The Ecological Future of the Salton Sea
By Timothy J. Bradley1 and Gregory M. Yanega2
1 Dept. of Ecology and Evolutionary Biology
University of California
Irvine, CA 92697-2525
2 Biology Department
Linfield College
McMinnville, OR 97128-6894
Send Correspondence to: Timothy Bradley [email protected]
May, 2017
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Executive Summary
The Salton Sea is one of the most important and productive avian feeding sites on the
Pacific Flyway. The Sea has been maintained for the past 110 years by runoff from agricultural
fields irrigated with water from the Colorado River. In January 2018, mitigation water which
has been flowing to the Sea to maintain its level and salinity will be diverted, causing the Sea to
shrink in volume and increase in salinity. These changes will eliminate fish from the Sea,
causing, in turn, serious negative consequences for multiple species of fish-eating birds. These
negative consequences will reverberate across bird populations in both North and South
America. Beginning in 2018, the Salton Sea will shift from a system in which fish are the
trophic level on which birds feed, to one in which birds feed on invertebrates (Figure 1). The
presence of brine shrimp and brine flies grazing intensely on algae, will promote a deeper photic
and aerobic zone. This may have profound positive effects on tourism at the Sea, but will also
pose challenges for toxic selenium management in the ecosystem.
Wetlands and salt lakes in the arid West are shrinking and or disappearing due to drought
and water diversions. The ecological transition of the Salton Sea therefore has increasingly
profound implications for continent-wide avian populations and communities. It will be
essential that managers and regulators responsible for the Sea promote the establishment of brine
shrimp and brine flies in order to exclude more noxious species. In addition, care must be taken
to monitor and control selenium levels to protect the millions of migratory birds that feed at the
Sea. Finally, great care must be taken to reduce the emission of dust from the newly formed
playa, to avoid a public health catastrophe similar to, but far more extensive than, that at Owen’s
Lake.
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Background
The Salton Sea, the largest inland body of water in California, plays a vital role in the
ecology of the northern Sonoran Desert. The Salton Sea was formed in 1905-06 when the
Colorado River flooded, escaped its banks, and flowed into the Salton Sink for nearly a year and
a half. It is a matter of conjecture whether the flood of 1905 was caused by the newly
constructed canals or was simply a repeat of a pattern of flooding thousands of years old. What
is clear is that a vast lake, nearly 50 miles long, was formed in the desert (deBuys 1999). In the
latter part of the 20th century, the lake began to get more and more saline. Evaporation from the
lake’s surface has caused the salinity to increase to a value 1.7 times that of the ocean.
As salts have accumulated in the Sea over more than a century, so also have nutrients.
As a result, the Sea is hyperproductive , with abundant primary productivity due to algae. These
algae form the base of a prolific food chain which supports an extraordinary concentration of
fish, invertebrates, and birds. Bird species dependent on aquatic habitats traverse the arid
western U.S. on stepping stones of wetlands and salt lakes scattered along the Pacific Flyway.
The most important and biologically productive of these is the Salton Sea. Through 2016, the
Sea has supported a large introduced fishery of Tilapia. As a result, the Salton Sea and the
region around it have supported multitudes of birds from over 300 species (Shuford et al. 2002).
With the increases in the Sea’s salinity expected beginning in 2018, all biologically relevant fish
populations are expected to die off, leading to a profound change in the lake’s animal
communities.
Through intentional irrigation and some unintentional flooding, the Salton Sea is an
entirely managed body of water whose water levels have been largely unchanged for over 100
years. This is about to change. Due to implementation of the Quantitative Settlement
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Agreement (QSA), an agreement entered into by the federal and state governments with regional
water districts, water currently used in the Imperial Valley will be transported to water districts
outside of the valley. This will cause water levels at the Salton Sea to begin dropping rapidly in
2018, resulting in receding shorelines (referred to as playa) and increasing salinity (Cohen 2014).
For these reasons, the Salton Sea of the future will be an ecosystem in transition.
Many hypersaline terminal lakes (70-120ppt) support a fauna of limited diversity but
great abundance where brine shrimp (Artemia spp.) and brine flies (Ephydridae) thrive as
primary consumers of algae. Lakes of this kind in Western North America (e.g. Great Salt Lake,
Mono Lake, and Lake Abert) support or have supported large bird populations whose focal
species include Wilson’s Phalaropes (Phalaropus tricolor), gull species, and Eared Grebes
(Podiceps nigricollis californicus).
We seek here to provide insights into the near future of the Salton Sea, and its ecological
trajectory. In addition, we point out management concerns that will need to be addressed as the
Sea undergoes its ecological transition.
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A Future in Transition
The Salton Sea, a major ecological feature in the Colorado region of the Sonoran Desert
is about to undergo a profound ecological shift. In fact as we write (April, 2017), evidence is
accumulating that the Tilapia may already be experiencing reduced reproductive success in the
main body of the Sea. We anticipate that the Sea will transition, if properly managed, into a salt
lake similar in ecology to Mono Lake and the Great Salt Lake. While this will benefit many bird
species, it will produce serious continent-wide deficits in feeding habitat for several species of
bird. To address this problem, state agencies in California are proposing the construction of
mitigation marshes on the shores of the receding Sea. As discussed below, this may create
problems with regard to selenium management.
In the past 15 years, a number of species of fish previously prevalent in the Salton Sea
have died off due to increasing salinity. These include Orangemouth Corvina (Cynoscion
xanthulus), Bairdiella (Bairdiella icistia), Sargo (Anisotremus davidsoni), and Threadfin Shad
(Dorosoma petenense). The only species of fish currently remaining in the Sea are hybrid
Tilapia, although Desert Pupfish (Cyprinodon macularis) , and Sailfin Molly (Poecilia latipinna)
are highly saline-tolerant species found in streams and canals contiguous with the Sea (Lorenzi
& Schlenk 2014, Riedel et al. 2002, Sutton 2002, Gonzalez et al. 2005).
Those species of fish that have died off at the Sea often survived the increasing salinity
for a number of years as adults, but eventually failed to successfully breed. The salinity
tolerance of the Tilapia in the Salton Sea is unfortunately far from clear. Some species of Tilapia
are restricted to fresh water, while other species can survive in waters with concentrations equal
to seawater or even higher. The Tilapia in the Salton Sea are hybrids of two species;
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(Oreochromis mossambicus X O. urolepis hornorum). For this reason there are relatively few
published data about their salinity tolerance. Sardella et al. (2004, a,b) and Lorenzi & Schlenk,
(2014) examined the salinity tolerance of hybrid Tilapia from the Salton Sea. Based on these
studies it has been proposed that the Tilapia at the Salton Sea will cease to reproduce and will be
eliminated from the main body of the Sea when salinities reach approximately 60-70 ppt. This
takes into account an increased sensitivity to high salinity at low temperatures. Currently, Desert
Pupfish and Sailfin Mollies are excluded from the main body of the Sea, presumably due to the
risk and/or occurrence of predation by the much larger Tilapia. It is anticipated that all three fish
species will be excluded from the main body of the Sea when salinity reaches 70 ppt. This will
occur in the next five years (by the year 2021) as result of transfers of water to urban centers as
mandated by the Quantitative Settlement Agreement which goes into effect in 2018 (Little
Hoover Commission Report, 2016).
Elimination of fish from the Sea will open the habitat to invertebrate species. These
include brine shrimp (Artemia sp.) and brine flies (Ephydra sp.). Two of the largest saline lakes
in North America, Mono Lake and the Great Salt Lake, have abundant brine shrimp populations
that feed on pelagic algae and large brine fly populations that feed on benthic algae. The brine
shrimp at Mono Lake are in the species Artemia mono, while those at the Great Salt Lake are
Artemia franciscana. The brine flies at Mono are principally Ephydra hians while those at the
Great Salt Lake are a mixture of E. hians and E. cineria. It is unclear which species will
eventually colonize and predominate at the Salton Sea, but it should be pointed out that the salt
composition at the Salton Sea is much more similar to the Great Salt Lake than it is to Mono, a
lake rich in sulfate and with a pH close to 10.
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Virtually all of the salt lakes, both ephemeral and permanent in the western U.S. contain
populations of brine shrimp and brine flies. Brine shrimp are readily transported as cysts by
water birds as these migrate up and down the Pacific flyway. Brine flies fly from one body of
water to another and are extremely tolerant of high salinities and high temperatures (Herbst et al.,
1988, Herbst & Bradley, 1989).
Studies we have carried out at our laboratory at the University of California, Irvine
indicate that cysts of the brine shrimp Artemia franciscana will hatch in Salton Sea water in
salinities ranging up to about 123 ppt. This result is similar to that of Clegg (1978) who
suggested a limit of 117 ppt based on metabolic studies of cysts of Artemia salina. Once
hatched, the nauplii can survive in water ranging up to 189 ppt. This is similar to results reported
by Dana & Lenz (1986). These results demonstrate that the ionic composition of the Salton Sea
does not prohibit colonization by brine shrimp once fish have been excluded. This supports
anecdotal observations of occasional brine shrimp colonization in isolated natural and
constructed ponds adjacent to the Sea (Jack Crayon, personal communications). Although
experience and direct experimentation strongly suggest that the Sea will transition to one
supporting an abundance of brine shrimp and brine flies, the stochastic nature of ecological
transitions argues for caution (Kuiper et al., 2015). We urge careful monitoring of the Sea
coupled with management to promote brine fly and brine shrimp colonization.
Elimination of fish species from the Salton Sea, coupled with the establishment of brine
shrimp and brine fly populations will have a profound effect on water quality. Currently, the
Salton Sea frequently has a Secchi depth of less than a meter due to the abundance of pelagic
algae (Holdren & Montano 2002). At Mono Lake, a salt lake lacking fish, algal blooms in the
Spring turn the water a bright green. After the brine shrimp hatch in the spring and begin filter
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feeding, the water becomes progressively more clear. By mid-summer the Secchi depth
increases to tens of meters (Lenz 1984; Lenz et al. 1986). The situation may be slightly different
at the Salton Sea. Mono Lake experiences cold winters and the brine shrimp populations
overwinter as cysts. At the Salton Sea, winter temperatures drop only to around 13oC. Brine
shrimp populations there may not show annual declines and cyst production. None the less, we
anticipate that brine shrimp populations will be sufficiently large to graze down the quantity of
pelagic algae. This will greatly increase the penetration of light into the water, allowing the
photic zone to increase in depth. This will allow benthic algae to proliferate over extensive
acreage. The downward extension of the photic zone for pelagic algae, and the proliferation of
benthic algae will serve to produce oxygen at greater depths. Currently, in the summer when
bacterial activity degrading organic matter is at its greatest, the aerobic zone in the Salton Sea
can be restricted to the upper few centimeters. We anticipate that the aerobic zone will be
substantially deeper when brine shrimp are present. The extremely high levels of nutrients in the
water should support abundant turnover of pelagic and benthic algae. The former will support
large populations of brine shrimp, the latter large populations of brine flies.
Abundant anaerobic metabolic byproducts are currently produced in the anoxic benthic
regions of the Salton Sea. These include ill-smelling organic gases, including hydrogen sulfide.
As a result, the Sea is unattractive for many recreational purposes due to a “rotting egg smell”.
We anticipate that once the aerobic zone achieves a greater depth, much of the organic
degradation will switch from anaerobic to aerobic processes, reducing the production of noxious
gases and smells.
Elimination of fish from the Salton Sea due to increasing salinity will produce a habitat
subject to invasion by saline-tolerant invertebrates. Concern has been raised that various
8
mosquito species may be among these invertebrates. Currently mosquitoes breed abundantly in
salt marshes adjacent to the Sea. These salinities can exceed those of the open ocean (personal
observations TB). Mosquito larvae are rarely found in large, open bodies of water, but this is
normally due to exclusion by fish. There is therefore a serious concern that mosquito production
in the Salton Sea could occur in the absence of fish. This would be a very serious outcome since
mosquitoes in these warm regions are excellent vectors of West Nile Virus, Western Equine
Encephalitis and St. Louis Encephalitis. The best way to prevent the establishment of
mosquitoes in the Salton Sea is to promote the establishment of brine shrimp following the loss
of fish from the Sea.
Millions of birds use the Salton Sea for staging during fall and spring migration (Shuford
et al. 2002). Although piscivorous birds comprise only 15% of the species at the Sea, they are an
abundant and charismatic component of the avifauna. Many of these species are colonial
waterbirds, e.g. Double-crested Cormorants (Phalacrocorax auritus), Black Skimmers
(Rynchops niger), Caspian Terns (Hydroprogne caspia), Gull-billed Terns (Gelochelidon
nilotica), and American White Pelicans (Pelecanus erythrorhynchos) for whom the Sea has been
an important breeding ground in the last 20 years (Shuford 2014). Declining lake levels have
exposed island breeding grounds to terrestrial predators. Although over 70% of California’s
8,300 Double-crested Cormorant bred at the Salton Sea in 2012; these colonies failed completely
and have not been re-established since 2013 (Shuford 2014).
The Sea will no longer support obligate piscivores when the Tilapia die off. There will
be fewer bird species. However, Eared Grebe, Ruddy Duck (Oxyura jamaicensis), Northern
Shoveler (Anas clypeata), California Gull (Larus californicus), and Ring-billed Gull (Larus
delawarensis) as well as populations of saline lake specialists such as Black-necked Stilts
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(Himantopus mexicanus) and American Avocets (Recurvirostra americana), will likely increase
in number. Wilson’s Phalarope, and Red-necked Phalarope (Phalaropus lobatus) currently do
not occupy a prominent place in the Salton Sea avifauna, though they’re numerous (tens of
thousands) at more northerly sites such as Mono Lake (CA), and the Klamath Lakes Complex.
A stopover site in the northern Sonoran Desert would reduce the distance between staging
grounds and wintering grounds in Ecuador for these species (Warnock et al. 1998).
The number of species that breed at the Salton Sea are few in comparison to those that
use the lake as a migratory stopover site, but they do include a number of species of special
concern at regional and Federal levels, including endangered (Yuma Clapper Rail, Rallus
longirostrus yumanensis), threatened (Snowy Plover (Charadrius nivosus)), and saline lake
specialists such as Black-necked Stilts and American Avocets.
The American West has already experienced enormous losses of wetland acreage
(estimated at 91% in California) as well as the loss of the Colorado River Delta due to upstream
water diversions. Many terminal lakes across the West are disappearing (Abert, Goose, Honey,
Klamath, Walker) due to drought and water diversions. This is severing the string of wetlands
and lakes that characterized inland valleys across the Great Basin and northern Sonoran region.
Loss of the Salton Sea will have a disproportionately large impact on the connectivity of bird
habitats due to increased distance between sites and scarcity of sites large enough to support
diverse food webs and consumer communities. The Salton Sea is effectively a managed
wetlands, highlighting the interdependence of ecosystem function and economic considerations
(agriculture, urban use, public health). The ecological transition of the Salton Sea has profound
implications for continent-wide avian populations and communities.
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Changes at the Sea will also affect humans. Removal of windrows of dead fish,
clarification of the water, and reduction of anoxic gas eruptions may well make the Sea once
again more attractive as a tourist destination. The abundance of brine shrimp may also prove to
be an economic boon for the harvesting of adult shrimp and/or cysts.
It is essential that levels of selenium in the water and in the food chain be carefully
monitored. Should the Sea become a place where millions of birds feed on invertebrates that are
bioaccumulating selenium, it could be an unprecedented biological disaster.
We have outlined above our reasons for believing that the Sea will have higher levels of
dissolved oxygen and deeper aerobic zones in the future once fish have been eliminated. The
water flowing to the Sea has, for the past 110 years, been almost exclusively derived from
agricultural runoff. This runoff contains total selenium concentrations ranging from 2 -30 parts
per billion (ppb) (Johnson et al., 2009). Maximum levels permitted in open waters by federal
regulation are 5 ppb. The water in the Salton Sea itself is low in dissolved selenium because
selenate and selenite become reduced in the anoxic regions of the Sea and precipitate into the
sediment (Schroeder et al. 2002). If the aerobic zone in the Sea should extend deeper into the
Sea, this selenium may be oxidized and thus solubilized. This could cause the entire Sea to
contain levels of selenium sufficient to cause dangerous bioaccumulation. We urge managers
and regulators to keep a careful eye on selenium levels in the Sea with the goal of minimizing its
possible effects on birds that feed at the Sea.
In 2002, the Quantitative Settlement Agreement was entered into by the State of
California, the federal government, and regional water districts. It stipulates that, beginning in
2018, agreed-upon volumes of water will be transferred from the Imperial Valley to more urban
water districts. As result, starting in 2018, the Sea will shrink and become more saline. The
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newly exposed bottom of the Sea (referred to as playa), will be the source of toxic dust
containing heavy metals and agricultural chemicals. This dust will greatly increase the risk of
asthma, chronic obstructive pulmonary disease, emphysema and cancer (Cohen 2014). To
reduce this risk, the California Natural Resources Agency proposes to build marshes on the
newly exposed playas to reduce dust emission. The water for these marshes will derive from the
rivers (i.e. agricultural runoff) mixed with water from the Sea. The river water poses extreme
risks for selenium bioaccumulation in these marshes. While the marshes are essential as a means
of protecting the health of hundreds of thousands of residents in the adjacent communities, they
also pose grave risks for bioaccumulation of selenium (Ohlendorf et al. 1987; Setmire et al.
1993). It will be essential that the selenium levels in the marshes be carefully monitored, and
that corrective measures be taken if selenium levels are found to reach a level of concern.
Acknowledgements: We thank the UC Irvine Salton Sea Initiative for financial support.
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Figure 1.
birds
fish
worms pelagicalgae barnacles
benthicalgae waterboatmen
bacteria
Foodchainpriorto
collapseofthefish
populations
birds
brineshrimp
pelagicalgae brineflies
benthicalgae waterboatmen
Foodchainaftercollapse
ofthefishpopulations
bacteria
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