Flooding and arsenic contamination: Influences on ecosystem structure and function in an Appalachian headwater stream

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 ± 39 μg L ^-1) were more than 30 times higher than in the reference reach (8 ± 1 μg L^-1). 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.