| Abstract: | Efforts to develop benthic macroinvertebrate sampling protocols for the bioassessment of lotic ecosystems have been focused largely on wadeable systems. As these methods became increasingly refined and accepted, a growing number of monitoring agencies expanded their work and are now developing sampling protocols for non‐wadeable large rivers. Large rivers can differ from wadeable streams in many ways that preclude the use of some wadeable stream sampling protocols. Hence, resource managers need clear and consistent large river bioassessment protocols for measuring ecological integrity that are cost effective, logistically feasible, and meet or are adaptable to the multi‐purpose sampling needs of researchers and managers. We conducted a study using an experimental macroinvertebrate sampling method that was designed to overcome limitations of several methods currently in use. Our objectives were to: (1) determine the appropriate number of sampling points needed; (2) determine an appropriate laboratory subsample size to use and (3) examine how varying reach length affects assemblage characteristics. For six reaches in each of two large rivers, we sampled the macroinvertebrates of both banks at 12 transects separated by increasingly larger distances using a multi‐habitat, semi‐quantitative technique. Interpretation of results relied on the values attained for nine benthic macroinvertebrate assemblage metrics. Results from Monte Carlo methods indicated that, using the sampling methods described herein, a representative sample of the assemblage was collected by sampling both banks on 6 transects. Across all sites, we did not observe a consistent relationship between transect spacing (i.e. total reach length) and metric values, indicating that our sampling protocol was relatively robust with respect to variation in reach length. Therefore, flexibility exists that permits the study reach length to be dictated by the spatial scale (e.g. repeating geomorphic units) in question. For those preferring to use a fixed reach length, we recommend that transects be spaced at a minimum of 100 m intervals over a 500 m distance. We recommend that the field method be coupled with a fixed laboratory subsample size of 300 organisms for bioassessment purposes, with the recognition that a subsample size of 500 organisms may be needed to meet the objectives of more rigorous studies. It is likely this approach will over‐sample sites of uniform composition, but the goal was to develop a robust sampling protocol that would perform well across sites of differing habitat composition. Possible modifications to the method to streamline its future application in the field are provided. Published in 2006 by John Wiley & Sons, Ltd. |
