Integrating observational and experimental approaches to demonstrate causation in stream biomonitoring studies

Routine biomonitoring of aquatic ecosystems generally is performed with the intent of demonstrating a causal relationship between stressors and responses. However, because it is impossible to eliminate other potential explanations for observed spatiotemporal correlation between stressors and responses, demonstrating causal relationships is highly tenuous in descriptive studies. In this research we show how results of descriptive and experimental approaches can be integrated to demonstrate a causal relationship between heavy metals and benthic community responses in a Rocky Mountain stream (CO, USA). By using a stressor identification process, we show that effects and exposure data collected from a contaminated site (Arkansas River, CO, USA) can be integrated with experimental data to support causal arguments. Analysis of the spatial co-occurrence of metals and benthic community responses in the Arkansas River provided support for the hypothesis that metals caused alterations in benthic community structure. Exposure pathways were quantified by measuring metal bioaccumulation in caddisflies (Trichoptera) collected upstream and downstream from metal inputs. A long-term (10-year) monitoring study showed that reductions in metal concentrations corresponded with improvements in benthic communities. These results were supported by microcosm and field experiments that quantified concentration-response relationships between heavy metals and benthic community composition. Consistency of these responses was demonstrated by comparing results to a spatially extensive survey of metal-polluted streams in Colorado. Our study demonstrates the power of integrating descriptive and experimental approaches for developing causal arguments in ecological assessments.