Unexpected outcomes in a macroinvertebrate drift experiment: The roles of habitat heterogeneity, discharge, and seasonality

Previous studies indicate that high habitat heterogeneity can promote temporal stability of aquatic macroinvertebrate communities.  I performed an experiment to test the hypothesis that higher heterogeneity stream substrates promote stability by decreasing drift rates, particularly during high flow events.  I manipulated the substrate heterogeneity of ten 0.75m x 0.5m plots in a New Hampshire stream and used an enclosed drift capture system to isolate drifting macroinvertebrate from manipulated areas on 4 dates during the late summer of 2003.  While there was a statistically significant effect of habitat heterogeneity on drifting macroinvertebrate abundances, the effect was neither strong nor consistent through time.  However, some unexpected relationships between drift and environmental variables emerged from this study.  As expected based on previous work, abundance of drifting macroinvertebrates was strongly affected by stream discharge.  However, while the relationship between drift and discharge was the predicted positive linear relationship during the first 3 sampling dates, the relationship on the final sampling date was a strong negative linear relationship.  I explored several possible explanations for this shift, including direct effects of temperature (which dropped significantly prior to the last sampling date) and indirect effects of temperature on predation-induced drift.  However, the most plausible explanation for the unexpected result based on available evidence appears to be that temporal changes in community composition led to a cumulative shift in the community response to increased discharge based on the behavior of a few species that increased dramatically in abundance prior to the final sampling date.