Disturbances that influence abiotic conditions, however, can mediate species interactions which may allow inferior competitors, but perhaps good pioneers, to thrive. In very harsh streams, frequent disturbances may severely limit the number of species to only those tolerant of the stress. Prior exposure to a disturbance regime will influence survival and species life history characteristics, and as such, has evolutionary consequences for the fitness of individuals. In essence, species may "fit" the environmental conditions of a stream type which acts as a filter.

The impact of climate change on freshwater ecosystems is an issue of international concern. Air temperatures are expected to rise in many parts of the world that will potentially affect both the amount and the seasonality of precipitation and evaporation. In turn, the flow and thermal regimes of streams, and thus, their disturbance regimes and ecological condition is expected to change. Flows are expected to become more intense and shift in seasonal timing. Extremely high and low flows are expected to become more common and stream temperatures increase with increasing air temperatures.

In this study we examine the relationship between extreme flow and temperature events and a variety of ecological measures frequently used to quantify fish communities in streams. Specifically we develop measures of disturbance based on high flow events and low flow events coupled with low and high temperature events. We also develop a severity index to describe all disturbances occurring over three time periods including annual, summer, and spring. We anticipate that ecological measures (e.g., fish density) will be affected during periods of increase severity.

Collaborators and Participants:
Environment Canada

Figure 1. The attached graphic illustrates the data sets in a format where each pixel represents a year (on the y-axis) and day during that year (on the x-axis). The stream flow data in the first panel are highlighted when the data are less than the 10th percentile (low flow conditions). The minimum temperature data are highlighted in the second panel when the data are less than or equal to the 10th percentile (very cold conditions) and the maximum temperature data are highlighted when the data are greater than or equal to the 90th percentile (very warm conditions). Intersecting the highlighted stream flow data with the highlighted temperature data, the third panel of the graphic, indicates concurrent low flow and very cold or very warm conditions - the extremes.