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.