By Andrew J. Lawrence and Andrew L. Rypel
Apocalyptic scenes of wildfires and floods are now familiar to Californians. However, the ecological impacts from these events remain understudied in California and across the world. Gaps in awareness and understanding on the issue are especially intense for freshwater mussels, whose cryptic and sedentary life-histories belie their importance to freshwater ecosystems and biodiversity (see previous post on freshwater mussels). One difficulty in studying effects of wildfire on freshwater ecosystems is that there is often a “right time in the right place” factor to appropriately conduct the science. For example, researchers and biologists often need to be studying a population or ecosystem before a burn so effects afterwards can be quantified – ideally alongside nearby unaffected control sites. Yet such natural experiments are rare because we never know when and where major wildfires will strike.
The 2020 Dolan Fire in Monterey County became an opportunity to document effects of this ‘one-two punch’ on freshwater mussels and some native fishes, described in our recently published paper (Lawrence et al. 2022). Shortly before the fire began, populations of native freshwater mussels were documented in headwater streams of the Nacimiento and San Antonio rivers in Monterey County, California (Fig. 2A). Species encountered included California floaters (Anodonta californiensis/nuttalliana; Fig. 2D) and western ridged mussels (Gonidea angulata), the latter being a current candidate for listing under the Endangered Species Act. Unfortunately, the excitement that accompanies finding previously undiscovered populations of imperiled species was quickly dampened by the catastrophic wildfires engulfing the landscape. The Dolan Fire burned about 20,432 ha (50,488 acres) above our study site in the San Antonio River headwaters region beginning in August 2020 (Fig. 3) and left behind a severely charred and exposed landscape. The fire was followed by an exceptionally dry period that was finally interrupted by a major atmospheric river that produced 38–51 cm (15–20 in) of rain over three days in late January 2021, resulting in significant hydric erosion across the burned area. Large unseasonal atmospheric rivers on their own (without burns) are already known to harm native fish diversity, including the induction of large fish kills (Rabidoux et al. 2022).
What happened to the mussels?
Following the major storm, sediment from wildfire ash and exposed soil nearly filled our entire study site pool, such that the pool’s thalweg (i.e., the deepest point) decreased from 1.77 m to just 0.2 m (Fig. 2B, 2C). While mussels can move throughout some substrates (Watters et al. 2001), the potential interactive effect of volatile compounds and substantial depth of new sediment seemed to exceed survival and movement capabilities. We found no live mussels during post-fire visual and tactile surveys and found no mussel eDNA at the study site (Fig. 3). Although California floaters inhabit diverse aquatic habitats (Jepsen et al. 2012), this population was apparently extirpated by wildfire runoff. The western ridged mussel was not identified at this particular site before the sedimentation; but limited access during the fire and short time until the post-fire survey does not eliminate the possibility that the mussel also occurred elsewhere in this system.
What about native fishes?
Before the wildfire, we found mature and juvenile Monterey sucker, (Catostomus occidentalis mnioltiltus), Monterey roach (Lavinia symmetricus subditus), and Sacramento pikeminnow (Ptychocheilus grandis) at the site. Even during periods of severe drought, the large and deep pool held stable water levels and was likely a refuge for fishes and other aquatic organisms. During post-wildfire surveys, we saw several dead mature Monterey suckers, while smaller-bodied fishes (Monterey roach and speckled dace [Rhinichthys osculus]) persisted in the shallow waters of the significantly altered stream.
Are freshwater mussels in California doomed?
We don’t think so…yet. But, keeping them around will require our help. More work is needed to identify broader regions and finer-scale stream characteristics most at-risk to wildfire runoff. This need is incidentally also true for other aquatic organisms including fishes, waterfowl, other freshwater invertebrates, aquatic herpetofauna, plants, aquatic mammals, and other dimensions of freshwater biodiversity. For freshwater mussels and other freshwater taxa eking out a living in California’s harsh Mediterranean climate, individuals often need remnant pools as refugia during the dry season (Bogan et al. 2019). Yet as we have seen in the San Antonio River, these pools can be rapidly filled by heavy sedimentation events. In these cases, previously beneficial refugia can quickly become “ecological traps” (Battin 2004). Climate change effects are never simple or straightforward.
Had the rainfall been spread out over weeks or months, perhaps runoff and corresponding impacts on the ecosystem would have been less severe. Unfortunately, this will be a common pattern in the coming century. California’s Mediterranean climate is becoming a more extreme version of itself, with increases in the frequency and intensity of precipitation events, coupled with larger and hotter wildfires overall (Touma et al. 2022). Will the habitat conditions and aquatic organisms of impacted streams return to their previous state? They might get close, but fully recovering would likely take a long time, especially given the long turnover rates of most mussel populations (Haag and Rypel 2011). Knowing how much sediment is too much for mussels to survive will also require further work, as effects can vary among species (Imlay 1972, Brim Box and Mossa 1999). Ultimately, California’s native freshwater mussels need much more research and extensive conservation work. Ironically, the unlisted status of California’s native mussels under the ESA has likely limited opportunities for research and conservation. This may change given petitions to list some native mussel taxa, and their reliance on native fishes in California to reproduce – these host fishes are also declining at alarming rates.
Andrew Lawrence is an interdisciplinary ecologist with Colorado State University’s Center for Environmental Management of Military Lands (CEMML). Andrew L. Rypel is a Professor and the Peter B. Moyle and California Trout Chair of coldwater fish ecology at the University of California, Davis. He is a faculty member in the Department of Wildlife, Fish & Conservation Biology and Director of the Center for Watershed Sciences.
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The strange, savage life of a freshwater mussel https://www.vox.com/down-to-earth/2022/10/27/23424362/freshwater-mussels-fish-lure-extinction
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