Influence of Multiple Disturbances on Stream Structure and Function

We investigated the influence of multiple disturbances on ecosystem structure and function in a headwater stream adjacent to an abandoned arsenic 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 arsenic mine was characterized as a press disturbance. Chronically elevated levels of arsenic were specifically addressed as a ramp disturbance. Stream ecosystem structure and biogeochemical functioning were characterized monthly over a period from July to December 2004 by determining benthic organic matter standing stocks, ecosystem metabolism, and by using solute additions to examine differences in phosphorus uptake and hydrology over the monitoring period. Influences of the press disturbance were evident in the mine-influenced reach where arsenic concentrations (254 ± 39 μg/L) were >30 higher than in the reference reach (8 ± 1 μg/L). However, in almost all cases the presence of the abandoned arsenic mine appeared to exert little influence on reach-scale measures of ecosystem structure and function (e.g., organic matter standing crops, phosphorus uptake). Conversely, floods (i.e., pulse disturbances) influenced organic matter standing stocks and hydrologic interactions between the stream and transient storage zones in both study reaches. Interactions between press and pulse disturbances were evident in several cases and illustrated by phosphorus uptake responses. Phosphorus uptake was best predicted by coarse particulate organic matter standing stocks in the reference reach. However, in the reach exposed to the press disturbance (i.e., mine-influenced reach), both coarse particulate organic matter standing stocks and characteristics of the pulse disturbance regime (i.e., number of days post-flood) were significant predictors of phosphorus uptake. Within the mine-influenced reach, arsenic concentrations increased from 16–600 μg/L and were addressed as a ramp disturbance. Analysis of phosphorus uptake in the mine-influenced reach across a gradient of arsenic concentrations correlated with Michaelis-Menton models of enzyme kinetics in the presence of a competitive inhibitor. These results suggest that arsenic appears to competitively inhibit phosphorus uptake by microbial assemblages in the mine-influenced reach. Results from this study highlight the fact that ecotoxilogical studies at the ecosystem scale should consider not only contaminant influences, but rather place its implications within the extant disturbance regime generated from both natural and anthropogenic sources.