Disrupting the flow: Effects of stream fragmentation on ecosystem processes and stream fish dynamics

Background/Question/Methods

Mediterranean-climate regions exhibit strong seasonality, with predictable patterns of dry and wet seasons. Consequently, streams in these regions undergo cycles of ecosystem contraction and expansion that coincide with seasonal cycles of precipitation. An extreme form of ecosystem contraction occurs in intermittent streams, which lose surface flow during the dry summer leaving behind isolated pools. Our goal is to understand how fish dynamics and ecosystem processes vary in intermittent streams across summers that differ in drought severity. Our approach has focused on quantifying changes in water quantity and quality, rates of leaf litter decomposition, and fish dynamics in a series of replicate riffle-pool habitats across four recent summers. We measured riffle and pool dimensions weekly to estimate changes in water quantity and deployed temperature loggers to track temporal changes in this aspect of water quality. We examined leaf litter decomposition by deploying 12 leaf packs per pool and retrieving a subset of packs at set intervals, which allowed us to calculate leaf breakdown rates (k). To examine fish dynamics, we implanted steelhead trout with uniquely-coded PIT-tags at the beginning of the summer and tracked their fates weekly across the summer using a portable antenna. Finally, we recaptured tagged fish at the end of each summer to re-measure their body size, which allowed us to quantify over-summer growth of individual fish.

Results/Conclusions

We found consistent reductions in riffle habitat and increasing stream fragmentation as the summer progressed, with more extreme fragmentation following a relatively dry winter (2008-09) than following relatively wet winters (2009-10, 2010-11). Leaf litter data indicated significant interannual differences in leaf breakdown rates with a relatively dry year having lower breakdown rates on average (2009: k = 0.0976) than a relatively wet year (2010: k = 0.1874). Our mark-recapture study suggested that drought severity affects the over-summer survival of steelhead, with relatively low survival following a dry winter (2009, 37%) than following wet winters (2010 and 2011, 65% and 61%, respectively). In contrast, our growth results were somewhat more complicated. In all years, a large proportion of fish showed negative over-summer growth (range among years 40-75%) though mass reduction did not correlate with drought severity. Rather, density appeared to be the major driver of growth variation among pools. Our findings suggest that seasonal drought can greatly influence the ecology of stream populations and ecosystems, with important among year differences due to interannual variation in precipitation.