In search of the vernal pools, living rainbows

Fairy shrimp, an invertebrate that requires the changing conditions of vernal pools for their life cycle.. Photo courtesy of USGS.

Every spring and summer, seemingly ephemeral ecosystems called Vernal Pools emerge in our region. Rings of wildflowers delineate their boundaries and can have up to six or seven distinct zones of different flowers creating a phenomenon akin to a living rainbow. During the moist spring, fairy shrimp and other enigmatic freshwater fauna swim through these seasonal waterbodies, depositing their eggs in the grayish silty clay soil. In the heat of our arid summers, these pools completely dry up but the eggs are so adapted to these seasonal desert pools that they can lie dormant for years — until the next rainstorm.

A vernal pool is basically an entire ecosystem that is dependent on seasonal fluctuations – from winter precipitation which fills up the ponds, to the dry summers which pull up all the water into the atmosphere leaving large swaths of arid lands crusty and dry. And in these vernal pools we can find invertebrates and plants that are rare in our area, and, of course, lichens too.

Vernal Pool at Turnbull National Wildlife Refuge

Vernal Pool at Turnbull National Wildlife Refuge. Photo courtesy of Douglas King and the Nature Conservancy.

But where do we find these ecosystems? We are lucky, they are all over our region here in Eastern Washington. They range from the size of a puddle to a football field. The easiest way to find them is to keep an eye out for a certain set of plants, and to look within areas that have certain geologic features.

A botanical study by Curtis Björk and Peter Dunwiddie published in 2004 gives some comprehensive information that can be used to identify the location of vernal pools in our region.

Firstly, the vernal pools in Eastern Washington occur on basalt bedrock within the channels that were scoured by the Missoula Floods. Björk and Dunwiddle additionally state that they are often found near or amongst Mima Mounds, which are the bizarre and mysterious prairie mounds found at Turnbull National Wildlife Refuge, over near Hog Canyon, and in many prairie lands throughout the inland northwest.

Navarretia leucocephala, a tiny white flowering plant that we'll usually only find in vernal pools. Photo by Gary Monroe, USDA

Secondly, and perhaps most importantly, we can distinguish a vernal pool from a seasonal wetland by three main criteria listed by Björk and Dunwiddle: 1) annuals are dominant and woody components like cattail and rush stems are minimal; 2) lack of surface salt deposits; 3) the presence of Plagiobothrys, Psilocarphus spp., and Navarretia leucocephala.

So now that we know what to look for, what did Björk and Dunwiddle find in their study? That our vernal pools are quite special and deserve alot more public attention. The species richness in the pools here is greater than the pools in California, and we have a very high density of vernal pools. In some areas, like the Swanson Lakes Wildlife Management Area, there are more than 200 vernal pools per square mile!

Björk and Dunwiddie also found that in the Spokane area the bryophytes associated with the vernal pools include Cratoneuron commutatum, C. filicinum, Riccia beyrichiana, R. cavernosa, Ricciocarpos natans, Fossombronia sp., and Sphaerocarpos texanus; these too create distinct zonation patterns.

Dermatocarpon miniatum. Photo by Jason Hollinger, MushroomObserver.

And lichens? Yes, our lichen friends are found in the vernal pools too! They are usually growing on the cobblestone basalt rocks found within the vernal pools, and these lichen include Dermatocarpon meiophyllizum, D. miniatum, Leptogium californicum, L. lichenoides, L. subaridum, and Aspicillia contorta.

Now that we’re equipped with the tools to know when we’re standing in the middle of a vernal pool – let’s go find them. As we all know here out in Spokane, 2012 has been another long cool spring, so right now is still a perfect time for searching out these magical pools.

– Nastassja Noell


Aspcillia contorta, a crustose lichen found in the vernal pool systems in E. Washington. Photo by Valter Jacinto, Encyclopedia of Life.

Floristics and distribution of Vernal Pools on the Columbia Plateau of Eastern Washington by C. Björk and P. Dunwiddie. Rhodora Volume 106 Number 928 p. 327-347 (2004).

Washington’s Vernal Pools The Nature Conservancy (May 2011).

Conservation Assessment for Dermatocarpon meiophyllizum USFS, BLM (2007).

Climate change and ephemeral pool ecosystems: Potholes and vernal pools as potential indicator systems by Tim B. Graham, USGS (1997).