Seasonal and spatial responses of macroinvertebrate secondary production to the removal of streamside forests

J.K. Jackson and B.W. Sweeney. Stroud Water Research Center, Avondale, PA 19311

Modification or removal of riparian vegetation is one of the most common land uses known to affect streams. Relative to forested reaches, meadow reaches are often characterized as being narrower, deeper, brighter, warmer, and faster, with greater algal productivity and lower allochthonous organic matter inputs. We examined the impact of riparian forest removal using secondary production to summarize survivorship and growth responses of benthic macroinvertebrates. Macroinvertebrate samples were collected bimonthly (Mar to Nov) from riffles and pools in 16 paired forest and meadow reaches of 1st to 4th order streams in the Piedmont of Pennsylvania. Daily, seasonal, and annual production were calculated based on benthic biomass and instantaneous growth rates estimated from mean individual body sizes and daily water temperature. Total macroinvertebrate production in these streams was primarily associated with Diptera, Ephemeroptera, and Trichoptera. Daily production in spring (Apr, May) was greater than in summer (Jul, Aug), and often greater than in autumn (Oct, Nov). Riffle production was generally 2-4 times greater than pool production (5 to 30 vs. 13 to 66 g DM/m²). Finally, annual meadow production was generally greater than in the forest, and much of this difference reflected greater meadow production during summer and autumn.