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.