may experience a high flow that exceeds a 10-year recurrence interval while the majority of the basin experiences no significant change. In these cases, the planned sequence of sampling sites may need to be altered to allow those sites that experienced the high flow to be sampled near the end of the field sampling period. Professional judgment and a fair amount of luck are required in making decisions about postponing field sampling. It is important to consider that peak flows do influence community structure and that this influence depends on factors such as the magnitude of the peak flow, duration of the high flow, and frequency of high flows that scour the channel. When sampling cannot be postponed after extreme flows, quality of the data can decline.
Sampling during normal low- and stable-flow periods, if compatible with life-history characteristics, is the preferred time for benthic invertebrate sampling. Sampling during low flow has a number of advantages including increased accessibility to the river, a reduced dependency on labor-intensive deep-water sampling techniques, and increased confidence that all parts of the wetted channel have been continuously part of the aquatic habitat. In contrast, some instream habitats, such as snags and stream m argins, that are accessible during high water may actually be out of water for most of the year. However, sampling during periods of abnormally low flow should be avoided because extreme low flows may affect invertebrate communities by reducing current velocities, decreasing the amount of instream habitat, increasing water temperature, decreasing dissolved-oxygen concentrations, increasing contaminant concentrations, and indirectly altering food resources and biotic interactions. Consequently, unusual l ow flows need to be identified and factored into the interpretation of the biological data in much the same manner as unusually high discharges. Regional characteristics must also be considered, particularly where streams are ephemeral or where flow characteristics, such as current velocity, change dramatically during normal summer low flows. For example, Coastal Plain swamp streams are best sampled at higher winter flows when conditions are suitable for supporting aquatic communities. Sampling these systems during normal summer low flows is generally not productive because conditions, such as current velocity, dissolved-oxygen concentration, and temperature, are not suitable for sustaining aquatic communities even at unimpaired reference sites (D.R. Lenat, North Carolina Division of Environmental Management, oral commun., 1992). To aid in interpreting discharge effects, all ecological survey sites associated with basic fixed sites should be continuously gaged for at least 6 months prior to sampling and throughout the intensive sampling period.
Other factors to consider in selecting the appropriate sampling season include life-history characteristics of other aquatic taxa, seasonal human activities, and site access. Spawning and migration periods of anadromous fish, especially threatened or end angered species, should probably be avoided. Seasonal agricultural practices, such as applications of pesticides and fertilizers, soil preparation, and irrigation patterns, that affect loads of sediment, nutrients, and pesticides must be considered in the selection of an appropriate sampling season. In addition, seasonal road conditions may limit site access during certain parts of the year. Subbasins with different climatic or hydrologic characteristics may need to be sampled at different times of the year. If this is the case, then an attempt must be made to link the subbasin sampling on the basis of temperature-dependent organism development (that is, sampling at periods of similar accumulated degree-days). The final selection of