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Oceanic Aerosols And Sodium Concentrations Of Sandplain Grassland Soils As Influencers Of Botanic Biodiversity On

Naushon Island, Massachusetts ———————————————————————————————————

Researcher: Luke Hasten O'Brien 1,2

In Collaboration With: Dr. Christopher Neill, Ph.D.2 and Lena Champlin3

1Boston College (Chestnut Hill, MA): Department of Earth and Environmental Geosciences. 2 The Marine Biological Laboratory (Woods Hole, MA): Semester in Environmental Science.

3Brown University (Providence, RI): Department of Earth, Environmental and Planetary Sciences.

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ABSTRACT Many factors contribute to the selection of individual plant species in coastal environments. The deposition of salts by oceanic aerosolization is investigated as an influencer to botanic biodiversity on Nashon Island, MA.

Keywords: Sandplain, Grassland, Naushon, Ocean Spray, Disturbance

I N T R O D U C T I O N

Naushon Island, the largest of the Elizabeth Islands, is positioned approximately 2 miles southwest of Cape Cod, MA. Prior to the arrival of colonials to North America, Naushon was heavily forested. With the arrival of European settlers in the 17th century, the forest was cut down to utilize the island’s agricultural potential in grazing livestock. The introduction of sheep insured a transformation of the island’s plant communities, facilitating the development of expansive grasslands. This drastic change in vegetation cover encouraged new flora and fauna (specifically avian) to take advantage of newly vacant ecological niches. The frequent grazing by sheep of Naushon’s grasslands maintained high biodiversity, on the island, with respect to new grass and avian populations (Foster and Motzkin 1999). By 1960, the owners of Naushon shifted focus away from the rearing of agricultural livestock, toward recreational purposes. By 1980, coyotes had truncating the sheep flock to a mere 30 individuals due to predation. The decline in grazing pressure allowed for ecological succession, as (roundleaf greenbrier) Smilax rotundifolia and (black huckleberry) Gaylussacia baccata began to expand in area, manifesting overwhelmingly homogenous regions of impassable monocultures (Motzkin and Foster 2002). Much of what was once grassland, maintained by large flocks of sheep until the 1970’s, has become encroached upon by these expanding shrub thickets, which threaten the plant and avian biodiversity of the island. Grasslands are known to harbor notably high floral and faunal biodiversity (White et al. 2000). These threatened ecosystems are vital to numerous avian species, with nearly 11% of endemic bird regions being grasslands (White et al. 2000). Grasslands have been declining in area across North America due to the conversion of grasslands to agriculture, fragmentation and islandization per road construction, and the introduction of invasive species (White et al. 2000). The conservation of Naushon’s historic, aesthetically pleasing, and ecologically pertinent sandplain grasslands is vital to protecting the species which inhabit them Several environmental factors determine the hindrance of plant growth, development, and productivity. Coastal sandplain grasslands and heathlands are exceptional systems that feature highly disturbance-dependent plant communities, scattered across the northeastern seaboard of the United States

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(Griffiths and Orians 2003). The vegetation in such coastal ecosystems is subjected to dissolved salts, as a result of seawater aerosolization per windstress during whitecap formation (Levin and Cotton 2009). Wind speed is pertinent to controlling the production of seawater aerosolization production; sea salt particle number concentrations can range from 50/cm-3 in high winds (>10 m • s-1), compared to 10, or less, particles per cubic centimeter (10/cm-3) under moderate wind regimes (Levin and Cotton 2009). Due to dependence upon wind speed, it may be rationalized that seawater aerosolization production rates could be impacted by climate change. Sea salt aerosols are primarily composed of sodium chloride (NaCl); however, are known to contain, but not limited to, additional ions such as: K+, Ca2+, Mg2+, and SO42- (Ma and von Salzen 2008). Per proximity to the ocean, the sandplain grassland soils of Naushon Island are exposed to the deposition of salty aerosols by prevailing winds. Thereby, altering the cation exchange capacity of these soils and ultimately soil-plant interactions. Sodium Chloride can cause osmotic stress and dehydration plants by inhibiting metabolic and protein production (Zhu 2001). Some plant species maintain homeostasis in stressful coastal environments, despite frequent exposure to salt spray (Zhu 2001). Plant species vary in tolerance to salt spray, and so could be selected against, thereby effecting plant community composition. The structure of sandplain heathland plant communities on Martha’s Vineyard, MA, were found to correlate with distance from the ocean shore (Griffiths and Orians 2003). Research indicates that coastal sandplain ecosystem plant community structure may be controlled by the selective repercussions of salt spray, limiting the production and growth of salt-intolerant vegetation (Griffiths and Orians 2003). As the effects of climate change worsen and sea-level continues to rise, it is important to investigate the potential implications that increase storm frequency and higher sea levels may have of coastal sandplain grasslands in the near future.

We hypothesize: (1) Plant species richness on Naushon is partially determined by soil salinity, specifically sodium (Na), and that a relationship exists between soil salinity and vegetation community structure; (2) Soil salinity will be greater in areas of greater exposure to the prevailing southwesterly prevailing salty winds; A correlation exists between the topography (slope aspect and elevation) of Naushon and mean soil salinity. METHODS Vegetation Cover Data

Two sites were chosen to test our hypotheses on Naushon Island, MA, Lighthouse Pasture and Protected Field (Figure 1). Positioned throughout Protected Field and Lighthouse Pasture, 3x3m vegetation monitoring quadrats allow for data collection to be conducted twice per growing season to account for

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warm and cool season flora cover. Between Lighthouse Pasture and Protected Field there are a total 390 plots. Soil Core Collection

Soil cores were taken to a depth of 13cm at each vegetation monitoring plot in Lighthouse Pasture and Protected Field, amounting to 390 soil cores. These soil cores were homogenized and de-rooted in mixing bowls, then incubated at 60ºC for two days. The dried soils were then allocated to coin envelopes in which approximately 10g of each dry, homogenized soil sample would be placed. The coin envelope samples were then stored at 60ºC until extraction chemistry was conducted. Ammonium Chloride Soil Extractions

In order to determine the quantity of exchangeable base sodium (Na) cations attached to clay and organic particles in the soils, a 1M solution of NH4Cl (ammonium chloride) was made. 53.49g of NH4Cl was measured out on balance with maximum mass of 4.1kg±0.1g, and mixed with 1L of distilled-deionized (DI) water. Each of the 10g, coin envelope soil samples were then transferred to urine cups into which 100mL of 1M NH4Cl was added. These samples were then shook on a shaker-table for 30 minuets and then refrigerated, overnight, at 1.6ºC (35ºF). Gravity Filtration

The supernatant NH4Cl solution was filtered through Whatman No. 5 filter paper(maker), by gravity filtration. The filtrate was captured into 25mL scintillation vials, and frozen until date of analysis for sodium ion concentration. Spectrophotometric Atomic Absorption

Prior to analysis by flame atomic absorption spectroscopy, the frozen scintillation samples were thawed to room temperature. A Perkin Elmer AAnalyst 200/400 AA Flame-Spectrometer was implemented to determine the absorbance of sodium in each sample. Standards were run to set calibration: 0.2 Na ppm, 1.0 Na ppm, and 2.0 Na ppm. If a sample was found to be above the highest calibration standard, the sample was diluted by a ratio of 1mLsample : 9mLDI(water). Absorption readings were then converted from Na ppm to mEq/100g soil in order to compare these data to mean species richness values obtained from vegetation monitoring cover data. Species Richness

Species richness of the vegetation monitoring plots was calculated by determining the number of species present within a given monitoring quadrat. Species richness was used to convey general plant diversity and abundance in order to compare these data to mean soil salinity.

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Topographic Aspect and Elevation

In collaboration with Lena Champlin, GIS software was utilized to determine elevation and slope aspects for Protected Field and Lighthouse Pasture. In GIS, a point-file was created for the locations of vegetation monitoring plots in both fields, which displayed XY coordinates in an Excel file for the respective geographic coordinates of the monitoring plots (Figure 2). A digital elevation model was obtained from the MassGIS database, produced as a result of the 1:5,000 Digital Orthophoto Project, to a 1m resolution. Geographic monitoring plot coordinates were overlaid onto the elevation map, wherein the Identify tool was used to record elevation with respect to plot coordinates (Figure 2). The Spatial Analysis tool was utilized in GIS to calculate topographic slope aspects from the elevation model raster (Figure 3). The topographic aspect was recorded with respect to each vegetation monitoring plot. Prevailing Wind Direction

Prevailing surface wind direction was determined from data collected by the National Digital Forecast Database. These data are near-term forecasts, revised hourly, which were used to confirm the prevailing southwesterly wind of the area study and direction of oceanic aerosol travel. RESULTS Mean Species Richness

From the vegetation cover data collected from monitoring plots during the summer of 2015, mean species richness was calculated through averaging the individual species richness of each monitoring plot in respect to Lighthouse Pasture and Protected Field. These data held that Lighthouse Pasture contains a mean species richness of 6.57 individuals per monitoring plot (Figure 4). Protected Field data found a mean species richness of 4.50 individual species per plot (Figure 4). Mean Soil Salinity

Spectrophotometric flame atomic absorption analyses were conducted on ammonium chloride (NH4Cl) soil extratant filtrate, to determine soil-sodium concentrations for each vegetation monitoring plot in Lighthouse Pasture and Protected Field. These data held that Lighthouse Pasture contains a mean soil sodium-salinity (Na) of 0.040996 mEq•100g soil (Figure 5). Whereas Protected Field was found to contain a mean soil sodium-salinity (Na) of 0.024262 mEq•100g soil (Figure 5). Soil Salinity and Species Richness

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Species Richness data were plotted against soil salinity concentration with respect to each plot. This analysis divided all predominantly shrub covered monitoring plots and all predominantly grassland covered monitoring plots from both fields, to investigated soil salinity impacts of these varying plots. The data were found to show no statistically significant correlation between soil-salinity and species richness in shrub plots (Figure 6). The trendline equation is as follows:

𝑦 = 6.58𝑥 + 4.3383 𝑅2 = 0.00356

The data were found to have no statistically significant correlation between soil-salinity and species richness in grassland plots (Figure 7). The trendline equation is as follows:

𝑦 = 15.956𝑥 + 5.6486 𝑅2 = 0.00689

Soil Salinity and Topographic Elevation Profiles

The topographic elevation of each vegetation monitoring plot in Lighthouse Pasture and Protected Field were recorded and plotted against mean soil salinity in each monitoring plot. The data were found to show no statistically significant correlation between soil salinity and plot elevation in Lighthouse Pasture (Figure 8). The trendline equation is as follows:

𝑦 = −0.0005𝑥 + 0.0568 𝑅2 = 0.00237

The data were found to show no statistically significant correlation between soil-salinity and plot elevation in Protected Field (Figure 9). The trendline equation is as follows:

𝑦 = −0.0008𝑥 + 0.0356 𝑅2 = 0.01389

Soil Salinity and Topographic Slope Aspect Orientation

The topographic slope aspect of each vegetation monitoring plot in Lighthouse Pasture and Protected Field were recorded and plotted against mean soil salinity. The data were found to show a correlation between soil salinity and a given monitoring plot’s aspect orientation in Lighthouse Pasture (Figure 10, 11). It is evident per the data analyzed that Southwestern aspect slopes receive the greatest oceanic aerosol deposition resulting in the largest mean soil salinity values. Southwestern aspect slopes in Lighthouse pasture contained, according to the data, a mean soil salinity of 0.069838 mEq•100g soil (Figure 10). It is interesting to note that the next highest mean soil salinity value, with respect to

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topographic aspect, were found to be of eastern-oriented slopes; 0.06149 mEq•100g soil (Figure 10).

The data were too, found to show a correlation between soil salinity and a given monitoring plot’s aspect orientation in Protected Field (Figure 12, 13). It is evident per the data analyzed that Southwestern aspect slopes receive the greatest oceanic aerosol deposition, resulting in the largest mean soil salinity values. Southwestern aspect slopes in Protected Field contained, according to the data, a mean soil salinity of 0.038312 mEq•100g soil (Figure 12). It is, again, interesting to note that the next highest mean soil salinity value, with respect to topographic aspect, were found the be of western-oriented slopes; 0.032929 mEq•100g soil (Figure 12). DISCUSSION

The purpose of this study was to determine if (1) plant species richness on Naushon Island is partially determined by soil salinity and weather a relationship exists between sodium concentrations in soils and the community structure of vegetation. It was also hypothesized that (2) soil salinity would be greater in areas of greater exposure to prevailing southwesterly winds which carry oceanic aerosols containing sodium. Lastly, this study sought out to determine (3) if a correlation exists between the topography of Naushon Island, slope aspect orientations and elevation above sea level, and mean soil salinity.

The study centered around two critical measurements: average species richness, and mean soil salinity. The data show species richness being, on average, higher in Lighthouse Pasture than Protected Field. This finding was the driving motivation to determine the impact of soil salinity by oceanic aerosol deposition. We hypothesized that species richness is lower in Protected Field, due its greater brunt exposure to prevailing southwesterly winds that carry oceanic aerosols and closure proximity to the ocean; increasing the mean soil salinity. However, the data yielded that Protected Field features less mean soil salinity compared to the more highly diverse, Lighthouse Pasture (Figure 5).

The unexpected results obtained from the investigation of mean soil salinity of Protected Field and Lighthouse Pasture, lead us to investigate weather ocean salts selected against shrub-dominated regions and grasslands. These data were plotted on a scatter chart, comparing soil salinity and species richness of all grassland plots and all shrub plots between the two fields. The data analysis held that no statistically significant trend existed between shrub/grassland species richness and mean soil salinity.

Topography was investigated as it may often determine several factors which impact the ecology and community structure of a given land area. Due the theory that Nauhson’s soil-sodium is mainly derived from wind-deposition of oceanic aerosols, it seemed pertinent to investigate weather elevation of a monitoring plot had an impact on its soil salinity. It was seen in the data that no

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statistically significant correlation exists between topographic elevation and mean soil salinity in Protected Field, nor that of Lighthouse Pasture (Figure 8, 9).

The most compelling evidence of this study was found in the topographic slope aspect orientation data. In support of hypotheses 2 and 3, it was determined that a relationship does exist between slope aspect orientation, prevailing wind direction, and mean soil salinity. These data from Lighthouse Pasture and Protected Field illuminate an intriguing trend. In Lighthouse Pasture We see that southwestern winds have yielded the highest mean salinity values on southwestern oriented aspects (Figure 11). Additionally, eastern-oriented aspects feature the second highest salinity values; it is thought that this is due to Lighthouse pasture shoring with the ocean on the eastern side (Figure 11). Furthermore, it can be said that this trend exists due to Lighthouse Pasture’s eastern proximity to the ocean, and its aerosol deposition to soils, regardless of prevailing wind direction. The same trend can be observed in data collected from Protected Field; southwestern oriented slope aspects still remain to be highest in mean soil salinity, while western oriented aspects are second highest in terms of soil sodium concentration (Figure 13). This may be explained by a similar, aforementioned logic. As Protected Field shores with the ocean (Buzzards Bay) on the western side, it logically held that western oriented slope aspects measure second in terms of highest soil sodium concentrations; whereas the eastern oriented slope aspects are significantly lower in soil-sodium concentrations (Figure 13).

Several thoughts have been developed in order to explain why this study yielded such unexpected results. I observed hat the soils of protected field were noticeably sandier than those of Lighthouse Pasture; incurring a lower cation exchange capacity (CEC). Considering Lighthouse Pasture, mean soil salinity was measured to be higher in these soils. It is hypothesized that this higher soil salinity in Lighthouse Pasture is due to the texture and composition of the soils themselves. While Protected Field’s Soils are sandier, I observed Lighthouse pasture to have a greater constituent makeup of organic matter. As the proportion of organic matter increases with respect to sand in these soils, so to do the total amount of cation exchange sites. Simply, a higher amount of organic matter allows these soils, in Lighthouse Pasture, to retain sodium ions longer. Whereas sandier soils allow sodium to percolate faster through the soil column.

It may be that measuring sodium concentrations in soils is not the beast measure of the impacts of salt spray in these environments. What is seen in the soils upon analysis, sodium concentrations, is less a function of aerosol deposition and more so a function of how well a given soil can retain sodium ions (soil texture). The vegetation on Naushon Island, specifically its plant diversity, appears to have no trends connecting plant species diversity and soil sodium concentrations despite the islands flora being exposed to a relatively high amount of salt spray. I Believe this may be due to the majority of Naushon’s

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vegetation being native to the area. The species that grow on Naushon have adapted to a tumultuous oceanic environment, and the salt spray associated with said climates. It is fair to conclude that, according to the evidence obtained by this study, sodium has no impact on determining plant species richness in the coastal ecosystems of Naushon Island. I would like to investigate other theories such as, land-use history, grazing history, storm events, avian activity, mycorrhizal communities, and other soil aspects. Studies such as this are important with respect to humanities future. As climate change progresses, sea-level continues to rise, and the frequency and hostility of storm events increases, it is logical to assume that oceanic aerosols will begin to impact coastal environments more so in the near future. This implies changes that must accounted for in the agricultural sector, tourism and recreation and the ecology of coastal ecosystems as a whole. It is my intention to investigate the potential impact of these imminent threats and changes to Earth’s climate and to develop methods in order to do so, before Cape Cod and its nearby islands and swallowed by the sea. AKNOWLADGEMENTS Dr. Christopher Neill, Ph.D.

For being a fantastically supportive mentor to me in my juvenile scientific career and for instilling the drive in me to see my own potential as a scientist.

Dr. Anne Giblin, Ph.D. For teaching me to run spectrophotometric atomic absorption on a very short notice.

Lena Champlin Who, while working on her bachelorette’s thesis, assisted with the GIS portion of this study and for assisting me in the field on Naushon during the summer of 2015.

The Department of Earth and Environmental Science at Boston College Whose Faculty allowed me to partake in the amazingly formative experience that is SES and for teaching me, though Jesuit tradition, to “set the world aflame”.

Dr. Danielle Taghian, Ph.D. For being by nonofficial academic advisor at Boston College; for seeing my true potential; for recommending me to the SES program.

Meredith Horan and Timothy Ring; For emotional, psychological, and support for sanity’s sake during the SES program.

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FIGURES

Figure 1: A Map of Naushon Island, MA, indicating the two sites of vegetation monitoring plots from which soil cores were collected “A”: Lighthouse Pasture. “B”: Protected Field. Source: Google Maps©.

Figure 2: This GIS-generated elevation contour map depicts the position of vegetation monitoring plots in Lighthouse Pasture on Naushon Island, MA. Created in Collaboration with Lena Champlin on ArcGIS software.

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Figure 3: ArcGIS-generated maps depicting topographic slope aspects, overlaid with vegetation monitoring plot locations for (A) Lighthouse Pasture and (B) Protected Field on Naushon Island, MA. Resolution: 1m.

Figure 4: Graphical representation of mean species richness between Lighthouse Pasture and Protected Field on Naushon Island, MA.

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Figure 5: Graphical representation of mean soil salinity (sodium) between Lighthouse Pasture and Protected Field on Naushon Island, MA.

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Figure 6: Scatter-plot diagram depicting soil salinity of shrub plots in Protected Field and Lighthouse Pasture on Naushon Island, MA, plotted against species richness

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Figure 7: Scatter-plot diagram depicting soil salinity of grassland plots in Protected Field and Lighthouse Pasture on Naushon Island, MA, ploted against species richness

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Figure 8: Scatter-plot diagram depicting soil salinity in Lighthouse pasture with respect to topographic elevation on Naushon Island, MA.

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Figure 9:Figure 8: Scatter-plot diagram depicting soil salinity in Protected Field with respect to topographic elevation on Naushon Island, MA.

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Figure 10: Bar-graph representation of mean soil salinity with respect to topographic slope aspect orientation in terms of coordinal heading in Lighthouse Pasture on Naushon Island, MA.

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Figure 11: Radial-graph representing mean soil salinity with respect to topographic slope aspect orientation in terms of coordinal heading in Lighthouse Pasture on Naushon Island, MA.

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Figure 12:Figure 10: Bar-graph representation of mean soil salinity with respect to topographic slope aspect orientation in terms of coordinal heading in Protected Field on Naushon Island, MA.

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Figure 13: Radial-graph representing mean soil salinity with respect to topographic slope aspect orientation in terms of coordinal heading in Protected Field on Naushon Island, MA.

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LITERATURE CITED Levin Z., Cotton W.R. (Eds), 2009, Aerosol pollution impact on precipitation: A scientific review McComiskey, A.(Editor), Andrews, E., et al., Aerosols and Radiation - NOAA Earth System Research Laboratory Ma, X., von Salzen, K., Li, J., 2008: Modelling sea salt aerosol and its direct and indirect effects on climate. Atmos. Chem. Phys., 8, 1311-1327 Griffiths M.E., and Orians C.M. 2003. Salt spray differentially affects water status, necrosis, and growth in coastal sandplain heathland species. American Journal of Botany. White, R., Murray, S., and Rohweder, M. 2000. Pilot Analysis of Global Ecosystems: Grassland Ecosystems. World Resources Institute: Washington D.C. Motzkin, G., and Foster, D.R. 2002. Grasslands, heathlands and shrublands in coastal New England: historical interpretations and approaches to conservation. Journal of Biogeography, 29, 1569–1590. Foster, D.R., and Motzkin, G. 1999. Historical Influences on the Landscape of Martha’s Vineyard: Perspectives on the Management of the Manuel R. Correllus State Forest. Harvard Forest, Harvard University: Petersham, Massachusetts. HU, J.-K. 2001. Plant salt tolerance. Trends in Plant Science 6: 66–71.