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Vernal pools: Rains bring to life mini-ecosystem of button celery, Otay Mesa mint and fairy shrimp

Vernal pools: Rains bring to life mini-ecosystem of button celery, Otay Mesa mint and fairy shrimp
Biologists Jason Allen (left) and Doug Allen take a closer look at one of the many vernal pools in a fenced off area of Otay Mesa designated as an Habitat Enhancement Site. (Nelvin C. Cepeda/San Diego Union-Tribune)

Alongside auto wrecking yards and shipping centers off state Route 905, a pop-up world has emerged with some of the strangest creatures to swim in six inches of water.

Here aquatic plants grow next to cacti, and animals that have waited for decades in the dust come to life. In this Otay Mesa preserve are some of San Diego's vernal pools, fleeting water bodies that appear and vanish over the course of a season.

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They don't fill up every year; just in wet ones, when San Diego's ubiquitous hard-pan soil turns shallow depressions into temporary holding ponds. This season, rainfall is well above average, and the Otay Mesa site is a water world of ephemeral pools.

With them come a host of rare and endangered species: San Diego and Riverside fairy shrimp, spadefoot toads, and numerous plants that occur only in or near San Diego. Pacific tree frogs, small crustaceans, dragonfly larvae and beetles also dwell in these miniature ecosystems.

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"Coming from an arid environment, it's really impressive to see the amount of life that appears in these pools in the wet season," said Mark Berninger, a senior planner with the City of San Diego's Parks and Recreation Department, Open Space Division.

Biologists collect a small grouping of Riverside Fairy Shrimp for closer examination.
Biologists collect a small grouping of Riverside Fairy Shrimp for closer examination. ((Nelvin C. Cepeda / San Diego Union-Tribune))
A biologist holds up a Baja California treefrog that was near one of the vernal pools.
A biologist holds up a Baja California treefrog that was near one of the vernal pools. (Nelvin C. Cepeda/San Diego Union-Tribune)
San Diego Button Celery growing in a fenced off area of Otay Mesa designated as an Habitat Enhancement Site.
San Diego Button Celery growing in a fenced off area of Otay Mesa designated as an Habitat Enhancement Site. (Nelvin C. Cepeda/San Diego Union-Tribune)

There are 2,591 vernal pools in the city of San Diego's preserves, and biologists actively monitor about about 650 of them, Berninger said. Others occur on Marine Corps Air Station Miramar and in San Marcos and Ramona. A preserve containing vernal pools is located near Fry's electronics in San Marcos, and other pools dot city and private properties at San Marcos Boulevard at La Mirada Drive.

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Biologists collect a Riverside Fairy Shrimp for a closer examination.
Biologists collect a Riverside Fairy Shrimp for a closer examination. (Nelvin C. Cepeda/San Diego Union-Tribune)
Kearny Mesa is among the places in the county were vernal pool basins are clustered.
Kearny Mesa is among the places in the county were vernal pool basins are clustered.

What remains, however, is dwarfed by what was lost. The existing pools are a small fraction of the estimated 200 square miles of vernal pool habitat that once existed in San Diego County; scientists calculate that about 97 percent of that habitat has been displaced by roads, ranches and buildings.

Biologists collect a small grouping of Riverside Fairy Shrimp for a closer examination.
Biologists collect a small grouping of Riverside Fairy Shrimp for a closer examination. (Nelvin C. Cepeda/San Diego Union-Tribune)

So environmental authorities have made efforts to preserve what's left. There are 740,000 acres in California and Oregon designated as critical habitat for 15 vernal pool species listed as threatened or endangered, according to the U.S. Fish and Wildlife Service.

In January 2018, the city of San Diego adopted a Vernal Pool Habitat Conservation Plan to manage the water bodies, and earmarked money for protections such as fencing, weed removal and monitoring. The plan kept some pools off-limits, while allowing development in other areas where they occur. Environmentalists, however, objected that the plan allows a net loss of the vanishing habitat, and doesn't conserve remaining vernal pools in areas slated for construction.

"Even after having lost over 97 percent of vernal pools in the city, the plan still allows more destruction of some of the last, best vernal pools anywhere on private property," said David Hogan, a San Diego environmentalist and vernal pool advocate.

The plan does ensure that biologists keep close watch on the protected pools. Earlier this month, Berninger and a team of other San Diego planners and biologists convened at Cal Terraces South in Otay Mesa, a 13-acre complex of small hills and depressions that includes 73 of the pools. The habitat there was improved as mitigation for state Route 905, which runs alongside it, mingling the thrum of traffic with the chirp of frogs and songbirds.

The plants form a community of maritime scrub, with barrel cactus, cholla, euphorbia and other succulents dotted among chaparral.

Next to desert wildflowers and cacti are thickets of duckweed, submerged in pools of water. Alongside those are even more specialized plants that grow exclusively near vernal pools. There's spiky San Diego button celery, a favorite of rabbits, and sweet-smelling San Diego and Otay Mesa mints. These plants are all endemic to the San Diego region, meaning they grow here, and nowhere else.

"Those plants are one part of the year in habitat as dry as the desert," Berninger said. "The other part of the year, they're standing inundated in water. There's not too many plants specialized to be really wet or really, really dry. So that's what makes them unique."

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As the biologists measured the area of a pool, a juvenile Pacific tree frog hopped alongside, alighting on a rock. The tiny golden froglet, with a black bandit's mask across its eyes, is common in California, and responsible for the characteristic croak we associate with frogs.

The pools also host rarer amphibians, including the Western spadefoot toad, which is now under review for possible endangered species protection, according to the U.S. Fish and Wildlife Service. The squat tan or greenish toad lives most of its life on land, but enters vernal pools to breed. Its tadpoles are purely aquatic, with chubby, iridescent gray bodies, golden eyes, and a whip-like tail.

"I love these toads; they have beautiful eyes," said Sara Allen, a biologist with the San Diego Multi Species Conservation Program.

Spadefoot tadpoles develop into one of two variations: grazers and killers. Omnivorous tadpoles scavenge algae and other detritus. Their carnivorous counterparts grow powerful jaws and tails, and prey on fairy shrimp, insects, and even fellow tadpoles.

"They're voracious little predators at this stage," Berninger said.

The pools in the Otay Mesa complex are a natural bouillabaisse of crustaceans. There are isopods, copapods and others. Seed shrimp, encrusted in an oval-shaped brown shell, paddle around in little, self-contained canoes.

What has made vernal pools famous are fairy shrimp, tiny translucent crustaceans that live only in the ephemeral water bodies. The pools at Otay Mesa host both San Diego and Riverside varieties. Both species are smaller than an inch long, but have slightly different body features, and different life strategies.

San Diego fairy shrimp hatch within three to five days after the pools fill with water, Berninger said. After 30 to 60 days, when the pool is more established, Riverside fairy shrimp emerge.

Like a timeshare for fairy shrimp, the different schedules represent "staggered use of limited resources," he said, and are cued to the presence of water.

"We're learning that the inundation triggers the hatching of the shrimp," said Doug Allen, a biologist with San Diego Parks and Recreation, Open Space Division.

Despite their resilience to severe conditions, fairy shrimp have dwindled as vernal pools have disappeared, and both species are federally endangered.

In 2007, the U.S. Fish and Wildlife service designated 3,082 acres of land in Orange and San Diego counties as critical habitat for San Diego fairy shrimp.

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Two years earlier, the service had assigned 306 acres in Ventura, Orange, and San Diego Counties for the Riverside fairy shrimp. It expanded that to 1,724 acres in 2012, after environmental groups including the Center for Biological Diversity sued, charging that the original area was far too small to protect the fragile species.

Those protection efforts provoked ridicule from developers, who asked how a creature smaller than a matchstick could hold up housing construction and other land uses. It's hard to justify saving space for a creature that appears just once a decade or so, they complained.

A 2005 opinion piece in the Orange County Register, for instance, denounced fairy shrimp as mascots for regulatory excess.

"For me, these tiny, hardy creatures symbolize the absurdity of the Endangered Species Act and the extremities to which property owners must go to accommodate even the most esoteric organisms," the author complained.

Before you dismiss fairy shrimp as insignificant, however, consider their extraordinary biology.

These delicate shrimp are some of the closest things to immortals that exist in the animal kingdom.Their cysts persist in dry soil for decades, and then hatch during a wet year. Wildfires can scorch the earth around them, and the shrimp emerge just fine the next rainy season, geologists with San Diego State University found.

Related brine shrimp cysts have survived exposure to liquid air at temperatures of negative 456 degrees Fahrenheit — close to absolute zero. And they have hatched after being heated to about 265 degrees Fahrenheit — the temperature you might use to slow-roast a chicken.

The bite-sized crustaceans also form a link in the food chain. Lizards, frogs, toads, insects, migratory birds and waterfowl visit vernal pools to dine on fairy shrimp. On a recent afternoon in Otay Mesa, a mallard duck floated contentedly in the water. And a California gnatcatcher, a type of threatened songbird, scolded visitors from a thicket of willow.

Despite their ability to withstand a high bandwidth of extreme conditions, fairy shrimp exist only in a narrow range of places, at specific times. The list of possible homes for the shrimp may be shrinking as San Diego acts to build new housing for people. Hogan said some of the last unprotected vernal pools occur on areas planned for future portions of Otay Ranch.

The San Diego County Board of Supervisors is expected to consider that project this spring, Hogan said. He and other environmentalists will ask them to require that developers conserve those ephemeral wetlands. As supervisors consider the plans, they should look to the teeming vernal pools that have sprung to life in San Diego this year, he said.

"A wet year like this reminds us how truly special these little wetlands are to the unique diversity of natural life in San Diego," Hogan said.

Once the weather dries up, the pools will shrivel into meadows, and then flats of cracked mud. Sealed in the soil will be the next generation of fairy shrimp, waiting for the rain.

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