Ecological studies of benthic macroinvertebrates for determining sedimentation impacts in Chattahoochee National Forest streams
Understanding sedimentation impacts to benthic macroinvertebrates in headwater, mountain streams is a top priority of watershed management programs in the Chattahoochee National Forest. Five studies involving the analysis of historical, biological survey data and current data were conducted to improve our understanding of macroinvertebrate response to sedimentation and to support the development of biological information for sediment load models to be applied in the Chattooga River watershed. An initial analysis of historical data involving a composited, macroinvertebrate reach-scale sample revealed weak relationships between assemblage metrics and sedimentation, which was similar to results of two recent macroinvertebrate studies that found biological ratings of good or excellent with reported physical impact attributed to sedimentation. Those findings and field reconnaissance in the Chattooga River watershed revealed that patchy, coarse sands may be the primary issue of concern regarding sedimentation impact to benthic macroinvertebrates. Therefore, a modified sampling approach was used to investigate relationships of macroinvertebrates and environmental conditions that included micro-habitat patches containing coarse sands, a product of erosion associated with Southern Blue Ridge, silicate parent geology. At the microhabitat, patch scale, flow velocity was the main environmental factor associated with a macroinvertebrate assemblage gradient, and was significantly correlated with percent deposited sediment across 264 samples. The high dominance of just a few macroinvertebrate genera, and the majority lack of individual macroinvertebrate associations with dominant substrate types may suggest that the dominant macroinvertebrates utilize a multi-microhabitat portion of the streambed at any given time, which may be due to the homogenization of streambeds due to sand (providing ease of movement) and its immobility (low bedload volume and sand patch shift). Because flow was the only significantly correlated environmental variable on an assemblage gradient produced by ordination (and was individually correlated with dominant substrate and percent deposited sediment), a subsequent study was conducted to determine macroinvertebrate sensitivity to deposited sediments among two flow-differentiated habitat types. Results showed that more taxa were related to a gradient of percent deposited sediment in fast water habitats, and no taxa were positively correlated with percent deposited sediment. Indicator species analysis found a number of taxa that were associated with a four-level grouping of percent deposited sediment levels. Therefore, a final study involved calculating deposited sediment tolerance values using indicator species associations and individual cumulative abundances across percent deposited sediment levels. The final index developed from cumulative abundances showed a relationship with deposited sediment within the range 0 – 30%, and that low range was due to the low deposited sediment levels at which all 50% cumulative abundances fell (1 - 10%). The sedimentation index produced from indicator species analysis produced a reach-scale index that was related to percent pool embeddedness. Key findings from these studies are: (1) sand is the primary deposited sediment type, with most streambed comprised of cobble-sand substrate, (2) few taxa are associated with deposited sand substrate, (3) there are high numbers of a relatively few dominant taxa across samples and streams, (4) macroinvertebrate response to deposited sediments is greatest in fast water habitats, and (5) the developed sedimentation biotic index is a potential, assemblage-level indicator of increasing sedimentation in these headwater systems. The functional and habit organization of the four most dominant taxa determined in recent studies suggest that they may be utilizing sand patches for crawling and collecting food, therefore structurally adapting to long-term, press disturbances due to historical and contemporary anthropogenic activities and natural erosion. In addition, macroinvertebrate assemblage composition in these streams indicates overall good "health" and suggests streambed stability in the presence of a large portion of coarse sand. However, an important question that remains involves sand movement along streambeds and the ecological consequences of continued sediment inputs to these headwater systems.