Once the appropriate habitat type has been identified for RTH and DTH sampling, then a suitable sampler is selected, based on the depth, velocity, and substrate conditions within the habitat type. The following recommendations on semi-quantitative sampling techniques and gear are organized on the basis of wadeability of sites and dominant substrate condition (coarse or fine-grained). Wadeable sites are generally considered to be areas where water depth is less than 1 m, whereas depths at nonwadeable sites are typically greater than 1 m. Coarse-grained substrates are defined as those that are typically dominated by medium to large gravel, cobbles, or boulders, or by bedrock. Fine-grained substrates are defined as those that are dominated by small grave l, sand, silt, or clay. As with the discussion of habitat types, the recommendations on quantitative sampling gear and procedures are matched to local conditions and modified as needed to better characterize the benthic invertebrate community. The proce dures and equipment recommended here are equally applicable to QMH sampling.
Disturbance-removal sampling techniques are the most appropriate method for sampling wadeable coarse-grained substrates with current velocities greater than 5 cm/s. These techniques involve defining a specific area, disturbing the substrate within that a rea to dislodge invertebrates into a sampler or downstream net, and then removing the larger substrate elements to acquire any specimens that are adhering tightly to the rocks. Hess samplers (fig. 3B), Surber samplers (fig. 3C), stovepipe corers (fig. 3E ), and box samplers (fig. 3O) are examples of the types of samplers that can be used.
The Slack sampler, a modification of the Surber sampler, proved very useful for sampling riffles and runs during the NAWQA Program pilot studies. This sampler, also referred to as a "Surber-on-a-stick," was developed by Keith Slack, USGS, Menlo Park, Calif. It requires a minimum of two people to operate and employs a 0.5-m wide rectangular kick-net frame to which a Nitex net with 425-Ám mesh openings is attached (fig. 5). The sampler is held perpendicular to the direction of flow and pressed tightly ag ainst the stream bottom. It may be necessary to move cobbles aside or to add a self-sticking foam strip to the bottom of the sampler (particularly when working on rock outcrops) in order to achieve a tight seal. Benthic invertebrates are collected from an area of approximately 0.25 m2 immediately upstream of the Slack sampler. The sampling area is delineated using either a guide rod or frame that attaches to the sampler. A combination guide rod and digging tool can be fashioned from 0.61 m (2 ft) of 9.5-mm (3/8-in.) diameter threaded rod (Karen Murray, U.S. Geological Survey, oral commun., 1992). Two nuts are threaded onto the rod to divide it into 0.5-m and 0.1-m lengths. One side of the sampling area is delineated by laying the rod perpendicular to the Slack sampler with the longer length facing upstream and the nuts in contact with the side of the sampler. The other dimensions of the sampling area are visually approximated. Alternatively, the sampling area can be delineated using a guide frame fashioned by bending a 6.35-mm (1/4-in.) wide by 3.18-mm (1/8-in.) thick flat aluminum strip to form three 0.5-m long arms joined at right angles. This frame is then attached to the front of the Slack sampler and used to define the sampling area.