Filtration Capabilities of The Eastern Oyster and Soft-Shell Clam
A flock of Canadian geese moves rapidly upriver, soaring over my head. Here, in the midst of the modern world, their sound calls to me from a distant age. Listening intently to their ethereal music, I cannot help but wonder if their cries will someday fade into extinction, stamped out by the changes imposed by society.
The Chesapeake Bay faces a crisis. Once described as "one of the richest agricultural regions of the earth, whose fertility can be compared only with that of the valleys of the Nile and the Ganges and other great rivers" (Brooks 1996), the state of this estuary has declined drastically from its original glory. Pollution, erosion, and other factors harm bay water quality, damaging or eliminating the habitats of native species. As populations of vital species decline, the diversity and abundance of this estuary is threatened.
In prior investigations, I analyzed the deteriorating water quality of the Chesapeake Bay. I determined that the next step was to search for a solution to this challenge. My attention was captured by the well-publicized water-filtration capabilities of the Eastern oyster ( Crassotrea virginica) . Oysters were once so plentiful in the Chesapeake Bay that they could filter the entire bay in just three to four days (Chesapeake Bay Program,American Oyster, 2005). Sadly, because of over-harvesting and pollution, oyster populations have declined to just 1 percent of their original abundance (Chesapeake Bay Program, American Oyster, 2005). As William K. Brooks, a former oyster commissioner of Maryland, noted in 1891, "our indifference and lack of foresight, and our blind trust in our natural advantages, have brought this great inheritance to the verge of ruin" (Brooks 1996). In recent years, oyster restoration projects have gained popularity as a means to improve water quality. I wondered to what extent the oyster's filtration capabilities could improve the ailing health of the bay.
I questioned whether other native species held similar promise for improving water quality. Since I have always been fascinated with clams, I wondered about the filtration capabilities of the soft-shell clam ( Mya arenaria ). I wondered whether M. arenaria could filter water as well or even better than C. virginica . I was determined to find out.
After directing my focus, I researched the habitats, behaviors, and filtration capabilities of C. virginica and M. arenaria . I confirmed that both species are native to my area and studied the abundance of phytoplankton in local waters, which both species rely on as a food source. My research indicated that a mature C. virginica filters five liters of water per hour (Chesapeake Bay Program, American Oyster, 2005), while a mature M. arenaria filters four liters per hour (Maryland Department of Natural Resources, Soft Shell Clam, 2007). After considering these figures, I formed my hypothesis: If equal numbers of the species C. virginica and M. arenaria are placed in identical simulated environments, then C. virginica will filter the water more effectively than M. arenaria.
Next, I constructed a basic experiment plan: three aquariums would be filled with water from the Severn River, a nearby bay tributary. One aquarium would hold 12 C. virginica, another aquarium would hold 12 M. arenaria,and the third aquarium would be a control of only river water . At predetermined intervals, I would measure the turbidity of each tank, allowing me to monitor changes in clarity. Since oysters are only available from September to April, I planned to conduct my experiment in winter. I was concerned that the effects of the filtration would not be evident since a scarcity of phytoplankton lowers winter filtration rates. The highest filtration rates occur in the warmer temperatures of the spring and summer months, so I conducted my experiment indoors at a room temperature of approximately 21 degrees Celsius. This decision necessitated an acclimation period of approximately six hours, during which the temperature of the river water would increase, enabling the animals to filter the water more effectively.
I began by collecting water from the Severn River, which I ferried to my house for preliminary experimentation and storage. During this process, I found that lifting large amounts of water challenged me physically, and that I was only able to collect 18 liters per trip to the river. Ultimately, I collected and transported 144 liters of water, weighing a total of 138 kilograms. To lessen the strain of this process, I spread the collection trips over several days.
The most daunting technical challenge I overcame was developing a method to measure turbidity. I sought a system that would provide quantitative measurements, as opposed to a vague comparison or undependable observations. Initially, I planned to count the number of particles on the bottom of the aquariums, but I dismissed this plan on several grounds. First, M. arenaria and C. virginica expel pseudofeces (waste particles) that would have been difficult to distinguish from suspended particles. Additionally, the presence of the animals in the aquarium would obscure the bottom of the tank, rendering it impossible to take reliable measurements. I also considered using a Secchi disk or a similar tool, but I concluded that the data provided from this method would be too qualitative to be consistent.
After extensive brainstorming, I decided to determine turbidity based on the amount of light able to pass through each aquarium; the more light that passed through the aquarium, the less turbid the water. After acquiring a light meter and a LCD lantern, I filled an aquarium with approximately 19 liters of river water, and another tank with 19 liters of tap water (see Appendix A) . I set the lantern to full power and placed it on the far side of the tank. I ensured that the lantern emitted a constant output of light by connecting it to an electrical outlet. I situated the light meter directly across the aquarium from the lantern and measured the illumination in lux that passed through the tank. The purpose of this test was to confirm the effectiveness of this method and establish the sensitivity of the light meter. The test was successful; the illumination was 12 lux higher for the tap water than the river water. I was satisfied with my measurement system, which provided me with reliable quantitative data.
Another major difficulty was the unavailability of M. arenaria. My research indicated that M. arenaria is a plentiful species, so I expected that locating 12 specimens would be a simple task. I was sadly mistaken. I contacted retail seafood markets, independent watermen, and commercial seafood distributors, all of whom informed me that M. arenaria were unavailable. When I inquired about the cause of the shortage, some informed me that the industry had not been successful for a while, and was not likely to recover soon. After I exhausted all the relevant listings in my phone book and received the same uninspiring answer each time, my confusion turned to frustration. I was witnessing firsthand the drastic decline of native species in the bay, a shocking realization. Was this species going to become extinct during my lifetime?
At one point, I was ready to terminate my entire investigation after months of planning and research. Just as I was about to admit defeat, my perseverance paid off: I was generously provided with 15 soft-shell clams by a local waterman, who spent hours locating the specimens for my research. Although this search ended successfully, my exposure to the profound decline of this species strengthened my resolve to improve the bay's water quality.
On the morning of the experiment, I collected cold river water for acclimation. At 11 a.m., I commenced my acclimation period: in a large tub containing 19 liters of river water, I placed 12 C. virginica. Another container held 12 M. arenaria and six liters of river water. I engineered a bubbler system to deliver oxygen to the tanks. During acclimation, I distributed water from each storage container evenly between the three waiting aquariums. Each tank contained 34 liters of water, allowing space for the water level to rise after the animals were introduced. Using the light meter, I ensured that all tanks were similarly turbid prior to experimentation. Throughout this process, I monitored the rising temperature of the acclimation tubs.
At 5:13 p.m., when acclimation concluded, I transferred the C. virginica into their tank, followed by M. arenariaat 5:23 p.m. At 5:33 p.m., I began monitoring the control tank. I measured the turbidity of each tank 25 minutes after the previous measurement for that tank. This schedule necessitated measurements every five to 10 minutes, which required a continual awareness of my schedule. Between measurements, I observed the animals and readied my experiment station for the next round of measurements.
I measured turbidity according to the method I previously established (see Appendix B ). Since I carried out my experiment at night, I was able to take measurements in total darkness, eliminating ambient light. Initially, I measured the turbidity of the tank at rest and the turbidity after the water was gently agitated. This second test allowed me to gauge matter introduced during filtration, such as pseudofeces.
Observing the animals was the most fascinating aspect of the investigation. Early on, two of the 12 C. virginica had open shells ( see Appendix C ). I knew from research that this behavior indicated filtration, so I was encouraged to see visible evidence that the animals were cleaning the water. Throughout the course of the experiment, every C. virginica opened its shell at one point or another, indicating filtration. The oysters were covered in a thin layer of mud and debris, which introduced additional sediments into the tank ( see Appendix D). Attached to almost every C. virginica was a tiny barnacle or mussel, indicating the abundance of life they support in the bay. I also noted an interesting phenomenon when one appeared to "spit" something out—I saw a disturbance in the water and a sudden movement from one of the animals. The discharged object appeared to be a small brownish particle of dirt, which I deduced was likely pseudofeces.
My observations of M. arenaria were as interesting as those of C. virginica . After an hour, the siphons of M. arenaria extended, as well as the translucent "foot" used to burrow into river-bottom sediments ( see Appendix E). Many M. arenaria moved the "foot" in a back-and forth motion, attempting to dig under mud. Upon close examination, tiny cilia became visible at the ends of the siphons. M. arenaria sometimes expel water from their siphons with surprising force, as I had the privilege to observe. While transferring a M. arenaria, a jet of water landed unexpectedly on my hand, causing me to direct the siphon away from me in the future. I also noted that small brown "blobs" accumulated on the bottom of the aquarium, which I inferred were pseudofeces. My observations of M. arenaria intrigued me and gave me a more complete understanding of their filtration behaviors.
During experimentation, I did not see drastic changes in the data, and I wondered if this was due to my experiment setup. After two hours of filtration, I made several carefully considered decisions: I stopped the bubbler system at 7:38 p.m. because it noticeably disturbed the water and the bubbles appeared to reflect light. I increased my attention to the animals following this decision and noted no problems; air bubbles continued to cling to the sides of the tank until the conclusion of the experiment, indicating an ample supply of oxygen. Beginning at 8:38 p.m., I stopped agitating the water during measurements, since I did not think this allowed suspended particles to settle between measurements. After making these adjustments, I continued diligently monitoring and measuring the aquariums until 11 p.m.
At the conclusion of the experiment, I placed white poster board behind all three tanks to visually observe turbidity ( see Appendix F ). From this analysis, I decided that the M. arenaria tank seemed clearest, although the numerical data indicated that the control tank was clearest. The final measurement of the C. virginica tank was 15 lux, the final M. arenariameasurement was 15 lux, and the final control measurement was 20 lux. While contemplating the reason for the inconsistency between my visual and numerical conclusions, I decided that the greater filtration capacity would be indicated by the amount of positive change over the course of the experiment, as opposed to the final clarity. This seemed the most unbiased analysis, since additional sediments were introduced from the exterior of the oysters' shells and pseudofeces were released.
Following this decision, I graphed my data and began to formulate my conclusion. (In my analysis, I ignored the "stirred" measurements, although they are included in my graphs.) The C. virginica data began at 14 lux and ended at 15 lux, rising and falling several times in the process ( see Appendix G ). The M. arenariadata was more consistent than that of C. virginica ; it began at 17 lux, dropped to 14, increased to 16, and stopped at 15 ( see Appendix G ). I regarded the initial measurement of 17 as an outlier because it was different from the rest of my data. Each measurement of the control tank was 20 lux, without exception (see Appendix G) . From the consistency of this tank, I concluded that my measurement system was sound, and that the data accurately reflect the changes in each aquarium.
My graphs and numerical data convinced me that both species equally improved the clarity of the water. Throughout the course of the experiment, the amount of light that passed through the C. virginica tank rose 1 lux, to 15 from 14. The tank of M. arenaria also increased to 15 lux from 14, disregarding the outlying measurement of 17. Since M. arenaria are much smaller than C. virginica, each M. arenaria filtered more water for its size than each C. virginica. A number of features might have impacted the data, including sediments from the C. virginica shells and pseudofeces. Although I expected C. virginica to filter water more effectively in my hypothesis, the conclusion that both species equally lowered turbidity was impressive.
Following my experiment, I released the animals into the Severn River, in an ideal location for their continued well-being. From the results of my experiment, I am confident these bivalves are cleaning the water in the Severn River.
This investigation provided me with the opportunity to study a nearby ecosystem that I am profoundly interested in—the Chesapeake Bay. Through research and experimentation, I established a solid methodology to improve the bay's water quality through the capabilities of native species. Most importantly, I gained a more comprehensive understanding of the Chesapeake Bay as an ecosystem, not simply a body of water. I now appreciate in greater depth the complex and ever-evolving cycle of life present in the Chesapeake Bay. My hope for the bay is embodied in the ancient song of the geese—it beckons from the past, lingers in the present, and continues, unconquerable and unvanquished, into the future.
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More About This Resource...
This winning entry in the Museum's Young Naturalist Awards 2008 is from a Maryland 9th grader. Alexandra investigated whether oyster's filtration capabilities could improve the ailing health of the Chesapeake Bay. Her essay includes:
- details about her previous investigations into the deteriorating water quality of the Chesapeake Bay;
- how she tested the filtration abilities of soft-shell clams (native to the bay) alongside the abilities of oysters; and
- the results of her investigation, which showed that both species equally improved the clarity of the water.
Supplement a study of ecology with an activity drawn from this winning student essay.
- Ask the class to brainstorm a list of methods that can be used to filter water.
- Send students to this online article, or print copies of the essay for them to read.
- Have them create a poster that details the role oysters/clams play in their ecosystem. Additional research will be needed.