The Impact of Dams on the Quantity and Quality of Suspended Organic Material in Four Dutchess County Streams (New York)

Small dams, which are ubiquitous in streams throughout the world, may influence riverine suspended organic matter, and by extension, alter the food sources of filter-feeding organisms. One recent study found that a small dam actually enhanced food availability for fresh-water mussels, suggesting that dams might make some positive contribution to stream ecosystems. In this study, we addressed the question of whether dams affect the quantity and quality of suspended organic material downstream of dams. We predicted that suspended organic material below dams would be qualitatively improved by algae flowing downstream after growing in the dams’ impoundments. We sampled for suspended organic material upstream (control) and downstream of dams in four streams in Dutchess County, New York. Sampling was repeated five times throughout the fall and winter. Chlorophyll-a concentrations were affected differently by dams depending upon the season, and showed that dams may exert a positive influence on suspended organic material downstream, at least in the fall. Future work is needed to establish the exact relationship between dam characteristics (such as size) and the magnitude of the changes induced in downstream suspended organic material