We investigated the influence of flooding and chronic arsenic contamination on ecosystem structure and function in a headwater stream adjacent to an abandoned arsenic (As) mine using an upstream (reference) and downstream (mine-influenced) comparative reach approach. In this study, floods were addressed as a pulse disturbance, and the abandoned As mine was characterized as a press disturbance. We further addressed chronically elevated As concentrations as a ramp disturbance, in which disturbance intensity was ramped by increasing proximity to the As source. Stream ecosystem structure and biogeochemical functioning were characterized monthly over a period ranging from July to December 2004. Influence of the press disturbance was evident in the mine-influenced reach, where As concentrations (254 6 39 m gL 21) were more than 30 times higher than in the reference reach (8 6 1 m gL 21). However, in almost all cases the presence of the abandoned As mine appeared to exert little influence on reach-scale measures of ecosystem structure and function (e.g., organic matter (OM) standing crops, phosphorus (P) uptake). Conversely, floods significantly influenced OM standing stock in both study reaches. Interactions between press and pulse disturbances influenced P uptake in the mine-influenced reach. Within the mine-influenced reach, P uptake across a gradient of As concentrations correlated with Michaelis-Menton models of enzyme kinetics in the presence of a competitive inhibitor. These results indicate that As competitively inhibits P uptake by microbial assemblages.