Dams slow decomposition of leaf detritus by eliminating shrimp.
Dams can cause dramatic changes to streams and rivers. In Puerto Rico most native freshwater shrimps and fishes migrate from estuaries to headwaters at some point in their life cycle. However, migratory species have been completely extirpated from about 27% of rivers due to dams. Without dams, high elevation streams are dominated by shrimps (up to 25 per square meter) that control decomposition and other ecosystem processes. When shrimp are blocked from migrating upstream, the stream ecosystem changes dramatically. Forest Service sponsored research with the University of Georgia found that the absence of shrimp causes drastic changes to stream ecosystems. Streams above large dams show higher algae biomass, nitrogen concentrations, and fine benthic organic matter. Leaf litter decomposition rates were 66% higher in undammed streams. An in-stream exclosure experiment provided a test for the actual effects of shrimp absence on leaf litter decomposition. In the undammed streams, shrimp exclosure treatments had slower decomposition rates than shrimp access treatments. No difference was found between treatments in the dammed stream. These experimental findings indicated that shrimp extirpation is a significant factor in the reduced leaf breakdown rates for streams above large dams. Slower decomposition will result in more detritus being washed downstream and out to the ocean where it will interfere with coral reef dynamics. The shift to detrital breakdown by microbes from shrimp also releases more nitrogen to the water to enhance algal blooms, and changes food web relationships.
Forest Service Partners