If 50 percent or more of a rock lies within the sampling area, it is removed and held in front of the net opening, and attached organisms are dislodged into the net by gently brushing the surface of the rock with the hand and then with a fingernail brush. After a rock is brushed, it is examined to determine if any closely adhering organisms, such as Leucotrichia (microcaddisfly) or Parargyractis (aquatic lepidopteran), are present. Such organisms are removed from the rock surfaces using fo rceps and placed into a separate vial holding the large-rare sample component. This sample component contains large organisms that can interfere with sample splitting and rare organisms that might be lost during sample splitting. After the large rocks (fist size and larger) are removed, the sampling area is dug to a depth of about 0.1 m. The guide rod makes an effective tool for digging to this depth. The short end of the guide rod is set to the depth criterion (0.1 m), and the long end provides subst antial leverage for digging into consolidated materials. When it is not possible to achieve the 0.1-m depth, digging is done as deeply as is practical. Any remaining organisms are dislodged into the net by kicking the substrate within the sample area fo r a period of 30 seconds. The material collected in the net is then rinsed into the bottle attached to the sampler and transferred to an appropriate container, usually a 19-L (5-gal) plastic bucket or dishpan, for further field processing. Subsequent elements of the composite sample are added to this container and then processed, or the separate elements may be processed and then composited.
Coarse substrates in water deeper than approximately 0.50-0.75 m cannot be effectively sampled using most disturbance-removal type samplers. A diver-operated dome sampler (fig. 3K) can be used in such situations. This sampler contains a battery-operated pump that empties material into a Nitex bag with 425-Ám mesh openings. The material in the dome sampler is dislodged by the diver, sucked up by the pump, and deposited in the mesh bag. As with the Slack sampler, the diver brushes invertebrates from the surfaces of the rocks first using his hands and then a fingernail brush. These rocks are taken out of the sampler and returned to the surface to remove tightly attached invertebrates. The substrate remaining in the sampler is then disturbed by hand for 30 seconds to a depth of about 0.1 m. Substrate samples are collected and returned to the surface for substrate characterization and inspection for remaining invertebrates. After the pump has cleared the dome sampler of suspended debris and invertebrates, the sampler is returned to the surface where the Nitex bag is removed and the contents washed into a suitable sample container and held for field processing.
Artificial substrates, such as rock-filled barbecue baskets (fig. 3M), may also be used in sampling coarse substrates in nonwadeable areas (Britton and Greeson, 1988). The barbecue baskets are filled uniformly with indigenous rocks from the river or with rocks that are geologically similar to the local river rocks. Filled baskets are placed on the bottom of the river within the appropriate instream habitat type and tied to floats or stream-side vegetation. The baskets are allowed to colonize for a mini mum of 6 weeks and then are removed with the aid of a 425-Ám net to catch organisms dislodged during transfer of the