Perspectives on the functional assessment of multi-stressed stream ecosystems

Abstract

Past research has examined how anthropogenic stressors affect both structural and functional attributes of stream ecosystems. Nevertheless, biomonitoring programs rely mostly on structural metrics for surface-water quality and status assessments, and few studies have examined the extent to which functional metrics can strengthen these structure-based assessments. We reviewed studies that combined the response of stream-ecosystem structural and functional measures to single and multiple stressors. These studies illustrate that structural and functional measures can respond in similar, complementary, or even contradictory magnitude and direction to different stressors. Because of this complexity, we suggest that a combination of structural and functional measures may provide added information on surface-water quality and status, especially when ecosystems are affected by multiple stressors. Better knowledge about trophic and non-trophic roles of dominant taxa and the effects of changes in species diversity on stream ecosystem processes could improve understanding of the relationships among structural and functional measures. Based on our review, we suggest that future research should be designed to: 1) increase understanding of the roles of individual species and communities in the functioning of stream ecosystems, and 2) quantify the responses of individual species and communities to individual stressors and combinations of multiple stressors. We propose that increasing the knowledge base about the suites of traits that occur in different species, how these traits coevolved under local environmental abiotic and biotic conditions, and how they interact is needed to understand how multiple stressors affect ecosystem structure and function. This trait-based knowledge is essential to understanding the relationship between structure and function in multi-stressed stream ecosystems and could help managers to make stronger inferences about the combined effects of multiple stressors on water quality and status.