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Harvard Forest Symposium Abstract 2005

• Title: Subsurface Flowpaths Drive Spatial Variation in the Macroinvertebrate Community of an Intermittent Headwater Stream in Central New England
• Primary Author: Bridget Collins (College of the Holy Cross)
• Additional Authors: Betsy Colburn (Harvard Forest); William Sobczak (College of the Holy Cross)
• Abstract:

  Introduction. Macroinvertebrate assemblages in many New England headwater streams are highly diverse and energetically linked to allochthonous inputs. However, intermittent headwater streams with predominantly subsurface flow have received less attention and may have atypical trophic and community structures. The aim of this study is to contribute to an emerging conceptual model for intermittent streams and their influence over spatial patterns of macroinvertebrate diversity. Furthermore, our study stream is in a catchment of predominantly Eastern hemlock (Tsuga canadensis), a dominant tree species which strongly influences understory microclimate and is experiencing severe mortality throughout its range due to an introduced pest (Orwig et al., 2002). As such, this ecosystem is likely to undergo dramatic changes in the coming years, and this study may provide valuable baseline data for future studies.
  
  
  
  
  
  
  Study Site. We studied a 500 m reach of Prospect Hill A, or PHA, a headwater of Bigelow Brook, on the Prospect Hill Tract of the Harvard Forest, Petersham, MA, USA (Figure 1), during the summer-fall of 2004. PHA is a first-order, intermittent stream dominated by subsurface flow and with a watershed largely dominated by mature hemlock forest. Detailed records of past land use indicate that the catchment was historically kept as woodland, but some areas were clear cut in 1793 and 1895 (Harvard Forest Archives).
  
  
  
  
  
  
  Methods. Sixteen PVC wells were installed in four transects (Figure 2), and water temperature, specific conductivity, pH, and dissolved oxygen were measured in both wells and surface flow (SF) sections of channel. We dug pits to compare macroinvertebrates in subsurface habitats to invertebrates collected with kickseines in SF sections.
  
  
  
  
  
  
  Results and Discussion. Water was found in isolated chutes in subsurface flow paths (Figure 3). When stream ecologists have studied subsurface flowpaths and the biota within them, it has traditionally been within the context of a hyporheic zone. In our study system, however, the shallow subsurface flow does not appear to percolate through sediment, but instead seems more akin to the soil pipeflow often investigated by geomorphologists in connection to storm events. Water chemistry was constant along the reach, with oxygen at or near saturation in surface and subsurface sections (Figure 4). Macroinvertebrates found in subsurface habitats differed in abundance, taxa, and functional feeding group relative abundances from those in SF. These subsurface flow regions had diversity and evenness statistics comparable to surface flow sections (Table 1), and Elmidae, Hydrophilidae, and Hydropsychidae (Homoplectra) were found only in subsurface reaches. Subsurface abundance of macroinvertebrates was lower than abundance in surface-flow areas; however, the ecosystem-level importance of subsurface reaches appears to be increased relative to surface flow as a function of the habitat’s total coverage (approximately 70% of the reach).
  
  
  
  
  
  
  
  
  

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