Resource use by detritivorous macroinvertebrates in southern Appalachian headwater streams
Eggert, Susan Lynn
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Terrestrially derived resources drive secondary production in Appalachian headwater streams. Seven years of litter exclusion and wood removal in a headwater stream resulted in a 93% reduction in shredder production. However, production did not decline at the same rate for each taxon during litter exclusion. I conducted leaf and wood breakdown studies, analyzed seasonal diets, and ran assimilation studies to examine why certain shredder taxa responded differently to the step-wise reduction of detrital resources in the exclusion stream. Pycnopsyche gentilis production declined to zero within the first three years of litter exclusion. Production of Tipula abdominalis and Tallaperla spp. did not decline significantly until after wood removal in year 4. Trophic basis of production calculations showed shifts in organic matter flows in the exclusion stream from leaves (70%) to wood (56%) after leaf standing crops declined. Following wood removal, large flows shifted from wood to amorphous detritus (62%). T. abdominalis and Tallaperla spp. shifted diets according to resource availability, while P. gentilis did not. Survivorship and growth of P. gentilis was significantly reduced at leaf standing crops <25-50 g AFDM m-2. Breakdown rates of maple leaves were significantly slower in the exclusion stream compared to the reference stream. Slower breakdown rates for red maple leaves were associated with lower shredder production. Shredder production in litterbags was 3-4x greater than in benthic substrates in the litter exclusion stream suggesting that litterbags acted as resources islands. Wood breakdown rates were significantly faster (1.4x) in the exclusion stream. All three taxa assimilated epixylon more efficiently (26-36%) than a mixed-species leaf diet (9-17%). Tallaperla spp. and T. abdominalis were better able to utilize epixylic biofilm than leaves, whereas P. gentilis was more proficient at assimilating leaves. These results show that detritivores vary in their ability to use different detrital resources and may not be functionally identical. Detrital resources such as wood, epixylon, amorphous detritus, and slow-processed leaves may allow some detritivores to survive for a short time in streams disconnected from riparian habitats. Over the long-term, however, a fully functioning stream ecosystem requires all forms of organic matter inputs.