The Thirsty Eel: Summer and Winter Flow Thresholds that Tilt the Eel River of Northwestern California from Salmon-Supporting to Cyanobacterially Degraded States
Although it flows through regions of northwestern California that are thought to be relatively well watered, the Eel River is increasingly stressed by drought and water withdrawals. We discuss how critical threshold changes in summer discharge can potentially tilt the Eel from a recovering salmon-supporting ecosystem toward a cyanobacterially degraded one. To maintain food webs and habitats that support salmonids and suppress harmful cyanobacteria, summer discharge must be sufficient to connect mainstem pools hydrologically with gently moving, cool base flow. Rearing salmon and steelhead can survive even in pools that become isolated during summer low flows if hyporheic exchange is sufficient. But if the ground water discharge that sustains river flow during summer drought drops below critical levels, warm stagnant conditions will kill salmonids, and cyanobacteria will thrive. Challenges and opportunities for restoring the Eel and increasing its resilience to climate extremes, water diversions, and excessive loading of fine sediments point toward exploring how land use and terrestrial vegetation affect delivery from uplands of water, heat, sediments, solutes, organic matter, and organisms—in ways that either heal or damage rivers.
Map of the entire Eel watershed, showing Potter Valley diversion where headwaters of the mainstem Eel are diverted through a 3.2 km tunnel south across the basin divide to the Russian River. The small Van Arsdale reservoir is just above the diversion on the Eel. To the west and slightly north, the larger reservoir serving the diversion, Lake Pillsbury lies at the confluence of the mainstem Eel headwaters and its small Rice Fork tributary, joining from the southeast. These are the only two reservoirs on the Eel. Squares show locations of dog deaths associated with cyanobacterial blooms since 2002.
Temperature (iButton) records from the river bed where benthic algal turfs remained attached, and adjacent floating mats of Cladophora from two adjacent river pools, GirlyMon and Merganser Pool, downstream. Floating mats, which trap sun-warmed water, showing the high amplitude fluctuations and high peak day-time temperatures in floating mat microsites, compared to the greater thermal stability of the benthic turfs. Floating mats attained temperatures above 30°C that stress or kill common green algae and diatoms, but are tolerated by or even favorable for some cyanobacteria (Paerl and Huisman, 2009).
Water year 2012–2013, which produced the largest proliferations of Cladophora glomerata, followed by the largest cyanobacterial blooms, ever observed in 25 years of field work in the upper South Fork Eel River within the Angelo Coast Range Reserve. On 2 December 2012, the first and last scouring flood of the 2012–2013 water year (that exceeded the 120 m3/s bankfull threshold at this site) scoured out many early instar Dicosmoecus gilvipes, reducing subsequent summer densities of the large, armored instars that, if abundant, can suppress macroalgae blooms. After the single flood, stable, clear, warming flows gave Cladophora a long seasonal ‘head start.’ But on 26 June, a small spate (that elevated stage ∼15–20 cm and discharge by 2.5 cm [inset upper right]) detached Cladophora streamers that had attained lengths of up to 8–10 m. This biomass was not exported far downstream, but accumulated as large floating mats along the slack-water margins of pools, and at pool tails where they stranded around emergent rocks (Fig. 4A).
(A) Decaying mat of Cladophora and its epiphytes, dominated by diatoms, at the tail of GirlyMon Pool. Healthy fresh Cladophora is a bright green color, and becomes yellow, then red as it gets encrusted with deeper and deeper layers of epiphytic diatoms. Diatom-covered assemblages of Cladophora are extremely high in food quality for riverine grazers. As these mats warmed in the sun, however, Cladophora hosts and diatom epiphytes perished. (B) Cell contents leaking from a dying diatom, Epithemia adnata, one of the dominant summer epiphytes on Cladophora, and highly preferred by vertebrate and invertebrate grazers (micrograph taken at 400X). (C) In the organic matter released by dying diatom and filamentous cells (orange debris is due to released carotenoids), balls of dark, olive-green filamentous cyanobacteria appeared (micrograph taken at 100X). (D) The terminal positions of their heterocytes suggest that these were colonies of Cylindrospermum sp., a cyanobacteria known to be neurotoxic (micrograph taken at 400X). Photographs by M. E. Power.
Merganser Pool, downstream from the floating mats in Figure 4, where a stubble of Cladophora epiphytized by diatoms remained attached. These remnant turfs were initially green (new Cladophora growth), and became yellow or rusty-red over the summer, depending on stage of epiphyte succession and thickness on hosts. After floating mats proliferated upstream, these turfs turned dark olive-green to black as they became thickly overgrown with cyanobacterial mats dominated by Cylindrospermum. Photographs by M. E. Power. Inset upper right: Late summer Cladophora-epiphyte filaments completely overgrown with dense dark colored cyanobacterial mats, bouyed by oxygen bubbles from photosynthesis. Some clumps have pulled loose and are floating downstream as potential propagules that can be advected by river flow or wind for many meters to infect downstream habitats. The blue thermometer lying on the riverbed is 15 cm long. Photograph by K. Bouma-Gregson.
Contributor Notes
Associate Editor: J. F. Schaefer.