Native Chesapeake Bay Grasses: A Solution to Nutrient Pollution?
Part of the Young Naturalist Awards Curriculum Collection.
Watching the sun rise over Baltimore’s skyline, it is difficult for me to realize that this bustling metropolis lies on a tributary of the second largest estuary in the world. Waterfowl and litter float together in the shadows of factories below giant exhaust towers sending smoke skyward. The interruption of the modern world into one of the most beautiful natural settings I know is difficult to grasp, and I worry anew about the difficulties facing this unique and ancient bay.
The Chesapeake Bay is a natural treasure; a 64,000-square-mile watershed that covers six states and has a rich cultural history (Chesapeake Bay Program 2008). Historically, the bay was so impressive that the 17th-century explorer Captain John Smith described it as “a country which may have the prerogative over the most pleasant places of Europe, Asia, Africa, or America” (Smith 1612). Now, however, the Chesapeake faces a multitude of environmental problems that threaten its health and future. Once abundant native species have fallen prey to the effects of pollution, excess fishing, and habitat destruction. The repercussions of these losses have spread throughout the bay community, damaging a successful seafood industry and compromising the bay’s health. In an effort to restore the bay, numerous organizations and programs are working to increase awareness about the bay’s ecosystem and implement restoration projects for the watershed.
Despite this support, emerging problems pose new threats to the bay’s health. Agricultural runoff and erosion generate an overwhelming amount of excess nutrients and sediment that inevitably find their way into the waters of the Chesapeake. According to the Chesapeake Bay Foundation, an estimated 300 million pounds of nitrogen enters the bay every year (Chesapeake Bay Foundation 2003). This astounding influx of harmful compounds causes algae blooms that prevent light from reaching plant and animal life on the bottom of the bay. As the algae decompose, oxygen is leached out of the water, creating “dead zones” with dangerously low levels of oxygen that endanger aquatic species.
In particular, this overabundance of nutrients poses a severe threat to native populations of submerged aquatic vegetation. While expansive grass beds once flourished throughout the bay, excess nutrients and other factors have significantly harmed native populations. In fact, grass levels have declined to approximately 10% of their historic abundance (Chesapeake Bay Foundation, Chesapeake Bay Foundation Guide to Underwater Grasses). As I searched for information on bay health, the unfortunate plight of native grasses captured my attention.
For the past four years, I have conducted research projects related to Chesapeake Bay health. My initial study of the bay analyzed fluctuations in water quality as a result of rainfall and seasonal change. This two-year investigation fueled my innate passion for the bay and increased my concern for this estuary and its future. Next, I decided to search for natural solutions to the bay’s declining health. Expanding upon this idea, I investigated the extent to which native oyster and clam species are capable of reducing suspended sediment particles in bay water. As a result of my research, I was motivated to voice my disapproval of the introduction of a non-native oyster species into the bay at a recent public hearing. This experience allowed me the opportunity to meet citizens and scientists who share my concern for the bay and reaffirmed my dedication to improve water quality through the capabilities of native species. Therefore, as I learned more about bay grasses, I became interested in studying the potential benefits these species might have on water quality in the Chesapeake.
Underwater grasses provide a multitude of benefits to the bay’s ecosystem. In addition to providing habitat for aquatic life, grass beds decrease wave energy and turbidity by anchoring sediment in root structures. Grasses also release life-sustaining oxygen into the water via photosynthesis. However, the fact that ensnared my curiosity is that grasses naturally reduce nutrient levels in water. I was intrigued: Could native grasses provide an answer to one of the most urgent and pressing dilemmas in bay health?
Further research both confirmed and complicated my original thoughts. As previously stated, populations of native grasses have all but vanished. Therefore, while grasses possess the capability to improve bay water quality, they lack the sizeable population necessary to provide any considerable benefit. However, I was heartened by the fact that others shared my speculation about the potential of grasses to restore water quality. Merrill Leffler plainly states, “If the bay is to be returned to a semblance of its former integrity, grasses must flourish once more” (Leffler 2001). Leffler describes bay grasses’ predicament as a Catch-22: Levels of excess nutrients must be lowered in order for grasses to assist in nutrient reduction (Leffler 2001). Also of particular interest was the mention of joint oyster and grass restoration projects as an effective method of improving water quality (Leffler 2001). This perspective mirrored the scope and focus of my own investigations. Following this research, I was curious to see how significantly bay grasses could help remedy the nutrient-laden waters of the bay.
After I focused the topic of my research to nutrient reduction by native grasses, I contacted the Smithsonian Environmental Research Center to gain professional guidance and advice. Josh Falk, an education specialist at the center, offered me feedback and remained a valuable resource throughout the duration of my project. With Mr. Falk’s assistance, I met with Dr. Thomas Jordan, a nutrient ecologist at the center who offered me valuable wisdom about nutrients in the bay. As a result of this conversation, I made several important decisions regarding the nature and direction of my research.
Since nitrogen is one of the primary pollutants in the bay, I resolved to measure how significantly grasses could reduce nitrate and nitrite in a contained simulated environment. Following my discussion with Dr. Jordan, I also decided to measure the initial and final biomass of the plants to estimate their growth over the duration of the experiment. Following these decisions, I developed my hypothesis: If native Chesapeake Bay grasses are placed in a simulated environment, then the grasses will reduce nitrate and nitrite concentrations to at least 80% of their original concentrations.
Next, I began to learn more about grasses in order to plan my experiment setup effectively. The Chesapeake Bay is home to a variety of native and non-native species, some of which thrive in the freshwater tributaries of the upper bay. Others grow in the lower bay, where saltwater input from the Atlantic Ocean raises salinity levels to approximately 25 parts per thousand (Chesapeake Bay Foundation, Chesapeake Bay Foundation Guide to Underwater Grasses). I contacted Mark Lewandowski, a natural resources biologist from the Maryland Department of Natural Resources, who directed me to the online resources for the “Bay Grasses in Classes” program, and provided me with adult water stargrass (Heteranthera dubia).
At this point, I began to set up my equipment and materials in the Upper School science prep room at Roland Park Country School. First, I placed two large carts with tiers of fluorescent lighting near two large windows. Under the lowest level of each cart, I placed two glass 10-gallon aquariums that would later be filled with eight liters of dissipated tap water (to remove chlorine) and a soil substrate. Two of the aquariums would contain grasses, and the other two would be controls with substrate but no grasses (Figure 1). By including two tanks for each type of environment, I hoped to increase accuracy and avoid any differences that could be potentially caused by uncontrollable variables.
When I received the grasses from Mr. Lewandowski, I placed them in a tub with approximately 18 liters of dissipated tap water, while the four aquariums were filled with water and substrate. Each aquarium contained 14 cups of substrate, which was a mixture of two parts all-purpose sand and one part garden soil. This mixture both anchored the grasses and provided the necessary nutrients for growth and measurement purposes. The sediment in the substrate was given 72 hours to settle before grasses were planted.
On planting day, I measured the initial wet biomass of the grasses and planted 88 g of H. dubia in each aquarium. After planting, the grasses were given seven days to acclimate to their environment before measurements began. Every other afternoon, I measured the nitrate and nitrite concentrations of the water using WaterWorks™ Nitrate/Nitrite Water Quality Test Strips over the course of three weeks.
After I collected my initial measurements, I was interested to see how the concentrations in my experiment related to nitrogen concentrations in bay water. As a result, I frequently measured nitrite and nitrate concentrations of the Severn River as a reference to the natural world. These measurements followed the methodology as my formal experiment and lasted for two weeks.
After data collection and preliminary analysis, a few general trends began to emerge. The initial nitrate measurement for all tanks was 10 mg/L, which offered me a consistent baseline to which I could compare future measurements. Over the course of the experiment, nitrate levels for the tanks containing H. dubia alternated between 10mg/L and 5mg/L, with declining numbers toward the end of the experiment. Nitrate levels for the tanks without H. dubia alternated between 10 and 15 mg/L, and rose to 20 mg/L in one of the tanks. These extremely high measurements in this particular tank caught my attention, and I investigated potential causes for this situation.
Throughout data collection, I had noticed what appeared to be a film of dirt or other particles on the two tanks without grasses, with particular abundance in the tank that I noticed contained 20mg/L of nitrate. David Brock, my school mentor and A.P. biology teacher, studied the tanks and informed me that this was an algae bloom, which I had suspected might occur because of the high nutrient concentrations of the tanks. Given this conclusion, the fact that algae were virtually nonexistent in both tanks containing H. dubia suggested that the grasses prevented algae blooms. Additionally, the water clarity in the tanks with H. dubia was significantly higher than in the two without grasses, so it can be concluded that grasses also decreased turbidity.
It was about this time that I noted that evaporation had reduced the water level to approximately half of its original volume. Although I recognized that evaporation would factor into my data, I had not expected the
change to be this drastic. Therefore, I decided to slowly re-introduce water into the aquariums. I began taking measurements daily for four days to better observe any changes. After I had been taking measurements for 13 days, I added 1.5 liters of water to each aquarium. The next day I added another 2.5 liters to each aquarium, bringing the water level back to its original level. Although I had exercised caution to avoid stirring up sediment during this process, some of the soil was inevitably disturbed and the water became cloudy. After the sediment settled for 48 hours, nitrate readings significantly declined for all tanks, as expected. Not only were overall nitrate concentrations lower, but also the tanks that held H. dubia contained considerably lower nitrate readings than those without grasses.
From these results, I am certain that restoring the water volume to its original level helped produce accurate representations of nitrogen concentrations in each tank. Overall, the data confirms my hypothesis: H. dubia decreased the amount of nitrate in the water by 65% of initial levels, which is 40% more than the nitrate reduction in the tanks without H. dubia.
Only three out of the four tanks had measurable nitrite at the beginning of the experiment, which declined to 0 mg/L of nitrite in all aquariums over the course of the experiment.
However, the initial measurements for nitrite were much higher in tanks without H. dubia than in those with grasses. After speculating why the initial concentrations were different, I inferred that some of the nitrite had been removed by the grasses in the few days after the experiment was set up but before measurements began.
The data collected from the Severn River yielded a constant trend: Both nitrate and nitrite measurements were repeatedly very low. However, it is likely that nitrogen levels would be higher during spring and summer months when seasonal factors such as increased precipitation and fertilizer runoff introduce nutrients into the water.
At the conclusion of the experiment, the grasses were green and healthy. Over the duration of the experiment, well-developed root systems formed and new growth became visible. The final biomass for one tank was 86 g and the other was 92 g.
Ultimately, the outcome of my experiment confirms the results of my research. The data supports the conclusion that H. dubia can successfully reduce nitrogen concentrations in water, confirming that a natural component of the Chesapeake could remedy a serious problem. Furthermore, my observations suggest that presence of H. dubia prevented algae blooms and reduced turbidity in both tanks containing grasses. Therefore, it appears that native bay grasses can provide significant benefit to water quality by decreasing three adverse conditions facing the bay.
Throughout the course of this investigation, I have gained a deeper appreciation of the Chesapeake Bay and the challenges it faces. Meeting with other scientists and individuals who share my interest inspired me to apply my natural curiosity to address current and critical problems. Through research and experimentation, I learned about one of the most threatened and promising aspects of the bay, and quantified the benefit that one species could have on an urgent environmental crisis. While grass species no longer grow in abundance throughout the Chesapeake, awareness is growing and restoration efforts are underway. The predicament this situation presents is eloquently expressed through the words of Tom Horton, who reflects on “that tension between how much we have lost of our Chesapeake heritage, and how incredibly much remains, was… sometimes dispiriting, often delightful” (Horton 1992). My simple question—Can native Chesapeake Bay grasses present a solution to nutrient pollution?—then evolved into a deeper investigation of the many dynamics at work in this complex and unique estuary.
Bibliography
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Acknowledgments
This investigation would not have been possible without the guidance and generosity of many scientists and educators. In particular, I would like to thank the following for their assistance: Mrs. Martha Barss, Environmental Education and Sustainability Coordinator, Roland Park Country School; Mrs. Ereni Malfa, Upper School Science, Roland Park Country School; Mr. David Brock, Environmental Science Summer Research Experience Director, Roland Park Country School; Dr. Thomas Jordan, Nutrient Lab Ecologist, Smithsonian Research Center; Mr. Mark Lewandowski, Natural Resources Biologist, Maryland Department of Natural Resources; Mr. Josh Falk, Education Specialist, Smithsonian Environmental Research Center.