Linear and Scaled Bioremediation in the Chesapeake Watershed
In the modern era of environmental consciousness, a great deal of emphasis has been placed on the water quality of the Chesapeake Bay Watershed. This watershed covers over 64,000 square miles of land and over 18 million people live within its limits. Literature pertaining to watershed bioremediation is broad-based and spans several different fields of research. Four solutions which have shown promise as pollution mitigators and have been especially recurrent in the literature are 1) the reintroduction of freshwater mussels to the rivers, 2) the restoration of riparian zones in areas where the zones have diminished, 3) the establishment and maintenance of forests in urban areas, and 4) the establishment of vegetated buffers in agricultural and residential areas. Additionally, a fifth, less proven but nonetheless intriguing strategy of bioengineered food products as a means of collapsing demand for destruction of natural resources was highlighted. The project concluded that the effectiveness of each of these strategies varies greatly, both overall and situationally, but their effectiveness would be maximized by including each strategy as part of a unified plan to be enacted across the watershed.
The purpose of this literary review was to explore a diverse collection of potential avenues for pollution mitigation in the rivers of the watershed and analyze their roles as part of a comprehensive action plan. The avenues were selected based on the stages in pollution mitigation in which they operate (e.g. source, air, runoff, in-river, groundwater). The chosen solutions were: Mussel population restoration (in-river), riparian zone restoration (runoff and groundwater), urban forest establishment (air and groundwater), and agricultural and residential buffer zones (source). The fifth solution which was only highlighted was the use of bioengineered food products as a means of collapsing demand for destruction of natural resources. This would not mitigate pollution directly, but would decrease pollution from farmland, making it a preventative solution.
Linear and Scaled Solutions
The secondary purpose of this review was to address the problem of human population expansion in ecology, and to distinguish between linear solutions (i.e. solutions that do not expand in effectiveness as human population grows) and scaled solutions (i.e. solutions that do account for human population expansion). The human population is growing in every state in the watershed except West Virginia (Figure 1) (U.S. Census Bureau, 2018). This requires agricultural expansion to grow food for new people, and residential expansion to house them. This leads to increased agricultural runoff and increased residential pollution, coupled with decreasing natural forest. As a result, not all solutions are equally helpful, because some solutions have carrying capacities. Bioremedial solutions which can be enacted in every residential area and agricultural area, buffer zones between developed areas and waterways or other unmanaged areas, and bio-engineered consumer products to collapse the demand for newly developed agricultural areas, were evaluated as scaled solutions.
Reintroduction of Freshwater Mussels
The American Eel restocking project in the Susquehanna River began in 2010 with the exact goal of re-establishing the populations of American Eels (Anguilla rostrata) in the Susquehanna watershed (Galbraith et al, 2010-2014). Equally importantly, the project was designed to reintroduce the Eastern Elliptio Mussel (Elliptio complanata), to the tributaries of the Susquehanna River which lie above the dams. These mussels, among other bivalve species of the order Unionoida, are filter feeders who play an instrumental role in maintaining a healthy aquatic ecosystem by consuming plankton and bacteria, as well as filtering organic matter from both the water and sediment (Vaughn and Hakenkamp, 2008).
Relationship Between Elliptio complanata and Anguilla rostrata
rostrata is an important host species for the parasitic larvae of E. complanata. The larvae of the mussel, known as glochidia, are dispersed by the adults and attach to the fins and gills of the eel. Later, the glochidia drop off at different points onto the riverbed, where they mature and grow into sedentary adults. While other host species exist, eels are a favored host by Elliptio mussels, primarily because the glochidia of E. complanata have the highest metamorphosis success rate on A. rostrata of all of the different host species (Galbraith et al, 2010-2014). These mussels are very specialized, and the fact that some of their host fish are not universally found in the Chesapeake Watershed puts additional emphasis on A. rostrata as a host species of utmost importance.
American Eel Conservation Status
The Eastern Elliptio Mussel’s decline in population in the upper tributaries of the rivers of North America is largely due to the downfall of the population of American Eels nationwide. The American Eel is a catadromous fish species, which means they grow up in rivers and streams before migrating to the ocean, where they spawn and promptly die. The baby eels, known as “glass eels” or “elvers”, float along ocean currents from their hatching grounds in the Sargasso Sea, and eventually make their way up a river as far as they can move. Elvers choose these rivers at random, and they are prevented from travelling up many rivers by dams. The continuation of these hindrances and the decrease in eel populations nationwide have landed the American Eel on the IUCN Red List of Threatened Species (Jacoby et al, 2017).
This reintroduction project had four major components: 1) infestation of elvers with glochidia, 2) reintroduction of elvers to four sites in two tributaries upstream of the Conowingo Dam, 3) a fish survey, and 4) a mussel survey (Galbraith et al, 2010-2014). Buffalo Creek and Pine Creek served as the receptacles of the infested elvers.
Glochidia Infestation and Elver Restocking
Adult mussels release glochidia when temperature is increased. In a laboratory, the temperature of water tanks containing adult mussels was raised, inducing the release of glochidia from the adults. The glochidia were then tested for viability, and viable individuals were transferred to separate aquaria containing elvers. After 30-40 days, the eels were ready to be transplanted into the streams. The eels were taken to the two tributaries and 120,000 eels were split between the two, just more than carrying capacity, which the project lists as 55,000 and 45,000 between Buffalo and Pine Creek, respectively (Galbraith et al, 2010-2014).
Results of Mussel and Eel Surveys
In subsequent surveys of mussel populations, researchers found that in both sites, mussel populations improved in different magnitudes. Mussel populations in Buffalo Creek improved in subsequent surveys, including doubling between 2012 and 2013 (Reese et al, 2014). The juvenile E. complanata population in Pine Creek increased 150-fold during surveys between 2010 and 2013 as well, a much more drastic increase in population (Galbraith et al, 2013). Juvenile eel populations also experienced improvement in both stocked tributaries. With each annual survey, the overall abundance of eels grew, and between 2010 and 2014 experienced population growth in all four stocking locations (Figure 2) (Galbraith et al, 2014). The reestablishment of their populations as a result of this survey (Galbraith et al, 2018) could prove to be a critical method of biological filtration of the pollutants which have already made their way into the rivers and streams (McKenzie and Ozbay, 2009).
Riparian Zone Restoration
The restoration of riparian zones has been a topic of extreme scientific interest for several decades. Riparian zones, in addition to preventing sediment buildup and erosion, also filter runoff water of excess nutrients. An idealized definition of riparian restoration simply entails returning damaged or destroyed riparian zones to their original state. This goal is not universally achievable, such as in places which have been developed or altered in irreversible ways (e.g. dams, bridges). Due to the limited restorable land and limited effectiveness of riparian restoration, this process was evaluated as a linear solution.
Riparian Filtering Effectiveness
Because of the aforementioned scientific interest in the topic, the effectiveness of riparian zones is very well-documented and heavily studied. One study conducted on Maryland’s coastal plain in 1989 found that while results varied wildly, vegetated riparian zones were generally effective as buffers for phosphorous, but had a negligible effect when employed as nitrogen filters (Magette et al, 1989). Another, more recent study reported that while great variation was present in results from different sites, the effects support the notion that riparian zones contribute to denitrification, both directly and through their effects on moisture and organic carbon (Schnabel et al, 1995). An even more conclusive study two years prior concluded that as much as 95% of NO3– runoff can be filtered out by riparian zones (Jordan et al, 1993). Study of this topic is broad-based, and results vary greatly, but a sample of 15 different studies in addition to the previously mentioned three shows that riparian zones do play a role in the retention of nitrates (Hill, 1996) (Figure 3). These results vary further depending on soil type and geography of the region.
Physiography of the Chesapeake Watershed
The Chesapeake Watershed is composed of six states: Maryland, Delaware, Virginia, West Virginia, Pennsylvania, and New York. Each physiographic region in the watershed is comprised of different soils, topography, and hydrology, and this means that riparian zones will be more effective in some areas versus others. A major consideration to make is an area’s potential for groundwater to move downward instead of laterally. Lateral water movement is essential for the water to come into contact with the roots of the vegetation of riparian zones. The land within the watershed can be divided into nine settings (Figure 4) (Hanson et al, 2016). Generalizations can be made about these areas by dividing the area into four physiographic super-provinces to be further broken down at a later time. The four areas of utmost focus are: Appalachian Plateau, Appalachian Valley and Ridge, Piedmont, and Coastal Plain.
Physiographic Suitability of the Four Largest Provinces
Coastal plain areas generally benefit significantly from riparian restoration because most of these areas are characterized by underlying aquitards (Klapproth and Johnson, 2009). This forces water in coastal plain physiographic provinces to come into contact with the roots of riparian vegetation. Similarly, piedmont regions benefit in many areas from riparian restoration because of low inter-basin groundwater flow (Maryland Geological Survey, 2019). Studies of the Ridge and Valley regions of Pennsylvania suggest that riparian zones do not necessarily provide a consistently more favorable environment for denitrification in these areas (Schnabel et al, 1995). These areas, however, can benefit from deep-rooted trees and restoration around springs or small streams where groundwater is near or at the surface. The regions of the Appalachian Plateau aquifers will benefit the least from riparian restoration projects, as the aquifers are not as fractured as in the Valley and Ridge, leading to less surface discharge than in other regions (Trapp, Jr. and Horn, 1997).
The broad spectrum of hydrologic behavior and soil type between physiographic settings makes riparian restoration inherently variable, strategically. Approximate generalizations, however, are possible and useful in identifying trends which can be employed in determining areas of paramount focus. For example, steeper slopes or higher sediment loads generally require wider buffer ranges to ensure that the water makes contact with the root systems of the vegetation for adequate amounts of time. A general recommendation is for the buffer zone to expand 5 feet for every one percent increase in slope (Palone and Todd, 1997). Generally, however, shallower slopes should have the widest possible buffers to ensure that they remove as much sediment and chemical and nutrient pollution as possible. The most rapid and effective way to restore these zones is by protecting seedlings with tree shelters and controlling competing vegetation (Sweeney et al, 2002). With the high variability of effectiveness due to physiographic, hydrologic, and other environmental factors between each potential site, the most reasonable conclusion seems to be that riparian zone restoration is effective, though wildly variable in this effectiveness, and should be considered principally as one component of a unified action plan. This would allow riparian zones to act as secondary catches for runoff from buffered agricultural zones and other primary mitigators such as forests as well as allowing them to lighten the pollution load which reaches the final mitigators such as mussels and other filter feeders.
Urban Forest Establishment
Forests are well-documented regulators of pollutants. Nutrient pollution, pesticides, and other airborne pollutants can be counteracted to a degree with the use of forests planted and maintained in urban areas within the watershed. While it is true that urban forests do have potential for expansion as urban areas expand, they were evaluated as a linear solution because they exist in areas that otherwise would be forested. Their role in an action plan should be as buffers in currently urbanized areas which do not already have forests.
Pollution Benefits of Urban Forests
Trees remove air pollutants via absorption through the stomata of leaves, and also reduce water pollutants via uptake of ground water through the roots. Samples of five different dry-deposited pollutants on trees from 15 cities around the country shows this effect, also highlighting monetary value of pollution removal (Figure 5) (Kuser, 2007). In addition to their capacity as pollution regulators, trees also function as microclimate regulators which causes them to have indirect effects on pollution as well. This has two distinct effects on pollution: reduced per-tree output of volatile organic compounds, and an overall reduction in energy expenses of surrounding buildings. Public opinion also seems to favor managed forested areas in cities (Tyrväinen et al, 2003).
Volatile Organic Compound Mitigation
Trees naturally release volatile organic compounds (VOC) as part of their natural processes, and these compounds contribute to pollutants such as O3 and CO. VOC release is also temperature-dependent. According to computer simulations, VOC output from forests can be reduced with more tree cover to reduce temperature, even though there are more trees to contribute (Cardelino and Chameides, 1990). This suggests that the rate of reduction of VOC outputs related to temperature is greater than the rate of increase of outputs related to tree population size. Consequently, a larger forest would result in a lower output of VOCs, despite this seeming paradoxical.
Reduced Air Pollution from Temperature Reduction
An indirect effect of urban forestry on pollution is through decreased energy demand of buildings in surrounding areas. The effect these forests have on reduced temperature during the warmer months of the year means that buildings do not require as much energy to reach their desired temperature (Kuser, 2007). Decreased energy need ultimately leads to decreased energy production, and this means that less fuel is required to generate this power, and therefore less pollution from fuel consumption at the source.
Vegetated Buffer Zones
Irrigation is an essential component of agricultural land. Open drainage ditches serve as a diversion to protect land from overflow (USSCS, 2008). Vegetated drainage ditches are simply drainage ditches which have been planted, and the vegetation acts like a riparian zone on a natural stream. The real differences between riparian zone restoration and vegetation of agricultural drainage ditches are 1) drainage ditches can be vegetated on any farm with drainage ditches, allowing the number of ditches to grow as the acreage of agricultural land expands, and 2) drainage ditches are not restored, they are created in areas where riparian zones do not historically exist. They are also highly effective in filtering pollutants such as the pesticide esfenvalerate, mitigating the pollutant to as little as 0.1% of initial concentrations within 510 meters of the ditch (Cooper et al, 2004). Esfenvalerate is a pesticide which has devastating, sometimes even eliminating effects, on several species of invertebrates and vertebrates alike (Lozano et al, 1992). Buffer zones are not limited only to drainage ditches. Any organic area bordering land where inorganic substances are used constitutes a buffer zone. Vegetating drainage ditches and other buffer zones from both residential and agricultural areas is a Best Management Practice and plays a role in nutrient and sediment removal from runoff (Jayakaran et al, 2010). Reducing the nutrient load to the drainage ditches by allowing natural grassed benches to be established along unmaintained ditches is another way to improve the effectiveness of the vegetated drainage ditch strategy (Kalcic et al, 2018).
A more recent, less-proven development which could have major implications in the future of this problem is the emergence of bioengineered foods. Reducing the need for farm expansion by introducing tissue-cultured meat, for example, could prove to be a viable additional method for handling this crisis. By culturing loose myosatellite cells on a substrate, it is possible to produce cultured meat by harvesting mature muscle cells after differentiation and processing them into various meat products (Bhat and Fayaz, 2010). This in-vitro meat production does have its limits, however, and more research is needed before it becomes a viable option for the broader marketplace (Datar and Betti, 2009).
It is impossible to attack the problems facing the Chesapeake Watershed’s health through purely restorative means. Any comprehensive plan to address the Chesapeake’s situation must be thoroughly comprehensive, and must take future developments into consideration during the planning stages. Broad-based reviews such as this paper are useful for highlighting strategies to be explored in greater detail by others in the aforementioned fields. By simultaneously collapsing the volume of pollutants released, filtering pollution from water sources through buffers, and restoring biological water filtration mechanisms such as mussels and other shellfish to allow for the leftover pollution to be cleaned, the overall water chemistry and ecological health of the bay can be significantly improved.
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Feature Image Credit: NASA, taken by MODIS (Public Domain)