribution in schools, and has worked with several teachers to incorporate watershed biology into the curriculum. One elementary school class raised steelhead in the classroom for release. Friends of the Napa River and Vintage High School Environmental Science collaborated to produce a 25 minute video tape featuring benthic macro-invertebrates under a microscope and covering the collection process.
The following materials are available to educators through the Friends of the Napa River office.
- all equipment necessary to collect benthic macro-invertebrates
- a 25 minute educational video tape
- a microscope
- worksheet packets
- books...
- chemical water quality testing equipment
For more information on the educational program or for educational materials,
please contact Friends of the Napa River at (707) 254-8520.
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