Groundwater Is Key to Salmonid Persistence and Recruitment in Intermittent Mediterranean‐Climate Streams

Abstract

Juvenile coho salmon thrive in intermittent streams of the Pacific Northwest yet are in danger of increased mortality from drought and rising temperatures. With warmer temperatures and more frequent climate extremes projected, the need to understand how intermittent stream hydrology and biogeochemistry impact juvenile salmonid habitat and behavior is imperative. Previous investigations indicated that dissolved oxygen limits the persistence of coho salmon in intermittent streams, leading to the hypothesis that groundwater inflow would ultimately control patterns of salmon recruitment and persistence. Here we tested that hypothesis in paired tributaries of Salmon Creek, Sonoma County, CA during California's extreme drought of 2011–2017. We used the fluorescent fingerprint of dissolved organic carbon, together with a parallel factor analysis, to estimate groundwater influence in individual stream pools, corroborating those estimates with stable isotope and radon analyses. Repeat snorkel surveys provided fish counts in those pools at the beginning and end of the period of surface‐water disconnection. Results suggested that coho salmon fry preferentially selected pools with a groundwater inflow signal and persisted in pools maintaining that signal through the dry season. This groundwater inflow signal was distinct from hyporheic influence, which exhibited little correlation with fish distribution. Groundwater within pools was young, and spot measurements suggested that it was relatively oxygenated. Proportional groundwater contributions to pools increased as drought deepened. Results suggest that maintaining relatively high groundwater levels in coastal aquifers may be imperative to the persistence of vulnerable salmonid populations in a changing climate.

Refuges and ecological traps: Extreme drought threatens persistence of an endangered fish in intermittent streams

“…Heterogeneity in physical catchment characteristics is a defining feature of Mediterranean climate regions (Cid et al., 2017). Indeed, adjacent catchments within our broader study region have remarkably distinct hydrologic characteristics owing to differences in lithology that influence subsurface water storage (Dralle et al., 2018) and over‐summer streamflow conditions (Larsen & Woelfle‐Erskine, 2018). However, there are currently no regional data available that describe variation in catchment lithology in relation to catchment hydraulics (water storage and runoff properties) and field methods to characterize local substrate and subsurface hydrologic properties within streams remain time‐ and resource‐intensive.…”

Rapid and Accurate Estimates of Streamflow Depletion Caused by Groundwater Pumping Using Analytical Depletion Functions

“…Groundwater pumping reduces streamflow ("streamflow depletion") by capturing groundwater which would have otherwise discharged into streams or inducing infiltration from the stream into the aquifer Bredehoeft et al, 1982). This can have negative consequences on surface water users and aquatic ecosystems, both of which depend on a stable contribution of groundwater to streamflow (Gleeson & Richter, 2017;Larsen & Woelfle-Erskine, 2018;Perkin et al, 2017;Rohde et al, 2017Rohde et al, , 2018. However, quantifying streamflow depletion is challenging due to the complexity of modeling groundwater-surface water interactions (Barlow & Leake, 2012).…”

Comparing Streamflow Depletion Estimation Approaches in a Heavily Stressed, Conjunctively Managed Aquifer

“…Groundwater is an essential contributor to streamflow around the world (Beck et al., 2013), providing a relatively cool and stable supply of water particularly during dry periods. Groundwater inflow to streams (“baseflow”) is essential for a number of aquatic and groundwater‐dependent ecosystems (Larsen & Woelfle‐Erskine, 2018; Rohde et al., 2017). However, groundwater abstractions can lead to reductions in streamflow (“streamflow depletion”) via the capture of discharge, which includes interception of water which otherwise would have discharged into a stream or, in extreme cases, induced infiltration from a previously gaining stream (Barlow et al., 2018; Bredehoeft, 2002; Bredehoeft et al., 1982).…”