Biology Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2749
Browse
3 results
Search Results
Item Discerning the roles of ocean acidification, eutrophication, and river alkalization in driving long-term pH trends in the Chesapeake Bay(2022) Guo, Yijun; Li, Ming; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Rising anthropogenic CO2 in the atmosphere and oceanic uptake of CO2 have led to a gradual decrease in seawater pH and ocean acidification, but pH changes in estuaries and coastal systems are more complicated due to a multitude of global and regional environmental drivers. Increasing global fertilizer use due to agricultural production has led to a doubling of riverine nutrient loading since the 1950s, leading to widespread eutrophication in estuarine and coastal waters. Excessive nutrient loading stimulates primary production in the surface euphotic layer, which consumes CO2 and elevates pH, but unassimilated organic matter sinks and decomposes in bottom waters, producing CO2 and reducing pH. In the meantime, human-accelerated chemical weathering, such as acid rain and mining, has resulted in rising alkalinity in many rivers and basification in estuarine and coastal waters. To discern how these environmental drivers influence long-term pH trends in coastal waters, a coupled hydrodynamic-biogeochemical-carbonate chemistry model was used to conduct hindcast simulations of the Chesapeake Bay between 1951 and 2010. The model reproduced the observed chlorophyll increase and hypoxia expansion due to the increased nutrient loading. In contrast, low-pH bottom waters and acidic volume shrank from 1950 to 1980. GAM analysis of long-term pH trends in different regions of Chesapeake Bay revealed increasing pH in the upper Bay driven by the river alkalinization, a peak pH in the mid-Bay in the 1980s coincident with the peak nutrient loading and decreasing pH in the lower Bay driven by ocean acidification. Four scenario runs were performed to assess the individual effects of rising pCO2, river alkalinization, riverine nutrient loading, and climate change (warming and sea-level rise) on long-term pH changes in the Chesapeake Bay. The model results suggested that river alkalinization was more important than ocean acidification in driving the long-term pH changes in the estuary.Item HYDROLOGY, SOIL REDOX, AND PORE-WATER IRON REGULATE CARBON CYCLING IN NATURAL AND RESTORED TIDAL FRESHWATER WETLANDS IN THE CHESAPEAKE BAY, MARYLAND, USA(2017) Keshta, Amr El Shahat Sedik; Baldwin, Andrew H; Yarwood, Stephanie A; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Tidal freshwater wetlands are key sites for carbon (C) sequestration and main component in the global C budget. The overall research objective of my dissertation was to examine the physical and biogeochemical processes that impact C cycling in tidal freshwater wetlands. One natural and one restored tidal freshwater wetland (salinity < 0.3 ppt) were selected in Maryland, USA along the Patuxent River. Data logging water recorders were installed in wells at each habitat in February 2014 for monitoring water level at 10-minutes interval and for two years. Soil organic matter and C stocks were estimated and a novel soil C bioassay (CARBIO) was developed and tested to assess C stability (change of soil organic matter concentration over time) and decomposition rates in both sites. A total of 162 CARBIO units were deployed in the natural and restored sites, and 81 were retrieved after 1 year while the others were retrieved after 2 years. Static chambers were used to quantify methane (CH4) and carbon dioxide (CO2) flux rates during day and nighttime. My results indicated that the natural wetland had significantly higher soil C stocks than the restored site (14.8±0.50 and 8.9±0.99 kg C m-2, respectively, P <0.0001). The swamp habitat had the highest soil organic matter (36.8%), while restored mudflat has the lowest (2.8%). Higher soil organic matter was partially correlated with shallower groundwater level relative to soil surface. Soil redox data with soil pH indicated that the soil of the natural wetland habitats was more reducing than the soil at the restored habitats. Based on CARBIO index, the soils in CARBIO units that were deployed in the natural wetland was significantly higher in C sequestration rate than the restored wetland (535±291.5 and -1095±429.4 g C m-2 year-1, respectively, P site<0.05). Under the current hydrological conditions, the restored wetland habitats were not able to accumulate C inside the CARBIO units after 1 or 2 years from deployment. In-situ CARBIO units can be employed in the newly constructed wetlands as in-situ sensors that reflect the C biogeochemical processes in the ambient soil to help better understanding C stability. The restored wetland had significantly higher annual CH4 emission rates than the natural wetland (1372.1±35.89 and 880.7±144.73 g CH4 m-2 y-1, respectively, P <0.05) and the log CH4 flux rate had a significant and strong negative correlation with the pore-water total available iron. Nighttime CH4 fluxes had very low concentration (<3650 µmole m-2 h-1). Future restoration efforts should focus on soil properties that will help increase C accumulation in newly constructed wetlands, but even more important every effort should be made to conserve the natural wetlands so that ecosystem function and services including wildlife habitat, water quality improvement, and offsetting the greenhouse gas emissions are maintained.Item EXPOSURE TO EXTREME HEAT EVENTS AND CHRONIC RESPIRATORY DISEASES AMONG A NATIONALLY REPRESENTATIVE SAMPLE OF THE UNITED STATES POPULATION(2016) Romeo, Crystal Eloma; Sapkota, Amir; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Previous studies have shown that extreme weather events are on the rise in response to our changing climate. Such events are projected to become more frequent, more intense, and longer lasting. A consistent exposure metric for measuring these extreme events as well as information regarding how these events lead to ill health are needed to inform meaningful adaptation strategies that are specific to the needs of local communities. Using federal meteorological data corresponding to 17 years (1997-2013) of the National Health Interview Survey, this research: 1) developed a location-specific exposure metric that captures individuals’ “exposure” at a spatial scale that is consistent with publicly available county-level health outcome data; 2) characterized the United States’ population in counties that have experienced higher numbers of extreme heat events and thus identified population groups likely to experience future events; and 3) developed an empirical model describing the association between exposure to extreme heat events and hay fever. This research confirmed that the natural modes of forcing (e.g., El Niño-Southern Oscillation), seasonality, urban-rural classification, and division of country have an impact on the number extreme heat events recorded. Also, many of the areas affected by extreme heat events are shown to have a variety of vulnerable populations including women of childbearing age, people who are poor, and older adults. Lastly, this research showed that adults in the highest quartile of exposure to extreme heat events had a 7% increased odds of hay fever compared to those in the lowest quartile, suggesting that exposure to extreme heat events increases risk of hay fever among US adults.