OS13E-01 INVITED
How Will the San Francisco Bay-Delta Ecosystem Respond to Climate Change and Continued Population Growth?
Programs to ensure sustainability of coastal ecosystems and the biological diversity they harbor require ecological forecasting to assess habitat transformations from the coupled effects of climate change and human population growth. A multidisciplinary modeling project (CASCaDE) was launched in 2007 to develop 21st-century visions of the Sacramento-San Joaquin Delta and San Francisco Bay under four scenarios of climate change and increasing demand for California's water resource. The process begins with downscaled projections of daily weather from GCM's and routes these to a watershed model that computes runoff and an operations model that computes inflows to the Bay-Delta. Hydrologic and climatic outputs, including sea level rise, drive models of tidal hydrodynamics-salinity-temperature in the Delta, sediment inputs and evolving geomorphology of San Francisco Bay. These projected habitat changes are being used to address priority questions asked by resource managers: How will changes in seasonal streamflow, salinity and water temperature, frequency of extreme weather and hydrologic events, and geomorphology influence the sustainability of native species that depend upon the Bay-Delta and the ecosystem services it provides?
OS13E-02 INVITED
Climatic Perturbations of Phytoplankton Dynamics in Mid-Atlantic Estuaries
Climatic perturbations by drought-flood cycles, tropical storms, and hurricanes are increasingly important in Mid-Atlantic estuaries, leading to ecosystem-scale responses of the plankton system that have significant trophic implications. Recent observations support an emerging paradigm that climate dominates nutrient enrichment in these ecosystems, explaining seasonal and interannual variability of phytoplankton floral composition, biomass (chl-a), and primary productivity (PP). We present historical and recent data for the Chesapeake Bay and Albemarle-Pamlico Sound - Neuse River estuaries to quantify long-term trends against a backdrop of strong climatic forcing that evokes a high degree of interannual variability in these dynamic estuaries. Data sources include historical observations, monitoring cruises, individual research programs, and aircraft remote sensing of chlorophyll biomass. We describe climatic forcing of phytoplankton dynamics that principally reflects variability of freshwater flow and commensurate variability of nutrient loading and light availability. Analyses consist of spatial/temporal variability of chl-a; interannual variability of PP; statistical methods to classify regional climate; coincident forcing of floral composition, biomass, and PP by flow/climate. Data from these sources are being combined with climate analysis and biogeochemical modeling to support our current understanding, leading to predictive capabilities for phytoplankton dynamics in these rich ecosystems.
OS13E-03
Hydrologic and biogeochemical impacts of a period of elevated hurricane activity on the Pamlico Sound system, NC: The challenges for nutrient and habitat management
Since the mid-1990's, US Coastal regions have experienced a sudden rise in hurricane and tropical storm
landfalls; this elevated frequency is expected to continue for the next several decades. The North Carolina
coast has been impacted by at least eight hurricanes and six tropical storms during this time. Each of these
storms exhibited unique hydrologic and nutrient loading scenarios. This variability represents a formidable
challenge to management of eutrophication and fisheries habitats of the Pamlico Sound system, the US's
largest lagoonal ecosystem and a key fisheries resource. Different rainfall amounts among hurricanes led to
variable freshwater and nutrient discharge and hence variable nutrient, organic matter, and sediment
enrichment. These enrichments differentially affected physical-chemical properties (salinity, water residence
time, transparency, stratification, dissolved oxygen), phytoplankton community production and composition.
The contrasting effects were accompanied by biogeochemical perturbations (hypoxia, enhanced nutrient
cycling), habitat alterations, and food web disturbances. Floodwaters from the two largest hurricanes, Fran
(1996) and Floyd (1999), exerted multi-month to multi-annual hydrologic and biogeochemical effects. In
contrast, relatively low rainfall coastal hurricanes like Isabel (2003) and Ophelia (2005) caused strong vertical
mixing and storm surges, but relatively minor hydrologic, nutrient, and biotic impacts. Both hydrologic and
wind forcing are important drivers and must be integrated with nutrient loading in assessing short- and long-
term ecological impacts of these storms. These climatic forcings cannot be managed but must be considered
when developing water quality and habitat management strategies for these and other large estuarine
ecosystems faced with increasing frequencies and intensities of hurricanes.
http://www.unc.edu/ims/paerllab/
OS13E-04
Relationship Between Pacific Decadal Oscillation and Paleo-Tracers of Hypoxia in Sediment Cores from Puget Sound, WA
Coastal eutrophication is often linked to increasing anthropogenic nutrient loading. While this cause-effect relationship holds true for many coastal regions, the Pacific Northwest also receives nutrient-rich waters from coastal upwelling. Sediment cores were collected in Puget Sound's Main Basin and the sub-basin of Hood Canal to reconstruct the historical record of hypoxia for circa 300 years. Paleo-tracers used to reconstruct the record included stable isotopes of carbon and nitrogen, biomarkers for terrestrial organic matter, and redox-sensitive metals (Mo, Re, U, Cd) that are enriched in sediment during periods of low/no oxygen. Reconstructions suggest evidence of hypoxia existing in Hood Canal prior to significant western-settlement (1900s). Therefore, the development of hypoxic bottom waters may be linked to natural processes. The relationship between Pacific Decadal Oscillation (PDO) and this historical record of hypoxia were evaluated. Sediment core profiles indicate a century-scale oscillation in hypoxia with periods of persistent hypoxia recorded circa 1700s, 1800s, and 1900s and more oxygenated conditions recorded around the middle of each century. The key features of this record include core segments highly enriched with redox-sensitive metals, encompassing ~20 cm, circa 1700s and little/no enrichment around the 1950s. These key features correspond with periods of intensive positive and negative PDO indices, respectively. The PDO results from a shift from west to east of the Aleutian Low with Pacific Northwest weather oscillating between relatively warm- dry and cool-wet conditions, respectively. These reconstructions suggest that atmospheric/oceanic conditions during positive PDO cycles may be linked to restricted ventilation and increased propensity for the occurrence of hypoxia in Hood Canal.
OS13E-05
Contrasts in the flux and composition of particulate organic matter in a temperate estuary as a function of river discharge
Climate change has the potential to affect the forcings that control general estuarine circulation and dynamics, including river discharge and wind, drastically. Changes in these forcings can lead to marked differences in the processing of materials within a given estuary. In order to explore the possible effects of climate- as well as human-induced changes on the carbon balance of estuaries, we examined the fluxes and composition of particulate organic matter (POM) in Winyah Bay (SC, USA), a partially mixed, temperate estuary. Specifically, we investigated the transport and transformation of POM in the estuarine turbidity maximum region of Winyah Bay under contrasting river discharge regimes, focusing on the mechanisms and factors that explain the spatial and temporal variability in net material fluxes in this system. Our results show marked differences in the concentrations and net fluxes of POM under contrasting discharge conditions. For example, higher concentrations and net landward fluxes characterized low discharge periods whereas lower concentrations and net seaward fluxes were observed during high river discharge periods. Compositionally, the nature of the particles in suspension varied significantly, with much higher carbon contents and distinct stable isotopic ratios measured during periods of high discharge. We attribute these marked contrasts to differences in the source of particles (river vs. estuarine sediments) and in the relative importance of advective vs. tidal components of the flux. Changes in the lateral circulation of the system under contrasting discharge conditions underlie these variations with significant implications for the residence time and cycling of particulate organic matter in this and other estuaries.
OS13E-06
Use of Sediment Core Records to Understand Anthropogenic Impacts on Carbon Delivery to the Sacramento-San Joaquin River Delta, CA
Anthropogenic activities, including climate change, will influence connections between the hydrologic and carbon cycles as well as the exchange of materials between terrestrial and aquatic systems. Altered precipitation will influence the delivery of water, suspended sediment and carbon, while construction of dams and reservoirs and changes in land use alter the flow paths and transport of sediment and associated materials to downstream ecosystems. We used the Sacramento-San Joaquin River Delta CA (Delta, hereafter) as a model system for understanding how human activities influenced the delivery and composition of organic carbon (OC) over the past 50-60 years. Sediment cores from the Delta were used to examine human impacts on carbon sources, amounts, and ages. Sediment and carbon accumulation rates were four to eight-fold higher pre-1972 relative to post-1972, coincident with completion of several large reservoirs and increased agriculture and urbanization in the Delta watershed. Several classes of biomarkers demonstrate that terrigenous OC has decreased since the 1940s. Radiocarbon isotopes of TOC and fatty acids in surface sediments indicate that much of the OC is highly reworked (900-1400 years BP) and vascular plant biomarkers have the oldest ages suggesting erosion of soils. Together, these data suggest that human activities have altered the amount, sources, and ages of carbon accumulating in the Delta. Projected increases in aridity and changes in the timing and amounts of freshwater delivery associated with anthropogenic climate change are likely to exacerbate these modifications to the delivery of carbon and sediment.
OS13E-07
Effects of Climatic Shifts in River Discharge Magnitude and Intensity on Estuarine Sediment Transport
Estuarine sediment transport depends on both tidal and fluvial forcing, and climatic shifts in river discharge can alter sediment fluxes at event, seasonal and decadal time scales. To consider estuarine response to changes in forcing over long periods, we have developed a time-dependent, along-estuary model of estuarine dynamics and sediment transport. We apply the model to the Hudson River estuary over the past 100 years, validating with available data. The model incorporates bathymetric variability between channel and shoals, producing lateral gradients in baroclinic circulation and sediment trapping. The results indicate that the direction and magnitude of sediment transport depend largely on river discharge. Net sediment transport is modified by the timing of high discharge events with respect to the spring-neap tidal cycle and subtidal fluctuations in sea level, as well as the duration of events with respect to the estuarine response time. Assuming morphological equilibrium at century time scales, we find that the estuary can have an excess or deficit of sediment over periods of years to decades depending on discharge conditions. During high discharge periods, maximum export coincides with maximum sediment supply from the watershed, but the latter dominates the estuarine sediment budget due to the nearly cubic discharge dependence of fluvial sediment supply and the nearly linear dependence of estuarine sediment transport. Consequently, the estuary accumulates sediment during the highest flow conditions, and is there is net sediment export during periods of moderate discharge. Using the model, we simulate effects of potential climatic shifts in river discharge including increased frequency and intensity of high discharge events and seasonal shifts in the timing of the winter/spring freshet.
OS13E-08
How does sea-level rise affect stratification and circulation in Chesapeake Bay?
Despite the potentially large impacts of climate change on the physical state of estuaries, very little research has been conducted on this topic, particularly with regard to sea-level rise, one of the most certain consequences of climate change. Global sea level rose at a rate of 1.8 ± 0.5 mm/yr from 1961 to 2003. Climate models project that the rate of sea-level rise will further increase in the 21st century, with projected global mean increases by 2100 of 0.5 to 1.4 m, depending on the greenhouse gas scenario. Chesapeake Bay is particularly vulnerable because relative sea-level rise there during the past 50 years is large (2.7 – 4.5 mm/yr) compared to the global average. This is due to land subsidence as well as the greater rate of absolute sea-level rise in the middle latitudes of the Northwest Atlantic Ocean. We have conducted process- oriented modeling experiments to investigate the response of Chesapeake Bay to an abrupt increase in the offshore mean sea level. It is found that the sea-level rise affects estuarine salinity distribution and circulation in a variety of interesting and unexpected ways. Sea-level rise causes stronger salt intrusion, which may produce stronger stratification in the estuary. However, sea-level rise also increases the tidal range. In Chesapeake Bay, a 1-m rise in mean sea level moves diurnal tides into the resonant band, amplifying the tides inside the Bay. Larger tides produce stronger mixing and reduce stratification. The net effect of sea- level rise on estuarine stratification thus depends on the competition between these two opposing forces, which is explored by the numerical model. The classic steady-state theory by Hansen and Rattray predicts that neither the residual velocity nor the stratification depends on the vertical mixing rate. We use the numerical model to test this theoretical prediction and examine the hypothesis that the horizontal salinity gradient and vertical mixing work in opposition to produce the relative insensitivity of estuarine stratification and circulation to changes in tidal mixing.