Response of secondary production by macroinvertebrates to large wood addition in three Michigan streams

1. We measured responses in macroinvertebrate secondary production after large wood additions to three forested headwater streams in the Upper Peninsula of Michigan. These streams had fine-grained sediments and low retention capacity due to low amounts of in-channel wood from a legacy of past logging. We predicted that wood addition would increase macroinvertebrate secondary production by increasing exposed coarse substrate and retention of organic matter. 2. Large wood (25 logs) was added haphazardly to a 100-m reach in each stream, and a 100-m upstream reach served as control; each reach was sampled monthly, 1 year before and 2 years after wood addition (i.e. BACI design). Macroinvertebrate secondary production was measured 1 year after wood addition in two habitat types: inorganic sediments of the main channel and debris accumulations of leaf litter and small wood. 3. Overall macroinvertebrate production did not change significantly because each stream responded differently to wood addition. Production increased by 22% in the main-channel of one stream, and showed insignificant changes in the other two streams compared to values before wood addition. Changes in main-channel macroinvertebrate production were related to small changes in substrate composition, which probably affected habitat and periphyton abundance. Macroinvertebrate production was much greater in debris accumulations than in the main-channel, indicating the potential for increased retention of leaf litter to increase overall macroinvertebrate production, especially in autumn. 4. Surrounding land use, substrate composition, temperature and method of log placement are variables that interact to influence the response of stream biota to wood additions. In most studies, wood additions occur in altered catchments, are rarely monitored, and secondary production is not a common metric. Our results suggest that the time required for measurable changes in geomorphology, organic matter retention, or invertebrate production is likely to take years to achieve, so monitoring should span more than 5 years, and ecosystem metrics, such as macroinvertebrate secondary production, should be incorporated into restoration monitoring programs.