Stormwater retention ponds have the ability to alter stream flow, temperature and other properties. With this change of conditions the potential for shifts in leaf litter decay rate is possible. In freshwater streams, decomposition is caused by microbial activity, invertebrates, and physical fragmentation. For the present study, decay of mixed leaf litter was measured upstream and downstream of stormwater retention ponds in Williamsburg, Virginia from June to July 2006. Fifty-gram leaf litter bags with course mesh were constructed and placed in groups of five in the stream channel immediately upstream and downstream of each retention pond. The bags were collected on the first day of the experiment, two weeks later, and finally after five weeks had passed. The leaves were then dried, ashed, and decomposition rates calculated. The invertebrates found in the bags were counted and identified to the lowest possible taxa. Overall the decay rate of leaves upstream storm water retention ponds was found to be 3.91 g/week while decomposition downstream occurred at 2.18 g/week. On average the decay was faster upstream but after a regression analysis and a two-sample t-test, the difference was found to be insignificant. Leaf decay rate and number of invertebrates were positively correlated at most sites, suggesting that biological activity was responsible for leaf decay. One location not exhibiting this pattern was surrounded by impervious cover and other human impacts both upstream and downstream of the retention pond. At this location, physical fragmentation of leaves due to storm water runoff was suspected. On average storm water retention ponds did not have a significant effect on decomposition, but the effect varied within individual basins.
For additional documentation Shelby Burks provided a PowerPoint Presentation entitled "The Effect of Stormwater Retention Ponds on Leaf Decomposition" provided here in PDF form.