Looking for Thresholds in Coupled Biophysical River Ecosystems

N. LeRoy Poff
Department of Biology
Colorado State University

Ecological Thresholds Meeting
November 4-5, 2002



Biological productivity and diversity in rivers and associated floodplains are strongly regulated by the temporal dynamism of a few key environmental regimes: flow, temperature, sediment (and wood), chemical. The components of these regimes (frequency, magnitude, duration, and timing) act independently and interactively in this regulation. Abiotic thresholds on biological function are known, e.g., floodplain processes depend on a threshold flow of inundation. The potential for other ecological threshold responses to environmental regimes will be explored, particularly those related to components of the flow regime.

Ecological thresholds in flowing-water ecosystems may potentially result "endogeneously" from non-linear interactions among species. Regulation of benthic algae by invertebrate herbivores is a particularly interesting candidate to explore, because algae have the potential to outgrow and overwhelm their consumers. Because the gain and loss functions for benthic algae depend not only on herbivores, but also nutrient and flow regimes, threshold responses are likely to be dependent on the environmental context and therefore potentially restricted to a subset of river types. Thus, land use and flow thresholds may facilitate or inhibit the expression of "endogenous" biological thresholds.

Identification of ecological thresholds could clearly assist in river restoration (or conservation), by identifying "how much" natural process is required to sustain (or maintain) an acceptable ecological condition. Complex interactions among key regime components, environmental legacies, landscape structure, etc. pose significant problems for identifying thresholds. I will explore some promising settings for thresholds research in this context, focusing on ecological responses to flow alteration in dammed and diverted rivers.