Our recent measurements of long-term fluid flux rates and output fluid chemistry at seep sites on Hydrate Ridge, Cascadia, suggest that the mechanisms driving fluid flow in gas hydrate bearing forearc settings may be significantly more complex than previously thought. Temporal changes in the polarity and magnitude of flow and in seep fluid chemistry were observed. The nature of flow appears to generate a strong response in the chemosynthetic seep community ecology. We document here: 1) the frequency and range of temporal variability in flow rates, 2) the relatively rapid changes in outflow composition, 3) strong outflow of seawater-like fluids, 4) the association of Calyptogena (sp) clams with oscillatory and inflow settings, and 5) microbial mat communities associated with strong advection of altered fluids. These results suggest that a reappraisal may be required of the nature of the hydrologic system and its effect on related processes such as benthic seep community ecology and biogeochemical cycles. ¿ 2001 American Geophysical Union