sampling season is a compromise based on all of the above factors and considers retrospective data and input from the liaison committees, regional biologists, North Carolina Ecology Group, local biologists, and national synthesis teams. Once the appropriate sampling seas on has been determined for each subbasin, all sites are sampled within a 3- or 4-week period. Ecological surveys in subsequent years or subsequent NAWQA Program cycles are conducted within the chosen season (consistent accumulated degree-days) or include that season to ensure year-to-year data comparability. Eventually, sampling should be expanded to include two seasonal periods (fall-winter and spring-summer) to accommodate seasonal shifts in community structure, particularly in heavily forested stream basins.
Many types of sampling equipment and various techniques have been developed for the collection of benthic invertebrate samples. The proper choice of sampling equipment and technique depends on the water depth, current velocity, and type of bed material to be sampled and on whether the sample is intended to provide qualitative or semi-quantitative data. The following sections establish guidelines for collecting samples. The study-unit biologists are responsible for applying and modifying these guidelines as conditions within their study areas warrant. These modifications are made in consultation with regional biologists, North Carolina Ecology Group, and local invertebrate biologists.
The objective of qualitative multihabitat (QMH) sampling is to obtain as complete a list of invertebrate taxa present in a sampling reach as is possible in the time available, usually about 1 hour. To accomplish this, individual samples are composited f rom as many of the 51 instream habitat types (fig. 2) as are present and accessible within the sampling reach. This composited sample, together with the semi-quantitative samples, represents the aggregation of organisms that exist in the sampling reach.
The primary sampling gear used to collect QMH samples in wadeable
streams is a
D-frame kick net (fig. a3A) equipped with a 210-Ám mesh net. This net is used to collect samples by kicking, dipping, or sweeping in a manner appropriate for the instream habitat type being sampled. QMH sampling encompasses as many habitat types as possible, including those habitat types sampled by semi-quantitative methods. When possible, equal sampling effort is applied to each habitat type within the sampling reach. This is usually accomplished by dividing the available 1-hour sampling time equally among the instream habitat types. This strategy can be adjusted to accommodate complicated collection methods, such as deep-water and diver-assisted sampling. In these situations, dividing sampling effort on the basis of the area or number of samples collected is preferred.
aPhotographs of samplers A-C, F-J, and L are courtesy of Wildlife Supply Company, Saginaw, Mich.; sampler K is modified from Gale and Thompson (1975); samplers D-E and M-O are modified from Merritt and Cummins (1984).