River bugs: benthic macroinvertebrates

When trying to understand river health, aquatic biologists often look not at fish, but rather fish food: benthic macroinvertebrates (or BMI). BMI are effective integrators of physical, chemical, and biological processes and are valued as indicators of river ecosystem health due to various BMI species occupying primary, secondary, tertiary, and higher-level consumer levels in riverine food webs. They are, in turn, a critical food resource for a variety of vertebrates, including fish.

Over the last three years, Jeff Holmquist and Jutta Schmidt-Gengenbach of the University of California White Mountain Research Station have been studying BMI in the river reach downstream of O'Shaughnessy Dam as part of the Looking Downstream Project and UTREP. The goal is to develop an understanding of BMI species diversity and abundance in the Tuolumne, and characterize how BMI assemblages change with flow. Current and future work will focus on food web dynamics and long term changes in BMI assemblages resulting from implementation of ecosystem-based environmental flow recommendations for O'Shaughnessy Dam.

Jeff and Jutta recently completed the first survey in their fourth year of sampling with assistance from dam operators at Hetch Hetchy Water and Power who were able to reduce releases from O'Shaughnessy Dam to allow access to the river channel.

Top left: Larva of predaceous diving beetle from pond habitats. The larvae of some species can reach almost two inches in length. Adults are smaller, but are also aquatic predators. (Holmquist photo)

Top right: Drift net in place below O'Shaughnessy Dam in the Tuolumne River. The net catches insects that enter the water column. BMI floating downstream (known as "Drift") are a natural phenomenon and an important source of food for fishes. In addition to BMI, surprisingly large numbers of terrestrial insects are found in drift samples. A one-hour sample can include close to 1,000 insects and 125 species. (Holmquist photo)

Bottom right: A backswimmer, found in pond habitats. These insects swim on their backs and have one pair of legs specialized as oars. Backswimmers have a stinging beak that is used in prey capture. The beak is visible in the photo and runs along 3/4 of the length of the body. (Holmquist photo)

Bottom left: Tossing a throw trap in the Poopenaut pond, 3 miles downstream of O'Shaughnessy Dam. The trap encloses a known volume of water column and associated substrate, and animals are removed with a bar seine. The device is effective at catching fauna that might avoid other collecting devices. The pond has a high level of diversity and abundance, sometimes over 1,000 insects per square meter. (Schmidt-Gengenbach photo)