Tidal Pools: Bacterial Variability, Marine Life Stability
"It is advisable to look from the tide pool to the stars and then back to the tide pool again."
—John Steinbeck, The Log from the Sea of Cortez
Living in California is quite different from living anywhere else in the United States. We have a former movie star for a governor; everything is always time-delayed, though Californians are ingenious at figuring out the results of an event before it is even broadcast on the West Coast; we don't allow smoking indoors, anywhere, which is something that the East Coast could learn from us; and we live in a unique environment, providing a critical link between humans and the Pacific Ocean. Hundreds of miles of coastline allow economic, artistic, and scientific connections that have endured for decades, yet far too many Californians do not realize just what an idyllic, yet fragile situation they occupy, on the periphery of the largest body of water on Earth.
Since second grade, I have been fascinated by the zone between land and water, the area commonly known as the intertidal zone. Full of tide pools, visible only for that brief period of time between full low tide and perhaps a half hour before and after low tide, the intertidal zone has its own unique creatures, both flora and fauna. At one time, the tide pools were in danger of becoming extinct, primarily due to over-harvesting of animals by over-enthusiastic tourists. Strict environmental laws, and a raised level of consciousness provided by marine specialists and park rangers, have led to a flourishing intertidal zone once again.
As an amateur marine biologist with a special interest in tide pools, I have spent countless hours watching the ebb and flow of life in the pools in Laguna Beach, California, near my home. Heisler Park, right near Main Beach in the heart of Laguna Beach, is an ideal spot for observing tide pools despite the fact that it is in the most heavily visited tourist area of the city. Most tourists are respectful of nature and visit the pools with caution—caution that probably stems as much from a fear of falling from the rocky shoreline into the frigid waters of the Pacific Ocean as from a spirit of conservation. Fortunately, nature has provided the tide pools with a sort of buffer from curious visitors, since the pools at Laguna Beach are only truly visible for about an hour and a half twice a day, at full low tide. Thus, visitors determined to view the pools must check up on the tides.
The Heisler Park tidal pools are a perfect example of how nature allows a person to be alone even in a crowd of tourists. Whether visiting at sunset or in the middle part of the day, it is possible to perch on the edge of a rock outcropping, gaze off into the brilliant azure of the Pacific, and step away from the crowds of tourists, as well as natives, clustered around Main Beach. Gradually, the sounds of nature outweigh those of humankind, as the crash of the surf against the rocks becomes the tick-tock of the Earth's alarm clock.
The tidal pool that I chose for this project has always appealed to me, primarily because it is a bit further out than some of the other areas. Since it is located nearer the actual breaker line, it has an even more limited viewing period, and spends a great deal of time in the splash zone, with swells flooding the area around the pool more frequently. Fortunately, there is a large rock outcropping that allows me to sit and observe without fighting the tug of the tide. I chose a one-meter-by-one-meter area to observe since it would not be feasible to observe the entire tide pool. Once, I visited "my" tide pool about two hours after high tide and discovered that it was necessary to wade through waist-high water in order to reach the site. Tide pool creatures have an amazing stamina and adaptability to their environment; they will continue to survive and thrive as long as humans leave them to their own devices.
Despite their adaptability, tide pools, and in fact the entire ocean, are quite susceptible to the whims of humankind. Storm drain runoff after a significant rain can raise bacterial counts at the coast for weeks, closing beaches to swimmers for as long as a month. Erosion from major Pacific storms can completely alter the shape of the coastline, destroying one beach and forming a new one in a matter of days. El Niño weather patterns can cause ocean water temperatures to drop, which can completely alter the meteorological patterns of California and change feeding patterns for much of the native marine life. Even the changing of the seasons may cause ambient ocean temperatures to fall. Thus, the formation of my twofold hypothesis for this expedition depends on the flux of the temperatures of the ocean, as well as the resultant change in bacterial counts.
My hypothesis, simply stated, is that ocean bacterial counts will decline proportionately with falling ocean water temperatures. At the threshold of winter, this seemed to be an ideal time to test my theory that bacterial counts will decline as the water temperature falls. I further predict that as bacterial counts decline, tidal pool marine life will increase. As a result, these changes in bacterial counts will be inversely related to the abundance of tidal pool marine life. Certainly bacterial counts can rise and fall due to a number of causes, triggered primarily by the presence of humans. However, as we move closer to winter, and as the ocean water temperature drops, I expect to see a distinct increase in both the quantity and quality of marine life due to the fall in bacteria.
I visited my selected tide pool once a week for six weeks to officially record air temperature, water temperature, and overall weather conditions. My observations included the overall percentage of marine life in a one-meter-by-one-meter square in my selected pool. To calculate this percentage, I observed the numbers of mussels, hermit crabs, sea urchins, barnacles, chitons, and, in particular, the number of anemones visible at each observation. In the observation site, most of the area on the bottom of the tide pool was occupied by some form of marine life. A few areas were empty, with rock visible but no marine animals. The "percent marine life" is a semi-quantitative estimate of the percentage of the total area of the one-meter-square observation site that was occupied by marine animals. The times of my visits varied considerably due to the timing of low tides. I also took a 1cc sample of water from the tidal pool, which I would analyze in the second part of my expedition.
Gathering the samples and performing my counts was a tremendous amount of fun. Once, an older gentleman approached me while I was measuring the temperature of the water in my tide pool and commented that he thought my thermometer was "off" by up to 4°C! There was a great deal of interest in my project from both the locals and visiting tourists, all of whom seemed genuinely interested in the final results of my expedition.
Following the six wonderful weeks of the first part of my expedition, it was time for me to embark on the second part of the adventure—an analysis of my water samples in my father's research laboratory at the University of California - Irvine Medical Center. To prepare for this part of my project, I first made bacterial growth plates. I mixed tryptone, yeast extract, salt, and agar together and then autoclaved, or cooked, the mixture at 121°C for 30 minutes in order to make sure that it was sterile. After letting this media cool to 55°C, I poured the liquid into Petri dishes and let them solidify overnight. This was an important part of my expedition, because if the growth material was itself infected with bacteria, I would not be able to evaluate the amount of bacteria actually found in the ocean water.
Next, I nervously began my efforts to grow bacteria from the ocean water samples. I had collected the 1cc samples in little plastic tubes. I centrifuged these six tubes at 14,000 rpm for seven minutes. At first I wondered why it was necessary to separate out the bacteria from the ocean water in a centrifuge, but since I only need to use the bacteria, not the water itself, the centrifuge is an ideal method for separation so that I had easier access to the bacteria. In fact, I was able to throw away the seawater. I then placed the remaining pellet of bacteria in 50 microliters of LB bacterial growth broth and put it in a water bath at 37°C for 30 minutes in order to relax the bacteria and make it easier to work with. Finally, the fun began when I used sterile technique to spread bacteria from each sample over its own, individually labeled Petri plate. I placed each dish in a 37°C incubator to see if anything would grow overnight. Although I was not certain, I felt confident that if bacterial colonies did not grow in 24 hours, perhaps they would grow in 48 hours. Worrying continuously, I anxiously waited for 24 hours to pass. After all, the future of my whole expedition depended on the outcome of these six plates! Delighted and ecstatic, I discovered that large and small, beige and transparent bacterial colonies covered my plates after only 24 hours. I took pictures of all of the plates to document my results, and made an effort to describe the colonies and the environmental conditions that existed during the time that the sample was collected. Finally, it was time for a mental expedition—a trip that would cause me to think more deeply about life in the tide pools that I love so much.
Several results caught my attention almost immediately. First of all, the number of beige bacterial colonies changed quite a lot. (See Table 1.) In my earliest samples, there were a great many of these colonies, while there were far fewer in the later samples. On one visit, on October 30, there were many transparent colonies. What had caused the difference in numbers and types of colonies became one of my first expedition mysteries. I also noticed that the temperature of the ocean water declined steadily over the six weeks. This was not unexpected, but I was pleased to note that at least one of my predictions had come true. However, the air temperature varied without any particular pattern, probably because the times of my visits to the tide pools depended on the tides and not on the time of day. The average air temperature for the week preceding my visits declined steadily over the six weeks of my expedition, which was also expected. Two of my expedition dates, October 24 and October 30, were preceded by very heavy rainstorms. The estimated percentage of marine life in the one-meter-square area fluctuated between 65% and 80%, without any clear pattern. Finally, the number of anemones remained constant—between six and eight. All of these results were very interesting, but what did they mean for my expedition?
First of all, there was a direct correlation between the average air temperature and the number of bacterial colonies counted. (See Table 2: Bacterial colonies vs. avg. temperature and Table 3: Bacterial colonies vs. water temperatures.) When the air temperature was higher, at the beginning of my expedition, there were more colonies. When the air temperature was lower, there were fewer colonies. What surprised me about this relationship was that it was exponential, not linear. This does make sense, though, since bacteria growth is often observed in an exponential ratio. There was also a direct correlation between the water temperature and the number of bacterial colonies counted. When the ocean water temperature was higher, at the beginning of the expedition, there were more bacterial colonies; when the water temperature was lower, there were fewer. This relationship was also exponential.
Another completely unexpected surprise came when my data demonstrated that there was absolutely no correlation between the estimated percentage of marine life and (1) the number of bacterial colonies; (2) the air temperature; (3) the water temperature; (4) the average air temperature; or (5) the amount of precipitation. However, the lowest estimated percentage of marine life was recorded after two weeks of rain, at the exact same time that an extremely large number of transparent bacterial colonies were observed. My second surprise came when I realized that there was absolutely no correlation between the number of anemones and (1) the number of bacterial colonies; (2) the air temperature; (3) the water temperature; (4) the average air temperature; or (5) the amount of precipitation. What did these unexpected results mean for my hypotheses, which were: (a) that ocean bacterial counts would decline proportionately with falling ocean water temperatures; and (b) that as ocean bacterial counts declined, the estimated percentage of marine life would increase?
My first hypothesis proved to be true. As ocean water temperatures declined, so did ocean bacterial counts, from a high of 155 colonies to a low of one colony. I also discovered that the number of bacterial colonies was related to the average air temperature. As the average air temperature declined, the number of bacterial colonies declined. I thought that the relationship between water temperature, air temperature, and bacterial counts would be linear and would appear as a straight line on a graph. However, the relationship was actually exponential, which is not totally unexpected, since bacteria grow exponentially when they have sufficient nutrients.
My second hypothesis was that as the number of bacterial colonies declined, the estimated percentage of marine life in my tide pool would increase. I proved this hypothesis to be absolutely false! In fact, the estimated percentage of marine life and the number of anemones observed was remarkably stable over the six-week period of my expedition. These two numbers did not correlate with water temperature, air temperature, average air temperature, or bacterial colony count. However, there was a slight decline in the estimated percentage of marine life following the two weeks of heavy rain. As you may recall, this is when I observed a different type of bacterial colony, perhaps carried in runoff from the storm drains.
My overall conclusions about the tide pools are that bacterial counts vary greatly over time, and directly correlate with average air temperature and ocean water temperature. In addition, my expedition has led me to believe that marine life in tide pools is very stable and not generally affected by bacterial counts or air and water temperature. However, an increase in bacteria from storm drain runoff after heavy rainfalls may adversely affect the tide pool population. It would be interesting to conduct another expedition to study tide pools over a longer period of time to confirm my findings, since it rains relatively infrequently in southern California. Discovering the types of bacteria that grew in the transparent colonies, and seeing if they can be linked to human/land waste from storm drain runoff, would be an interesting expedition for another day.
I have returned to my beloved tide pools often since the completion of this expedition, not to conduct further research but to visit an area of unique beauty, stability, and importance to both the human world and the world of the ocean. My thoughts are often riveted on how a certain event may affect the animals and plants that dwell partially in each world, tied irreversibly to mine as well as their own unique environment. My expedition has brought them into my sphere; perhaps it has also allowed me to venture into theirs.
"Then one can come back to the microscope and the tide pool... It is really the understanding and attempt to say that man is related to the whole thing..."
—John Steinbeck, The Log from the Sea of Cortez
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More About This Resource...
Supplement a study of biology with an activity drawn from this winning student essay.
- Ask students: What are tide pools? Where are they found? What are some difficulties scientists face in studying them? On the blackboard or flip chart paper, keep track of student responses to the last question.
- Send students to this online article, or print copies of the essay for them to read.
- As a class, revisit the last question, What difficulties do scientists face in studying tide pools? Add answers to the list generated before reading the essay.
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