The Yellow Breeches, a tributary of the Susquehanna River, is a freestone stream flowing 49 miles through limestone-dominated valleys. The character of the stream changes as limestone streams join it at several points, altering the bedrock, formation and water source. Macroinvertebrate communities consequently change in conjunction with the physical and chemical transformations. As cornerstones of the food chain and ecosystem, shifts in these populations can have widespread effects on the stream community as a whole. It is essential to determine factors promoting community changes to be able to accurately determine the conservation measures that can be safely taken without changing the overall ecosystem structure. Therefore, this project strives to assess whether there is significant difference between macroinvertebrate communities in the two streams as they join and if one exists, to identify the chemical and physical parameters contributing to that shift. To accomplish this, visual assessments, nutrient analysis, and macroinvertebrate sampling were performed at eleven sites within thirty meters of the mixing site. Site location was determined by conductivity representing limestone and freestone conditions as well as an intermediate mixing zone.

Results/Conclusions

Preliminary data reveal significant differences in macroinvertebrate communities in the limestone and freestone influenced sites in several keystone genera. Gammarus was present in significantly higher levels in limestone conditions, whereas Brachycentrus and Ephemerella were more prevalent in freestone conditions; dynamics of the overall community changed with shifts in the feeding groups present as well. Significant differences occurred in nearly all chemical parameters and only two physical parameters: conductivity and substrate composition. Depth and velocity are not significant factors contributing to community changes, contrary to what literature reports about other types of stream habitats. Further testing will be performed at this site as well as two additional sites- one in which a limestone stream and sandstone stream intersect and another in which two freestone streams meet- to further specify the cause of the change in community structure and composition.