Intermittent streams lose surface flow for part of the year as shallow riffle habitats dry, leaving behind a series of residual pools that provide habitat for aquatic organisms. Historically, intermittent streams have been understudied when compared to perennial streams but this is changing. For example, there has been growing interest in the importance of intermittent streams for regional species richness, their use by fishes for spawning and rearing, their importance in providing refuge from winter spates, and their importance in nutrient dynamics and in maintaining the integrity of entire river networks. However, other studies have reported that physical conditions in intermittent streams can be harsh during the drying phase, which can result in large mortality events, including local extinctions during extreme drought. I hypothesized that drought intensity, which is driven by antecedent precipitation, plays an important role in determining whether conditions in intermittent streams are relatively benign or harsh. I tested this hypothesis by studying the ecology of a single intermittent stream, the John West Fork, a tributary of Olema Creek in Marin County, California, across four years that differed in drought intensity. Specific goals of my dissertation included examining the influence of antecedent winter precipitation on (1) patterns of stream fragmentation and pool habitat availability, (2) movement and survival of juvenile salmonid fishes, and (3) leaf decomposition, a key ecosystem process providing an important source of carbon in low-order streams.
In Chapter 2, I used long-term rainfall data to determine the drought intensity of each of my study years: two study years were relatively dry (2009 and 2012) and two years were relatively wet (2010 and 2011). I monitored changes in water level, including in both riffles and pools, at weekly intervals in each year. I then quantified the rate of riffle drying (as a measure of stream fragmentation) and the rate of pool drying (as a measure of habitat availability), and compared these rates between wet and dry years. Additionally, I track the movement of individually-marked juvenile steelhead trout (Oncorhynchus mykiss), and used a logistic regression to determine factors that influenced their movement among pools. Lastly, I estimated the difference between biological fragmentation (defined as the date when fish movement ceases) and physical fragmentation (defined as the date when riffles completely dry). I found that riffles dried earlier following dry winters when compared to wet winters, which limited the movement of juvenile steelhead to the early summer following dry winters (but not wet winters). I found that fish movement was positively associated with riffle volume and negatively associated with originating pool depth, riffle length, and day of year. Lastly, I found that biological fragmentation preceded fragmentation by many weeks during each year, suggesting that there is a minimum level of water required for fish to move despite the presence of streamflow.
In Chapter 3, I explored differences in survival of juvenile steelhead trout between wet and dry years using Program MARK. I marked individual trout in the early summer and tracked their fates each week across the summer dry season in each of four years. I found that survival was higher following wetter winters. Moreover, I found that survival was high through the midsummer during all years; however, during dry years, survival was considerably lower during the late summer. These findings suggest that steelhead in the study region are resistant to drying conditions in intermittent streams to an extent. During wet years, steelhead can experience relatively high rates of survival intermittent streams, suggesting that such systems may be important to the overall productivity of the watershed following wet winters. During dry years, however, harsh physical conditions during the late summer result in high rates of mortality. These results highlight the importance of this critical period during the late summer in shaping fish population dynamics in intermittent streams and suggest that monitoring efforts should be focused on late-summer surveys in such systems.
In Chapter 4, I compared differences in leaf decomposition rates among years that differed in drought intensity and across a single season, including the early, middle, and late summer. , I used generalized linear mixed-effects models (GLMM) to determine whether abiotic (water level and stream temperature) and/or biotic (shredder density) factors were important in determining leaf decomposition rates. I found that leaf decomposition was fastest during the wettest year and fastest during the early summer, suggesting that breakdown is generally faster when conditions are wetter. I also determined that the density of macroinvertebrate shredders was positively associated with leaf decomposition rates and that shredder decomposition rates were higher following wet winters and during the early summer, suggesting a possible indirect influence of drought intensity on leaf decomposition through its influence on shredder density.
In conclusion, the studies outlined in this dissertation highlight the role of antecedent precipitation in shaping physical conditions during the late summer, which have a strong influence on stream biota and ecosystem processes in an intermittent stream. Climate change is slated to heavily influence hydrologic regimes and will likely result in perennial streams shifting to intermittent flow states. As such, studies contributing to the body of research focused on intermittent streams are becoming increasingly important and findings from such studies can help guide management decisions and help prioritize conservation efforts.
