Doctor of Philosophy (Ph.D.)
Virginia Institute of Marine Science
Mark J. Brush
Carl H. Hershner
Harry V. Wang
The efficiencies of water exchanges in both vertical and horizontal directions reflect the overall impact of various physical processes and serve as important indicators of physical control over a variety of ecological and biogeochemical processes. The vertical exchange between surface layers and bottom layers of a waterbody has proved to exert great control over the hypoxic condition, while the horizontal exchange between an estuary and coastal ocean determines the flushing capacity of the estuary and the retention rate of riverine materials. Various processes, such as tidal flushing, tidal mixing, gravitational circulation, and lateral circulation, can affect water exchange. Therefore, water exchange processes are complex and varying in time and space in estuaries. Besides the impact of numerous forcing variables, large-scale climate oscillation, sea-level rise, and human activities can result in a change of estuarine dynamics. Two biologically relevant timescales, residence time (RT) and vertical exchange time (VET), are used in this study to quantify the overall horizontal and vertical exchange, aiming to understand the physical transport control over the ecosystem functioning in a simpler way.
A long-term simulation of VET in the Chesapeake Bay over the period of 1980-2012 revealed a high spatial and seasonal similarity between VET and the dissolved oxygen (DO) level in the mainstem of the Chesapeake Bay, suggesting a major control over the DO condition from the physical transport. Over the past three decades, a VET of about 20 days in the summer usually indicates a hypoxic condition in the mainstem. Strong correlation among southerly wind strength, North Atlantic Oscillation index, and VET demonstrates that the physical condition in the Chesapeake Bay is highly controlled by the large-scale climate variation. The relationship is most significant during the summer, during which time the southerly wind dominates throughout the Chesapeake Bay. By combining the observed DO data with modeled VET, decoupling the physical and biological effect on the DO condition becomes possible. Bottom DO consumption rate was estimated through a conceptual model that links DO with VET. Using observed DO data and modeled VET, the overall biological effect on the DO condition can be quantified. The estimated bottom DO consumption rate shows strong seasonal variation and its interannual variation is highly correlated with the nutrient loading.
The response of an estuary ecosystem to a change of nutrient loading depends on the flushing capacity of the estuary, which is related to the horizontal water exchange. The overall flushing capacity can be quantified by resident time, which determines the retention and export rates of materials discharged in the estuary. The horizontal exchange in Chesapeake Bay was investigated over the period of 1980-2012. Quantified by the residence time (RT), the horizontal exchange in Chesapeake Bay exhibits high interannual and spatial variability. The 33-year simulation results show that the mean RT of the entire Chesapeake Bay system ranges from 110 to 264 days, with an average value of 180 days, which is smaller than 7.6 months (approximately 230 days) reported in previous studies. There is significant lateral asymmetry of RT in the mainstem, with a larger RT along the eastern bank than that along the western bank in the lower Bay, which is mainly attributed to the horizontal shearing of estuarine circulation and large freshwater input along the western bank. Because of the persistent stratification and estuarine circulation, the vertical difference between the surface RT and bottom RT is dramatic, with a difference as large as 100 days. Relations among RT, river discharge, and strength of estuarine circulation reveal that the variation of horizontal exchange is mainly controlled by the river discharge and modulated by the estuarine circulation. A strengthened estuarine circulation will enhance the water exchange and reduce the RT. By affecting the estuarine circulation, wind forcing has a great impact on the horizontal exchange.
The horizontal and vertical exchanges, together, contribute to the unique pattern of riverine material redistribution in Chesapeake Bay. By conducting long-term numerical simulations using multiple passive tracers that are independently released in the headwater of five main rivers (i.e., Susquehanna, Potomac, Rappahannock, York, and James Rivers), the relative contribution of discharge from each river to the total material in the mainstem can be calculated. The results show that the discharge from Susquehanna River has the dominant control on the riverine material throughout the entire mainstem. Despite the smaller contribution from the lower-middle Bay tributaries to the total materials in the mainstem, materials released from these rivers have a high potential to be transported to the middle-upper Bay through the bottom inflow by the persistent estuarine circulation. Depending on the magnitude of river discharge and the location of the tributary, material released at the headwaters of the main five rivers contributes differently to the riverine material in the mainstem. Material released in the upper estuary tends to have a longer residence time and a larger contribution, while materials released near the mouth are subject to a rapid flushing process, a small retention time, and a strong shelf-current induced dilution. The results reveal three distinct spatial patterns for materials released from the main river, tributary, and coastal oceans.
One of the potential factors to change the exchange processes is the degree of human activities, such as construction of large infrastructures. With projected intensified hurricane and an accelerated sea-level rise in the 21st century, building storm surge barriers to mitigate the flooding risk has been considered as feasible climate change adaptation strategies in many coastal areas, which will surely affect the ecosystem functioning by affecting the water exchange. Two types of partially embanked storm surge barriers across the mouth of Chesapeake Bay were examined. Under modeled scenarios, surge barriers exert a significant influence on the tide, salinity, residual current, and transport processes. The vertical exchange is weakened, mainly due to the reduction of tidal range and tidal mixing. Even though the stratification is enhanced, the estuarine circulation is weakened due to accumulation of freshwater in the downstream and a decreased horizontal salinity gradient. The overall horizontal exchange is weakened due to a barrier, but the impact varies spatially.
© The Author
Du, Jiabi, "Impact of Climate Variation and Human Adaptation on the Physical Transport Processes and Water Exchange in Chesapeake Bay" (2017). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1516639465.