The response of Potamogeton crispus L. breakdown to controlled doses of different levels of chlorine and chlorine + ammonia was investigated over two years in outdoor experimental streams. In 1985, downstream riffles of 2 streams were dosed (observed in-stream concentrations) at ca. 10 micro g/L Total Residual Chlorine (TRC), one stream at 64 micro g/L TRC and one stream at 230 micro g/L TRC. Two control streams were not dosed and the upstream riffles of each stream served as within stream controls. In Aug-Sep, all three streams with chlorine + ammonia (6, 56 and 146 micro g/L TRC + 2.5 mg/L ammonia) and the 70 micro g/L TRC alone stream had significantly lower decomposition rates in the downstream dosed sites. For these streams, downstream decay rates ranged from 46% (high chlorine + ammonia) to 73% (low chlorine + ammonia) of the upstream control rates. No other up-down pairs were different during the trial. Up and downstream sites of the stream dosed with 2.5 mg/L ammonia alone were nearly identical for both trials (<3% difference). These results indicate that TRC at less than 250 micro g/L can significantly reduce litter decomposition and strongly suggest that addition of ammonia to chlorinated water can increase the toxic effect of chlorine. (Copyright (c) 1989 Kluwer Academic Publishers.)