Consequently, Vital Signs monitoring includes only four large embayments with drainage areas greater than 500 square miles and surface areas greater than 4,500 acres.
Locations for stream monitoring stations are chosen to sample as large a portion of each tributary watershed as possible. Stations generally are in unimpounded river reaches near the downstream end of each watershed.
The selection of ecological indicators is tailored to the type of monitoring location. Physical, chemical, and biological indicators are used to provide information on the health of various habitats or ecological compartments. In reservoirs, the status of the open-water or pelagic area is represented by physical and chemical characteristics of water (including chlorophyll-a, a measure of phytoplankton abundance) and measurements of fish communities in midchannel. The shoreline or littoral area is evaluated by sampling the fish community. Two indicators provide information on the bottom or benthic compartment: quality of surface sediments in midchannel (chemical analysis of sediments and acute toxicity of pore water) and benthic macroinvertebrate community (10 samples collected across the full width of the reservoir at each station). In streams, all available habitats are sampled to fully characterize the station by measuring the same basic indicators used for reservoirs. For both reservoirs and streams, information from each indicator is evaluated separately, and results are then combined without weighting to arrive at an overall evaluation of ecological health.