Quantifying abiotic habitat characteristics to determine thresholds for salmonid over-summer survival in intermittent stream

Background/Question/Methods

Fragmentation of streams into isolated pools during the dry season is prevalent in low-order coastal California streams that are spawning and rearing habitat for salmon, yet ecologists have only recently focused on how stream drying affects stream communities and food webs. Flow drives invertebrate drift, temperature regime, and dissolved oxygen mixing, and thus strongly influences salmonid energetic requirements and growth rates. Stream diversions can reduce hyporheic flow in intermitent streams below crucial temperature and oxygen thresholds. In this study, we investigated whether salmonid persistence during dry periods exhibits a threshold response that is governed by DO concentrations in semi-isolated pools and investigate differences in DOC availability that drive DO levels once surface flow has ceased. We conducted repeat surveys of juvenile salmonid (Oncorynchus mykiss and O. kisutch) abundance, water temperature, DO concentrations, water depth and velocity, pool volume, and organic carbon concentrations and quality on two tributaries of Salmon Creek (Sonoma Co., CA, USA). We hypothesize that readily bioavailable DOC will drive DO concentrations once surface flow ceases, with higher DOC leading to lower DO levels via microbial respiration.

Results/Conclusions

We instrumented study reaches with piezometers containing thermistor arrays, which we use to calculate vertical groundwater flux, and installed one continuous water quality sonde. We collected water samples for DOC analysis from piezometers and surface water once per month during the dry season (with additional samples collected during rapid drydown) and analyzed fluorescence and DOC composition. We used a hierarchical modeling framework to assess the survival of stream fish from paired sampling of discrete habitat units and investigated the relative importance of flow, fragmentation, and habitat quality to fish over-summer survival. Results indicate that dissolved oxygen drops to critical levels in many isolated pools, and that minimum DO levels are correlated with high organic matter content, long upstream subsurface flow paths, and length of time in the fragmented state. We found high salmonid survival in some isolated “sanctuary” pools that contained groundwater seeps, larger volume, and large woody debris or other cover. Our results demonstrate the importance of increasing of late-season groundwater flow to benefit juvenile salmonids, and suggest that salmonid recovery and stream restoration strategies should target “sanctuary reaches” that possess adequate flow and structural complexity to support summer rearing. Future work using DOC as a tracer can identify source aquifers that sustain late-summer baseflow, thus enabling managers to target aquifer recovery strategies in those areas.