Macroinvertebrate Study (Bugs, bugs, and more bugs!)

Home | Photo Gallery | Macroinvertebrate Diversity | Identification Key | Taxa List

Macro -- (From Greek makro, form makros long, large) Invertebrate -- (in ver te brit), biology -n. an animal without a backbone or vertebrae. Worms and insects are invertebrates, in contrast to such vertebrates as fishes, amphibians, and reptiles

Crayfish

Adult Mayfly

In short, the term macroinvertebrate refers to the group of animals without a defined backbone. Animals that are in this group include crustaceans, mollusks, worms, gastropods and insects. What makes this group of interest to the Las Vegas Wash scientists is that they serve as important biological indicators. Due to their abundance and varied sensitivities to environmental changes, they are used to assess the health of a stream. As grade control structures are built to restore wetland ecosystems in the Las Vegas Wash, we need to gauge the effectiveness of our efforts in re-creating habitat. Studying the macroinvertebrates in the stream is a great way to determine the health of the Las Vegas Wash.

Dragonfly

THE STUDY:

Dragonfly on bulrush

Purpose:
The purpose of the study is to determine how erosion control structures and associated wetlands impact the macroinvertebrate fauna of the Las Vegas Wash. This will be achieved through the documentation of the macroinvertebrate population at selected locations in the Las Vegas Wash. The results will be helpful for the prediction and planning of habitat improvement measures that may be incorporated with future grade control structures.

Sampling Sites:
Eight sites, as shown on this map, have been selected for sampling. A control site is located above the City of Las Vegas Wastewater Treatment Plant, (LW10.75). Monitoring will occur at 3 grade control structures, both up and downstream of the Pabco Weir (LW6.05 and LW5.9), Historic Lateral Weir (LW5.5 and LW5.3) and Demonstration Weir (LW3.85 and LW3.75). The eighth site is located downstream of Lake Las Vegas, near Northshore Road (LW 0.8).

Field Protocol:
During each sampling event the following parameters are documented: date, time, weather conditions, stream conditions (including estimate of flow, observed ecological factors, water temperature, dissolved oxygen, pH, specific conductance, and turbidity). Cooperating investigators collect water quality parameters for mainstream water quality monitoring simultaneously with the monthly sampling for macroinvertebrates.

Using a D-frame dip net biologists sample a variety of habitats that are present in the Wash such as, boulders, cobble, woody snags, aquatic vegetation, and sand. By 'kicking' or disturbing the substrate and sediments within approximately 1-2 ft. in front of the net, organisms are set loose and the current carries them into the net. Each sample is preserved in the field with a 70% alcohol solution. Macroinvertebrate samples are then brought back to the laboratory for identification and enumeration.

Woody debris and riffle habitats are just two of the many places macroinvertebrates can be collected

Results:
Monthly sampling began in July 2000. The results from the first 3 ½ years of the study are described below. Data collected during this period serves as a valuable baseline of the macroinvertebrate population in the Wash.

Bioassessment Year One - July 2000 - June 2001
During the first year of the study, we collected and identified 31 aquatic insect families, nine sensitive taxa and 72 total taxa. These sensitive taxa can be used to assess water quality conditions.

Bioassessment Year Two - July 2001 - June 2002
Within the second year of the study 48 families, 11 sensitive taxa and 141 total taxa were collected and identified. Cumulative macroinvertebrate results for the two study years can be seen in the table below. The two new sensitive taxa and the addition of 69 total taxa are a resultant of further baseline data yet to be gathered.

Bioassessment Year Three - July 2002 - June 2003
During the third year of the study 50 families, 11 sensitive taxa and 148 total taxa were collected and identified. Only seven new taxa were identified during this year and the number of sensitive taxa remained constant.

Bioassessment Year Four - July 2003 - January 2004
Through the fourth year of the study 50 families, 11 sensitive taxa and 148 total taxa were collected and identified. The number of total taxa between 2002-2003 remained constant and this demonstrates that a true baseline data set has been obtained.

The study is ongoing and is now conducted on a quarterly basis by Las Vegas Wash Coordination Committee member agency, U.S. Bureau of Reclamation, biologists.  Now that the baseline has been established, the continued collection and analysis of data will allow us to determine how the macroinvertebrate population in the Wash is changing as a result of stabilization and enhancement activities.