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The Saga of the Quagga
Below is a list of studies on the quagga and zebra mussels, and several links to studies referenced in the article. Click on the titles to view the full study or report.
Quagga Mussel Risk Assessment An experiment test of quagga mussel survival and reproductive status using Lake Tahoe water with a prediction of invasion into Western water bodies
The Ecological Society of America survey (pdf download) “A calcium-based invasion risk assessment for zebra and quagga mussels” commissioned by the U.S. Environmental Protection Agency: This study puts Lake Tahoe, Donner Lake, and surrounding lakes in the “very low risk” category for quagga and zebra mussel invasion. Less than 10 percent of the country is categorized as “very low risk.”
Potential Distribution of Zebra Mussels and Quagga Mussels Phase 1 Report A Report for the California Department of Fish and Game: “Researchers have generally used or recommended the use of 12-15 mg/L [ppm] of calcium or sometimes lower concentrations … However, the records at the lower calcium levels probably represent either misidentifications, limited or inaccurate calcium data, or non-reproducing sink populations recruited from upstream in higher calcium waters.”
Examination of Calcium and pH as Predictors of Dreissenid Mussel Survival in the California Water Project Prepared for the Department of Water Resources: “12 mg/L [ppm] is given as a threshold for possible long-term survival of adult dreissenid mussels, provided the pH is above 7.8. This conservative calcium value is based on experience from Lake Superior where the pH ranges from 7.9-8.2 but the average calcium level is 13 mg/L [ppm]. Lake Superior has only isolated pockets of dreissenid mussels along the shoreline, despite the fact that the first mussels were documented in Duluth Harbor more than 20 years ago”
CALCIUM CONTENT OF LOCAL LAKES
Lake Tahoe (springtime nearshore average) - 9.16 ppm
(Note: Nearshore calcium levels are typically higher than the rest of the lake, and springtime calcium levels can also be elevated, since this is the period where runoff loads the nearshore with minerals and nutrients.)
Donner Lake- 5.16 ppm
Boca Reservoir – 3.09 ppm; 6.6 ppm
Stampede Reservoir- 2.75 ppm; 6.1 ppm
Prosser Reservoir- 4 ppm; 6.7 ppm
Martis Creek Reservoir- 5.5 ppm; 8.7 ppm
Independence Lake- 4 ppm
Marlette Lake- 3 ppm; 3.9 ppm
Spooner Lake- 25.9 ppm; 28.6 ppm
(Note: Where two readings are shown, they represent an early summer average reading and a late summer/early fall average reading in 2012.)
Source: Inventory of aquatic invasive species and water quality in lakes in the Lower Truckee River Region: 2012, UNR
For nearly five years, the campaign against invasive species in the Lake Tahoe area has been spurred by the specter of the environmental devastation created by two mollusks — small, aggressively invasive bivalves called quagga and zebra mussels.
But an in-depth review of national scientific studies reveals that the poster child for local invasive species programs has almost no chance of surviving long-term in local waters. Millions of dollars have been spent trying to keep mussels out of Lake Tahoe, Donner Lake, and surrounding reservoirs, despite the fact that scientific studies suggest the prolific little bivalve can’t colonize these calcium-poor bodies of water.
A new examination of this science has derailed a mandatory boat inspection program that was scheduled to take effect this summer at Donner Lake. After passing an ordinance mandating fee-based inspections last August, Town of Truckee officials have backtracked and are now recommending that the mandatory inspections be put on hold as they investigate the scientific underpinnings of the program. Voluntary boat inspections are still scheduled to take place this summer at Donner Lake and Boca, Prosser, and Stampede reservoirs as they have in years past.
But the science also raises larger questions — questions about how limited and poorly funded local scientific research was able to trump national scientific studies on mussel invasion risks; and questions about how invasive species program advocates have taken limited and conflicting scientific information and, according to one critic, generated a distorted picture of a lake under imminent threat of mussel invasion.
The most critical factor for zebra and quagga mussel survival — and the reason Lake Tahoe and Donner Lake are among the nation’s most inhospitable lakes for mussel invasion — is the calcium content of the water.
Mussels thrive in calcium-rich waters, filtering out the element and using it to build their shells and support their basic metabolic function. Lake Mead, the southern Nevada water body where some of the dire, worst-case-scenario examples of mussel infestations have occurred, has calcium levels of 80 parts per million (ppm) and water hardness levels of 288 ppm. In these conditions, quagga mussels grow large, reproduce rapidly, and colonize aggressively.
But Lake Tahoe has calcium levels that are merely one eighth those of Lake Mead, and Donner Lake’s calcium levels are even lower — less than half of the nationally recognized minimum calcium threshold for mussel survival. An average of three calcium readings last fall put Donner Lake’s calcium levels at 5.2 ppm, or about 15 times lower than Lake Mead’s concentrations. Despite those average calcium readings, the concentrations do fluctuate throughout the season and vary in different parts of the lakes. Some small areas of Tahoe’s shoreline have registered higher concentrations, including readings of 12.4 ppm at the Tahoe Keys Marina and localized elevated calcium readings of 24.1 ppm taken from the sediment of Asian clam beds in the Ski Run Marina. Spooner Lake is the lone local lake with lake-wide calcium levels high enough to support mussel invasion.
Numerous scientific studies say that zebra and quagga mussels cannot survive at calcium levels below 12 ppm. And even at 12 ppm calcium levels, test cases across the nation show that mussels do not flourish. In places like Lake George and Lake Superior, where borderline calcium levels of 12 and 13 ppm exist, mussels have been introduced, but have not colonized either lake.
But for years, Tahoe residents heard little about the science that showed Lake Tahoe and Donner Lake were at very low risk of mussel invasion. Instead, they heard stories about Lake Mead, with photos of quagga-encrusted boat propellers from the southern Nevada reservoir that is a starkly different habitat than the Sierra Nevada, and stories about the tens of millions of dollars of damage the creatures create. And when the public did hear about scientific studies, they heard mostly about one 51-day, low-budget study that for the last four years has heavily influenced invasive species policy in the Lake Tahoe region.
THE MUSSEL RISK ASSESSMENT
In 2008, with the science pointing to Lake Tahoe as being at “very low risk” of mussel invasion, according to the Environmental Protection Agency-commissioned “A Calcium-Based Invasion Risk Assessment for Zebra and Quagga Mussels” (see sidebar), a team of scientists received $20,000 to do a local experiment on the mussel’s ability to survive in Lake Tahoe water. Led by Dr. Sudeep Chandra, an associate professor of limnology and conservation ecology at the University of Nevada, Reno, the team transported eight mussels from the waters of Lake Mead to the UNR lab, and placed them in water taken from the Tahoe Keys in South Lake Tahoe. Tahoe Keys water is among the warmest, most calcium-rich and nutrient-laden water you can find anywhere in Lake Tahoe. Despite the fact that one mussel died and the rest were losing body weight by the end of the experiment, the study reported that, “The possibility exists for at least adult quagga to survive, grow, and reproduce in the Lake Tahoe environment … the diligent monitoring of recreation vehicles (the major pathway for transfer of invasive mussels to inland lakes) putting into western lakes-of-interest is prudent. The assumption that western oligotrophic waterbodies low in calcium are at very low to low risk of quagga mussel invasion is not necessarily supported.”
The report did not research the full life cycle of quagga mussels, including the ability of vulnerable mussel offspring (called veligers) to survive in Tahoe. Studying the ability of mussels to reproduce in an environment is the true measure of risk, since the threat of mollusks is directly tied to their rapid colonization of an environment. A non-reproducing “sink population” of mussels would only survive for the animal’s three- to five-year lifespan.
Steve Urie, a long-time Donner Lake homeowner who has extensively researched the quagga mussel issue, studied the scientific process of Chandra’s laboratory experiment and found flaws in the process.
“The ‘study’ and its conclusion is equivalent to raising tomatoes in a Sacramento greenhouse, transplanting them in a Meyers’ garden in July, and noting that because the tomatoes were still alive on Labor Day that the Tahoe Basin would be a fine area for tomato farming,” wrote Urie in an article submitted to Moonshine Ink.
The report did recognize some of its own shortcomings. The risk assessment said: “It is possible that the individuals collected from Lake Mead had sufficient reserves for survival, and even moderate growth, in any (non-toxic) environment for the 51-day duration of the experiment.”
The fact that adult quagga mussels could survive in Lake Tahoe water for 51 days should not have been surprising. Tahoe’s own invasive species education material says that the mussels can survive for up to a month completely out of water.
“In Tahoe, adult quagga mussel can survive out of water for as long as 30 days!” says the website of Tahoe Keepers, a kayak and non-motorized watercraft inspection group affiliated with the Tahoe Regional Planning Agency.
The study even theorized that the mussels were possibly cannibalizing their own shells to generate enough calcium for survival.
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