As the timing and magnitude of snowfall is changing in the Sierra Nevada, so is the resulting snowmelt and water runoff. With a shift in peak snowfall and snowmelt to earlier in the year, many historically perennial streams have become functionally intermittent. The transition to a complete loss in surface water flow in alpine streams has implications to the lifecycle of many organisms, including aquatic insects. We utilized the 50x1m experimental channels at the Sierra Nevada Aquatic Research Laboratory, as well as Convict Creek for our study. The experimental channels are naturally fed by Convict Creek and are colonized via drift. Seeking to compare the natural fluctuations in flow regime to a more extreme experimental setup, we sampled Convict Creek through a 19 year period, and subjected the channels to different drought “treatments” in a before/after sampling scheme. We sampled for invertebrates and other water properties before, and several times after the drought treatments were set in place, over the span of 3 years. At the end of the third year, we restored natural flow conditions to all the experimental channels, and measured resulting recolonization in the fourth year.
Results/Conclusions
We were successful in establishing a gradient of drought treatments in the experimental channels. Flow reduction ranged from 0-100%, depending on the channel. Channels with 100% flow reduction lacked any surface flow and became functionally “intermittent”. Oxygen and temperature cycles in the intermittent channels were extreme compared to others. Insect communities in the moderately disturbed channels exhibited high resistance to flow reduction, which was contrary to our hypotheses. The community in the channels were a subset of that in Convict Creek. Convict creek communities exhibited higher resistance to flow fluctuation, and we suggest that this is because of a deeper hyporheic zone. Based on preliminary results, the main trigger for a collapse in the insect community is when surface flow is lost in the experimental channels, even if there was very little beforehand.