The importance of decadal-scale climate variability to wind-driven modulation of hypoxia in Chesapeake Bay

Millions of dollars are spent annually to reduce nutrient loading to Chesapeake Bay, with a fundamental goal of reducing the extent and severity of low dissolved oxygen (hypoxia) during the summertime months^1^. Yet despite recent reductions in nutrient loading, large volumes of the Bay continue to be impacted by hypoxia and anoxia during the summer months^2-3^. One obstacle to assessing efforts to improve water quality in the Bay and other estuarine systems is a complete understanding of the physical processes that modulate dissolved oxygen and the long-term variability of these processes. Here I analyze a 58-year data set of estimated hypoxic volume in the Bay^2^ and demonstrate the importance that wind direction plays in controlling the extent and severity of summertime hypoxia. This analysis indicates that wind direction explains a greater percentage of the observed inter-annual variation in hypoxic volume than estimates of nutrient loading. The implication is that physical processes play a dominant role in modulating hypoxia and that much of the increased hypoxia observed since the early 1980s can be attributed to changes in wind forcing that are the result of decadal-scale climate variability. These findings emphasize the importance of understanding the physical processes that modulate dissolved oxygen in coastal and estuarine systems and highlight the potential impact that climate change may have on water quality in Chesapeake Bay and other estuarine systems

The contribution of physical processes to inter-annual variations of hypoxia in Chesapeake Bay : a 30-yr modeling study

Author Posting. © Association for the Sciences of Limnology and Oceanography, 2016. This article is posted here by permission of Association for the Sciences of Limnology and Oceanography for personal use, not for redistribution. The definitive version was published in Limnology and Oceanography 61 (2016): 2243–2260, doi:10.1002/lno.10372.A numerical circulation model with a very simple representation of dissolved oxygen dynamics is used to simulate hypoxia in Chesapeake Bay for the 30-yr period 1984–2013. The model assumes that the biological utilization of dissolved oxygen is constant in both time and space in an attempt to isolate the role that physical processes play in modulating oxygen dynamics. Despite the simplicity of the model it demonstrates skill in simulating the observed inter-annual variability of hypoxic volume, capturing 50% of the observed variability in hypoxic volume (<2 mg L−1) for the month of July and 58% of the observed variability for the month of August, over the 30-yr period. Model skill increases throughout the summer suggesting that physical processes play a more important role in modulating hypoxia later in the summer. Model skill is better for hypoxic volumes than for anoxic volumes. In fact, a simple regression based on the integrated January–June Susquehanna River nitrogen load can explain more of the variability in the observed anoxic volumes than the model presented here. Model results suggest that the mean summer (June–August) wind speed is the single-most important physical variable contributing to variations in hypoxic volumes. Previous studies have failed to document the importance of summer wind speed because they have relied on winds measured at Patuxent Naval Air Station, which does not capture the observed inter-annual variations in wind speed that are observed by stations that directly measure wind over the waters of Chesapeake Bay.National Science Foundation Grant Number: OCE-1338518; National Oceanic and Atmospheric Association NOAA via the IOOS Office Award Grant Numbers: NA10NOS0120063, NA11NOS0120144

Mixing of dissolved oxygen in Chesapeake Bay driven by the interaction between wind-driven circulation and estuarine bathymetry

Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 121 (2016): 5639–5654, doi:10.1002/2016JC011924.Field observations collected in Chesapeake Bay demonstrate how wind-driven circulation interacts with estuarine bathymetry to control when and where the vertical mixing of dissolved oxygen occurs. In the across-Bay direction, the lateral Ekman response to along-Bay wind forcing contributes to the vertical mixing of dissolved oxygen in two ways. First, the lateral tilting of the pycnocline/oxycline, consistent with the thermal wind relationship, advects the region of high vertical gradient into the surface and bottom boundary layers where mixing can occur. Second, upwelling of low-oxygen water to the surface enhances the atmospheric influx. In the along-Bay direction, the abrupt change in bottom depth associated with Rappahannock Shoal results in surface convergence and downwelling, leading to localized vertical mixing. Water that is mixed on the shoal is entrained into the up-Bay residual bottom flow resulting in increases in bottom dissolved oxygen that propagate up the system. The increases in dissolved oxygen are often associated with increases in temperature and decreases in salinity, consistent with vertical mixing. However, the lagged arrival moving northward suggests that the propagation of this signal up the Bay is due to advection.National Science Foundation Grant Number: OCE-13385182017-02-0

Physical controls on hypoxia in Chesapeake Bay : a numerical modeling study

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 118 (2013): 1239–1256, doi:10.1002/jgrc.20138.A three-dimensional circulation model with a relatively simple dissolved oxygen model is used to examine the role that physical forcing has on controlling hypoxia and anoxia in Chesapeake Bay. The model assumes that the biological utilization of dissolved oxygen is constant in both time and space, isolating the role that physical forces play in modulating oxygen dynamics. Despite the simplicity of the model, it demonstrates skill in reproducing the observed variability of dissolved oxygen in the bay, highlighting the important role that variations in physical forcing have on the seasonal cycle of hypoxia. Model runs demonstrate significant changes in the annual integrated hypoxic volume as a function of river discharge, water temperature, and wind speed and direction. Variations in wind speed and direction had the greatest impact on the observed seasonal cycle of hypoxia and large impacts on the annually integrated hypoxic volume. The seasonal cycle of hypoxia was relatively insensitive to synoptic variability in river discharge, but integrated hypoxic volumes were sensitive to the overall magnitude of river discharge at annual time scales. Increases in river discharge were shown to increase hypoxic volumes, independent from the associated biological response to higher nutrient delivery. However, increases in hypoxic volume were limited at very high river discharge because increased advective fluxes limited the overall length of the hypoxic region. Changes in water temperature and its control on dissolved oxygen saturation were important to both the seasonal cycle of hypoxia and the overall magnitude of hypoxia in a given year.The funding for this research was obtained from NSF Grant OCE-0954690 and supported by NOAA via the U.S. IOOS Office (Award Numbers NA10NOS0120063 and NA11NOS0120141) and managed by the Southeastern Universities Research Association.2013-09-1

A diel method of estimating gross primary production: 1. Validation with a realistic numerical model of Chesapeake Bay

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 123(11), (2018): 8411-8429, doi: 10.1029/2018JC014178.A method for estimating gross primary production (GPP) is presented and validated against a numerical model of Chesapeake Bay that includes realistic physical and biological forcing. The method statistically fits a photosynthesis‐irradiance response curve using the observed near‐surface time rate of change of dissolved oxygen and the incoming solar radiation, yielding estimates of the light‐saturated photosynthetic rate and the initial slope of the photosynthesis‐irradiance response curve. This allows estimation of GPP with 15‐day temporal resolution. The method is applied to the output from a numerical model that has high skill at reproducing both surface and near‐bottom dissolved oxygen variations observed in Chesapeake Bay in 2013. The rate of GPP predicted by the numerical model is known, as are the contributions from physical processes, allowing the proposed diel method to be rigorously assessed. At locations throughout the main stem of the Bay, the method accurately extracts the underlying rate of GPP, including pronounced seasonal variability and spatial variability. Errors associated with the method are primarily the result of contributions by the divergence in turbulent oxygen flux, which changes sign over the surface mixed layer. As a result, there is an optimal vertical location with minimal bias where application of the method is most accurate.This paper is the result of research funded in part by NOAA's U.S. Integrated Ocean Observing System (IOOS) Program Office as a subcontract to the Woods Hole Oceanographic Institution under award NA13NOS120139 to the Southeastern University Research Association. All of the model output, as well as both the CBIBS data (2010–2016) and the bottom oxygen data of Scully (2016b), are publicly available through the THREDDS server associated with the IOOS Coastal Modeling Testbed site: https://comt.ioos.us/projects/cb_hypoxia.2019-05-2

A diel method of estimating gross primary production: 2. Application to 7 years of near-surface dissolved oxygen data in Chesapeake Bay.

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 123(11), (2018): 8430-8443, doi: 10.1029/2018JC014179.A diel method for estimating gross primary production (GPP) is applied to nearly continuous measurements of near‐surface dissolved oxygen collected at seven locations throughout the main stem of Chesapeake Bay. The data were collected through the Chesapeake Bay Interpretive Buoy System and span the period 2010–2016. At all locations, GPP exhibits pronounced seasonal variability consistent temperature‐dependent phytoplankton growth. At the Susquehanna Buoy, which is located within the estuarine turbidity maximum, rates of GPP are negatively correlated with uncalibrated turbidity data consistent with light limitation at this location. The highest rates of GPP are located immediately down Bay from the estuarine turbidity maximum and decrease moving seaward consistent with nutrient limitation. Rates of GPP at the mouth (First Landing Buoy) are roughly a factor of 3 lower than the rates in the upper Bay (Patapsco). At interannual time scales, the summer (June–July) rate of GPP averaged over all stations is positively correlated (r2 = 0.62) with the March Susquehanna River discharge and a multiple regression model that includes spring river discharge, and summer water temperature can explain most (r2 = 0.88) of the interannual variance in the observed rate of GPP. The correlation with river discharge is consistent with an increase in productivity fueled by increased nutrient loading. More generally, the spatial and temporal patterns inferred using this method are consistent with our current understanding of primary production in the Bay, demonstrating the potential this method has for making highly resolved measurements in less well studied estuarine systems.This paper is the result of research funded in part by NOAA's U.S. Integrated Ocean Observing System (IOOS) Program Office as a subcontract to the Woods Hole Oceanographic Institution under award NA13NOS120139 to the Southeastern University Research Association. All of the data analyzed in this paper are publicly available including the CBIBS data (http://buoybay.noaa.gov), the NCEP NARR data (https://www.esrl.noaa.gov/psd), and the Kd‐490 MODIS data (ftp://ftp.star.nesdis.noaa.gov/pub/socd1/ecn/data/modis/k490noaa/monthly/cd/). Model output analyzed in this paper is publicly available through the THREDDS server associated with the IOOS Coastal and Ocean Modeling Testbed (COMT) site (https://comt.ioos.us/projects/cb_hypoxia). Postprocessed and compiled data for all seven CBIBS locations including the interpolated values of incoming solar radiation and satellite‐derived Kd‐490 can also be download from the COMT site.2019-05-2

The interaction between stratification, circulation, and sediment transport in a partially-mixed estuary

Detailed field observations from the York River estuary, Virginia are used to examine the processes governing vertical density stratification and to evaluate the importance of spatial and temporal variations in turbulent mixing on estuarine dynamics and sediment transport. Contrary to previous findings that suggest wind stress acts predominantly as a source of energy to mix away stratification, this study demonstrates that the wind can play a more important role in straining the along-channel estuarine density gradient. as a result, down-estuary winds enhance the tidally-averaged vertical shear, which interacts with the along-channel density gradient to increase stratification. Conversely, up-estuary winds tend to reduce, or even reverse the vertical shear, reducing stratification. While wind straining can play a dominant role in governing the overall degree of turbulent mixing at sub-tidal time scales, tidal straining of the along-channel density gradient can result in asymmetries in turbulent mixing at the tidal time scale. In estuarine systems with channel-shoal morphologies, tidal straining can lead to asymmetries in turbulent mixing near the deeper channel while the neighboring shoals remain relatively well-mixed. These temporal and spatial variations in turbulent mixing result in a barotropically-induced estuarine residual flow that favors inflow over the shoal regions and outflow over the channel. This pattern of residual circulation can offset, or even reverse, the pattern of residual circulation typically associated with baroclinic estuarine circulation. These tidal asymmetries in mixing have the opposite influence on the patterns of sediment flux. The higher values of eddy viscosity that occur during the less-stratified flood tide resuspend sediment higher in the water column, favoring up-estuary pumping. The presence of strong density stratification significantly damps turbulence in the upper water column, and the lateral dynamical balance is largely geostrophic at tidal time scales. Even though friction does not contribute at lowest order to the lateral balance, the lateral circulation is frictionally-driven by Ekman transport in the bottom boundary layer. The interaction of the lateral circulation and the stratification acts to limit the strength of the lateral circulation and as a result, significantly stronger lateral circulation occurs during less stratified conditions

Modeling of Critically-Stratified Gravity Flows: Application to the Eel River Continental Shelf, Northern California

An analytical and numerical model are presented and applied to predict gravitydriven transport and deposition of fluid mud layers that form within the wave boundary layer on the continental shelf off the Eel River in northern California. Observations indicate that following floods of the Eel River down-slope transport of fluid mud trapped within the wave boundary layer is the dominant across-shelf transport mechanism. The models are based upon the assumption that following significant floods, an abundant supply of easily suspended fine sediment is delivered to the coastal ocean, allowing a negative feedback mechanism to maintain the near-bed Richardson number at its critical value. Thus, sediment-induced stratification effectively limits the amount of fine sediment that can be maintained in suspension, allowing the calculation of down-slope transport and deposition knowing only the appropriate near-bed velocity scale. Analytic predictions of mid-shelf mud transport and deposition are spatially and temporally consistent with field observations and provide strong evidence that gravitydriven processes control the emplacement and location of the Eel margin flood deposit. Analytic predictions of deposition suggest that the magnitude of wave energy is more important than the magnitude of the flood event in controlling the thickness of mid-shelf gravity-driven deposition following floods. Higher wave energy increases the capacity for critically-stratified gravity flows to transport sediment to the mid-shelf and results in greater deposition. The bathymetry of the Eel margin plays a critical role in gravitydriven transport and deposition. Analytic predictions indicate that gravity-driven deposition on the mid-shelf begins roughly 7-8 km north of the river mouth. Closer to the river mouth, the seaward increasing mid-shelf slope associated with the concave downward subaqueous delta causes gravity-driven flux divergence, preventing significant mid-shelf gravity-driven deposition and favoring sediment bypassing. Seaward decreases in shelf slope in the vicinity of the observed flood depo-center leads to greater flux convergence by gravity-driven flows, and hence greater deposition. The numerical model predicts gravity-driven deposition on the continental shelf for four consecutive flood seasons of the Eel River using realistic bathymetry, waves and river forcing. Results from the numerical model are consistent with observations of deposition on the mid-self and support the results of the analytical model that suggest wave intensity and bathymetry are the dominant factors controlling the location and magnitude of observed deposition. Despite significantly greater sediment input near the river mouth, little mid-shelf deposition is predicted in this region due to the increasing off-shelf slope. The numeric results suggest that gradients in the along-shelf components of bed-slope also favor gravity-driven deposition 10-30 km north of the river mouth. Including the influence of along-shelf currents had little impact on the location of midshelf deposition, providing further support for bathymetric control of flood sedimentation on the Eel margin. A significant fraction of sediment from the Eel River was predicted to leave the shelf as a gravity-driven flow during floods with large wave energy. However, in extremely large floods, gravity-driven processes were not capable of removing riverderived fine sediment from the inner-shelf

The importance of tidal and lateral asymmetries in stratification to residual circulation in partially mixed estuaries

Author Posting. © American Meteorological Society, 2007. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 37 (2007): 1496-1511, doi:10.1175/jpo3071.1.Measurements collected in the York River estuary, Virginia, demonstrate the important impact that tidal and lateral asymmetries in turbulent mixing have on the tidally averaged residual circulation. A reduction in turbulent mixing during the ebb phase of the tide caused by tidal straining of the axial density gradient results in increased vertical velocity shear throughout the water column during the ebb tide. In the absence of significant lateral differences in turbulent mixing, the enhanced ebb-directed transport caused by tidal straining is balanced by a reduction in the net seaward-directed barotropic pressure gradient, resulting in laterally uniform two-layer residual flow. However, the channel–shoal morphology of many drowned river valley estuaries often leads to lateral gradients in turbulent mixing. Tidal straining may then lead to tidal asymmetries in turbulent mixing near the deeper channel while the neighboring shoals remain relatively well mixed. As a result, the largest lateral asymmetries in turbulent mixing occur at the end of the ebb tide when the channel is significantly more stratified than the shoals. The reduced friction at the end of ebb delays the onset of the flood tide, increasing the duration of ebb in the channel. Conversely, over the shoal regions where stratification is more inhibited by tidal mixing, there is greater friction and the transition from ebb to flood occurs more rapidly. The resulting residual circulation is seaward over the channel and landward over the shoal. The shoal–channel segregation of this barotropically induced estuarine residual flow is opposite to that typically associated with baroclinic estuarine circulation over channel–shoal bathymetry.Support for this research was provided by the National Science Foundation Division of Ocean Sciences Grant OCE- 9984941

The role of advection, straining, and mixing on the tidal variability of estuarine stratification

Author Posting. © American Meteorological Society, 2012. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 42 (2012): 855–868, doi:10.1175/JPO-D-10-05010.1.Data from the Hudson River estuary demonstrate that the tidal variations in vertical salinity stratification are not consistent with the patterns associated with along-channel tidal straining. These observations result from three additional processes not accounted for in the traditional tidal straining model: 1) along-channel and 2) lateral advection of horizontal gradients in the vertical salinity gradient and 3) tidal asymmetries in the strength of vertical mixing. As a result, cross-sectionally averaged values of the vertical salinity gradient are shown to increase during the flood tide and decrease during the ebb. Only over a limited portion of the cross section does the observed stratification increase during the ebb and decrease during the flood. These observations highlight the three-dimensional nature of estuarine flows and demonstrate that lateral circulation provides an alternate mechanism that allows for the exchange of materials between surface and bottom waters, even when direct turbulent mixing through the pycnocline is prohibited by strong stratification.The funding for this research was obtained from NSF Grant OCE-08-25226.2012-11-0