Responses of hyporheic meiofauna to habitat manipulation

Interactions between interstitial meiofauna and physicochemical parameters of the hyporheic zone were examined via an in situ experiment on the Speed River, Ontario. The manipulation comprised reversing upwelllng and downwelling zones at the riffle scale, and was maintained for I month. Significant differences in physicochemical parameters were detected between zones and between treatments (control vs. manipulated). Depth-related variables, such as sediment particle size, were most important in structuring the hyporheic community during pre- and post-manipulation phases. Flow reversal was largely successful, with more significant changes occurring in the original downwelling zone. For example, change from downwelling to upwelling resulted in decreased larval chironomid dominance but an increase in the numbers of oligochaetes, nematodes, mites, and copepods. However, under field conditions, it was difficult to keep other variables, such as water temperature, constant and some of these may have contributed to the changes seen in the meiofauna.