Macroinvertebrates are collected following state protocols outlined in the document Macroinvertebrate Biological Assessment of Wadeable Streams in Georgia. The primary purpose of this document is to establish and maintain uniform methodology and quality control guidance for biological data collected within the state. Compliance with these procedures is essential to produce reliable biological data.
Macroinvertebrate sampling is conducted mid-September through February, which is called the Index Period. This Index Period is the biologically optimal sampling season, which occurs when the habitat is utilized most heavily by more mature larvae (late instars) and the food resources have stabilized to support a balanced indigenous community (Plafkin et al., 1989).
A stream reach of 100 meters is designated for sampling, which begins downstream and moves upstream so that habitat is not disturbed before sampling. Within the reach, 20 locations bearing productive and stable habitats are sampled by two team members. One individual collects samples by “jabbing” a D-frame net into the habitat, while the other member keeps up with the number of jabs and compiles collected material in a bucket. Six types of habitats are sampled, including (in order of sampling priority) fast and slow riffles, woody debris/snags, undercut banks/rootwads, leaf packs, soft sediment/sandy substrate, and submerged macrophytes (aquatic plants), if present. When habitats are not present, sampling locations will be reallocated using the priority listed above. The sample is composited, preserved, and returned to the laboratory for subsampling, sorting, and macroinvertebrate identification.
In the laboratory the sample is evenly distributed in a Caton subsampler, which divides the sample into 30 equal portions (squares). Squares are randomly selected and macroinvertebrates are sorted and identified until reaching the target sample size of 200 organisms, plus or minus 20 percent. Individual organisms are preserved and stored with site identification markers, including sampling location, date of collection and date of subsample.
Data collected is entered into a multi-metric index database for analysis. Individual metrics are combined to determine an index score which is used to help Cobb County evaluate stream health and aid future management decisions.