Life in a Vernal Pool

Part of the Young Naturalist Awards Curriculum Collection.

by Yan Hui, Grade 9, Texas - 2004 YNA Winner

When I enrolled in the Kinkaid School in the fall of this year, I had biology for my science subject. Biology is my favorite science subject because we can learn how the biosphere system works and how other living organisms like ourselves interact with the environment.

Map 1 (Click to enlarge)

Some of my biology lab experiments were carried out in the school's backyard (below), where we can observe nature at work. The school's backyard is a secondary forest beside the Buffalo Bayou (a river). There are three vernal pools on the riverbank, and they were dry when I first saw them. Why are they called vernal pools? The pools don't look like pools at all; they are just dried-up muddy earth. Later, when it rained, I observed that the vernal pools were flooded. I decided to research what a vernal pool really is. A few questions came into my mind. How long can a vernal pool last? What are the organisms that depend on the vernal pool? What are their life cycles? What is the water quality in the vernal pool? Does water quality affect the organisms in the vernal pool? 

The dry Kinkaid vernal pool before the rain.

In order to answer these questions, I prepared a procedure to collect data from one of the vernal pools. The data was to be collected in three categories. The first category was a description of the physical parameters in and around the pool. These included air and water temperatures, humidity, pool size, water depth, and water opacity. The second category was a description of the water quality of the pool. Water samples from two locations in the pool were collected and analyzed for pH, total alkalinity, total hardness, and the amounts of iron, copper, free chlorine, total chlorine, nitrate/nitrite, and nitrite present. Distilled water was used as a control for my water quality analysis. I also analyzed Houston tap water for reference. The two water sample locations were at the edge of the pool and at the deepest point of the pool. The third data category was to observe and record the living organisms in and around the pool area. This category included a description of the microorganisms in the water. The organisms in the pool were sampled using an aquarium fish net, while the microorganisms were observed from my water samples, using a microscope. This data was collected every afternoon for the duration of the vernal pool. These observations helped answer my questions about life in a vernal pool.

On October 9, the opportunity to answer all my questions came. It rained heavily in the vicinity of Kinkaid's backyard. The next day, October 10, I went to the school backyard to find out whether a vernal pool had formed. The vernal pool area was flooded with water. 

The lush green flooded Kincaid vernal pool.
The flooded Kinkaid vernal pool.

The duration of this flooded vernal pool was 10 days. It formed on October 10 and was completely dry by October 19. During this time, the air temperature ranged from 65 degrees F. to 84 degrees F., the water temperature was between 67 degrees F. and 77 degrees F., and the humidity range was from 58 percent to 75 percent. There was no additional rain during this period of time. The pool was 19 inches deep on the first day, and the depth increased by one inch on the second day as water was still draining into the pool from the surrounding area. However, the depth of the pool started to decline almost linearly after that, until it dried up. The atmospheric conditions do not seem to have affected the rate of water loss due to evaporation and seepage. The vernal pool swelled to a maximum size of approximately 65 feet long by 25 feet wide, or 1,625 square feet total, and then shrank slowly until it was dried up on the 10th day (Graph 1). 

Graph 1 (Click to enlarge)
Graph 2 (Click to enlarge)

I analyzed water samples taken from two locations in the pool, Location A and Location B (see Map), for water quality. I found that the water quality did not fluctuate very much for the duration of the pool until the day before the pool dried up (Graph 2). The water had a pH of 6, total alkalinity was 0 ppm (parts per million), total hardness was 50 ppm, the iron content was 0 ppm, copper was 0.5 ppm, free chlorine and total chlorine were both 0.1 ppm, nitrate was 0.5 ppm, and nitrogen was 0.15 ppm, in both locations. On the day before the water dried up, the water qualities changed slightly. The total hardness increased to 120 ppm, the total chlorine content increased to 0.2 ppm, and the concentrations of nitrate/nitrite nitrogen and nitrite nitrogen declined to 0 ppm. The changes in water quality on the last day may be due to the increased concentration of dissolved minerals in the water due to evaporation. I also tested distilled water and found that there were no dissolved minerals in it at all. Houston tap water had a total hardness of 120 ppm, and a free chlorine and total chlorine content of 1.0 ppm. Compared to the standards of the U.S. Environmental Protection Agency, the water quality of the vernal pool is within the EPA recommendations, and I deduced that it was suitable for organisms to live in. However, I did not observe any microorganisms under the microscope in the water samples taken from the pool. I only observed rotting tissues from vegetation. My observations showed that the rotting process was taking place in the water, and explained why I had seen gas bubbling from the pool.

Yan Hui collecting specimens and a water sample from the pool.

Every afternoon I went to the pool to look for organisms. I waded in the pool and tried to collect specimens using a fish net. During the first few days, I looked for frogs, salamanders, toads, and their eggs, as these are some of the animals that spend one or more stages of their life cycles in vernal pools. None were found. However, I observed a lot of mosquitoes flying above the surface of the pool on the first day, and a few larvae were found on the second day. As the days progressed, fewer mosquitoes were found, but more and more mosquito larvae were seen. Later, the larvae metamorphosed to pupae. On the eighth and ninth day, mosquitoes were again seen flying above the surface of the pool. 

I realized that I had witnessed the complete life cycle of the mosquito. The adult mosquito has unique white bands on its legs and body. Based on Internet literature, I identified the mosquito as the Asian tiger mosquito, Aedes albopictus. Some skaters were found, too, but they may have originated from the nearby Buffalo Bayou. About seven disk-shaped snails were seen in the pool. These are presumably water snails of the species Micromenetus d. dilatatus, also called the bugle sprite. They were about 0.3 inches in diameter, 0.2 inches thick, and light yellowish-brown in color. They were found on submerged vegetation in the pool.

Bugle Sprite, Micromenetus d. dilatatus, a species of freshwater snail found in the vernal pool.
A dobsonfly larvae caught by Yan Hui.
A guppy fish caught with the net as the pool shrinks.

As the vernal pool was drying up, insects and spiders were seen crawling around in the receded area. The receded area was moist and muddy. When the vernal pool was almost dried up, a lot more organisms were found. Dot-sized organisms were darting in the water. Small insects were flying over the surface of the water, too. I caught a dark, flattish, rod-like organism with a pinlike tail in my net. It was one inch long and was quite abundant in the water and could be the larvae of the dobsonfly. A lot of guppies, approximately one inch in length, were seen swimming in the water. I observed them because they were concentrated in a shrinking pool of water. I had not seen any eggs in the pool for the past nine days. I wondered where these fish had come from, because according to the Internet, a vernal pool is not supposed to have fish in it. Then I noticed that there was a small dry depression leading to the vernal pool. I assumed that the fish were washed into the vernal pool when the nearby Buffalo Bayou overflowed its banks after the rain. The pool had a strong smell of decaying vegetation.

Dwarf Salamander, Eurycea quadridigitata, found on the forest floor near the pool. (Click to Enlarge.)

Salamanders were spotted darting across the forest floor close to the pool area. They were seen either in pairs or alone. The salamanders were dark brown in color and about four inches in length, including their long tails. They had four toes on each of their hind legs and a dark brown stripe along their sides. There are also dark spots on their backs. Based on literature from the Internet, I identified these salamanders as dwarf salamanders, Eurycea quadridigitata. They are very common on the coastal plain, especially near pine savanna ponds. These salamanders court during the fall and lay their eggs in late fall to early winter. Since I had found no salamander eggs, I inferred that these salamanders might just be courting as it was early fall. 

A young smooth-skinned toad was seen near the pool. It was very light brown in color with a creamy underside. It was about 1.5 inches in length and 0.8 inches in width. A light yellowish stripe was present at the middorsal and along both sides of the toad. There were darker brown stripes on the hind legs and the front toes. Its cranial crests were well defined, forming ridges and valleys between them. Based on literature from the Internet, I identified the toad to be the Gulf Coast toad, Bufo valliceps. This is a very common toad in the Gulf Coast region that breeds during spring and summer.

A young Gulf Coast Toad, Bufo valliceps , found among the leaves near the vernal pool. (Click to enlarge.)

During the period when the vernal pool was flooded, I witnessed the full life cycle of the mosquito, from larvae to pupae and finally to adults. I also observed freshwater snails, dobsonfly larvae, and guppy fish in the water. However, I could not document the breeding activities of the bigger species like the salamander and the toad. No eggs, larvae, or tadpoles were found. Based on the documented breeding patterns of these species, the breeding season for the toad is already over, and for the salamander it has yet to come. My observations of a young toad and pairs of salamanders playing on the forest floor by the pool support this conclusion.Overall, I have learned a great deal from this project. I learned about the types of organisms that depend on the vernal pool for their life cycles, the breeding seasons of different species, and how organisms adapt to the variable duration of the vernal pool. The Kinkaid vernal pool is flooded for only a short amount of time, which limits the types of organisms that can breed successfully in the pool. Perhaps during the rainy season the pool will stay flooded for a longer time and allow larger species to breed successfully. The water quality was relatively good during the period of study and was hospitable to living organisms.

I went back to the vernal pool on October 25. The pool was dry, but moist earth was still present. Many salamanders were seen running about in the moist earth of the pool. A young toad was also seen near the dried-up pool. The fish and the insect larvae had since died.


Materials and Equipment Used

  • Microscope and slides
  • Thermometer
  • Hydrometer
  • 1 measuring stick
  • 1 measuring tape
  • 3 packets of "Shirt-Pocket Water Test Kit," by Carolina Biological Supply Company
  • 1 aquarium fish net
  • 20 water sample bottles
  • Notebook
  • Camera and film

Data Tables

Table1 (Click to enlarge)
Data table showing Kincaid School vernal pool water analysis, from project of 15-year-old student Yan Hui
Table 2 (Click to enlarge)




Web Sites 

An Online Encyclopedia of Life. NatureServe, Arlington, VA, 2003. Retrieved from the World Wide Web on 27 November 2003.

"Description of Dwarf Salamander." National Wildlife Federation.

Drees, B.M. and John Jackman.  Field Guide to Texas Insects. Houston: GulfPublishing Co., 1999.

Grantham, Richard. "Mosquito Biology." Department of Entomology and Plant Pathology, Oklahoma State University.

"Herps of Texas." University of Texas, College of Natural Sciences at Austin.

National Weather Service Forecast Office, Houston/Galveston, TX.

"Salamanders." U.S. Geological Survey, Florida Integrated Science Center, Gainesville, FL. quadridigitata.html

"Salamanders (Overview)." Alien Earth.

Thompson, Fred G. "Field Guide to the Freshwater Snails of Florida." Florida Museum of Natural History.

"Vernal Pool." The Vernal Pool Association, Reading Memorial High School, Reading, MA.