Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Related Publications
• Related Datasets
• Parameters
• Project Information
• Funding

This research was supported with funding from the National Science Foundation's Chemical Oceanography Program (OCE‐1536996 and OCE‐1537013). Additional support was provided by NASA through Grants NNX14AM37G and NNX14AF93G.

The data products are  calculated partial pressure and air-sea flux of carbon dioxide in the main stem of Chesapeake Bay; the dataset also includes all the inputs to the calculation.

The data were derived from the Chesapeake Bay program's Water Quality Database, United States Geological  Survey monitoring data, North American Regional Reanalysis,  World Data Centre for Greenhouse Gases, and 2016-2018 carbonate system measurements the Chesapeake (Shadwick et al., 2019). See Related Datasets section.

The data are in the form of seven netCDF files:

1. results-pco2-gasex.nc: calculated monthly pCO₂ and air-sea CO₂ flux in the Chesapeake Bay mainstem, separated into 8 segments and for the total bay (point estimates and Monte Carlo ensembles).
2. atmosph-apres-wspd2_xco2.nc: atmospheric data (squared wind speed, atmospheric pressure, and CO₂ mole fraction) used as inputs to the CO₂ flux calculation; also provided are intermediate calculation results (airpCO₂, CO₂ solubility, and gas transfer velocity).
3. nontidal-susquehanna-alk-dic: alkalinity and DIC at the non-tidal station in the Susquehanna River at Conowingo, Maryland (USGS gauge number 01578310).
4. tidal-33stations-database.nc: measured (Chesapeake Bay Program) and calculated variables at the 33 tidal stations in the Chesapeake Bay mainstem (temperature, salinity, oxygen, pH and alkalinity); at the surface- and at the native time resolution of the Chesapeake Bay Program.
5. shadwick2019a-buoy-pco2.nc: hourly pCO2 calculated from the high-frequency measurements of temperature, salinity, and pH at the York River buoy (Shadwick et al. 2019a).
6. shadwick2019b-cruises-alk-dic.nc: surface alkalinity and DIC measurements from 2016-2018 cruises in the Chesapeake Bay mainstem (Shadwick et al. 2019b).
7. cbp-cruises-alk.nc: surface alkalinity measurements from 1986-1991 cruises in the Chesapeake Bay mainstem (Chesapeake Bay Program)

pCO2 and the air–water CO2 flux were computed at monthly resolution from 1998 to 2018 from tidal fresh to polyhaline waters.

Detailed description of processing of pH, temperature, salinity, and dissolved oxygen data

Data from the beginning of the regular monitoring program in 1985 through 2018 were downloaded from the Chesapeake Bay Program’s Data Hub (see related datasets), with the original intention of producing a 33-year analysis of the air–water CO2 flux. However, a close examination of the pH data led us to question some measurements in the early part of the record. We found a dramatic difference in long-term trends between stations measured by institutions in the state of Virginia and stations measured by the state of Maryland, particularly from late spring to early fall. The boundary between the station groups runs east–west within the mesohaline portion of the bay in segment 5, where the Potomac River estuary intersects the mainstem bay. The boundary separates strong negative linear trends to the south (Virginia stations) from neutral and weakly positive linear trends to the north (Maryland stations). A closer examination of summer data at two stations straddling the boundary between the states (CB5.3 and CB5.4) revealed a distinct separation of the data before 1998 and convergence thereafter. The Virginia measurement protocols changed around this time (Chesapeake Bay Program, 2012). Specifically, Old Dominion University (ODU) took over sampling from the Virginia Institute of Marine Science (VIMS) in 1996. VIMS measured pH by first collecting a water sample and then inserting a pH probe into that sample whereas ODU and Maryland measured pH with an in situ probe. In 1997, ODU switched from using a Hydrolab Surveyor II sonde to a YSI 6000 sonde. It seems likely that the data quality of the Virginia measurements improved greatly after 1997 and we thus chose to limit our analysis to 1998–2018. For this shorter time period, the four parameters were measured exclusively with in situ probes from YSI® and Hydrolab®, and pH was reported on the NBS scale. The specific models used are described in Chesapeake Bay Program (2012) and probe accuracies can be found in the respective user manuals. As an example, the user manual for the YSI 6-series multiparameter water quality sondes provide accuracies of 0.15 ºC for temperature, 1% or 0.1 part per thousand (whichever is greater) for salinity, 0.2 for pH, and 2% or 6.25 mmol m–3 (whichever is greater) for dissolved oxygen measured with a polarographic sensor and half these values for an optical sensor.

Surface time series of pH, temperature, salinity, and dissolved oxygen were subset from the full Chesapeake Bay Program database depth profiles as the shallowest available measurement in the upper 2 m (36% at 0.5 m, 50% at 1 m, and 14% at 2 m). The native temporal resolution of the measurements was preserved in the surface subset. All data flagged with “Problem” by the Chesapeake Bay Program were removed (<0.1% of data for temperature and salinity, 0.6% for pH, and 0.4% for oxygen). The oxygen supersaturation was computed as the oxygen concentration minus the saturation concentration (computed from temperature and salinity, Garcia & Gordon, 1992). Surface values of pH, temperature, salinity, and dissolved oxygen supersaturation were quality controlled by removing gross outliers in three steps. First, pH was converted to hydrogen ion concentration. Second, the interquartile range for each variable for the whole 1998­­–­2018 data set was computed. Third, all data 20 interquartile ranges above or below the 75th and 25thpercentiles, respectively, were removed. pH was the only variable affected by this filtering, resulting in removal of 0.26% of the pH data. pH was subjected to a second round of quality control following the same procedure, but this time on a station-by-station basis using 10 interquartile ranges as the threshold, leading to removal of 0.5 to 3.5% of the pH data from 18 of the 33 stations. 

Because pH is a non-linear function of hydrogen ion, any time the pH data were manipulated in the analyses that follow (e.g., calculation of monthly averages), pH was converted to hydrogen ion concentration, the hydrogen ion data were processed, and the results were converted back to pH. 

The Matlab programming language was used for the subsetting and quality control of input data and for the calculation of pCO2 and the air–water CO2 flux. NetCDF files were created from the native Matlab files.

NSF Award Abstract:

Understanding the vulnerability of estuarine ecosystems to anthropogenic impacts requires a quantitative assessment of the dynamic drivers of change to the estuarine carbonate system. Estuaries are currently experiencing multiple environmental stressors that have significant impacts on their carbonate chemistry, making this assessment a major challenge. Although the effects of changes in nutrient run-off (i.e. eutrophication and hypoxia) have been long-studied in many estuaries, much less attention has been given to the impacts of global change on these systems. In this study, a team of field scientists and modelers will attempt to distinguish natural interannual variability in a major US estuary from the impacts of local anthropogenic changes (e.g., nutrient inputs, changing freshwater end member characteristics) and global change (increases in atmospheric temperature, atmospheric carbon dioxide, and sea level), by using numerical models calibrated with CO2-system observations at appropriate spatial and temporal scales. If successful, this will be the first study to quantitatively distinguish between local and global anthropogenic impacts on the CO2 system in an estuary. The results are expected to have important implications for management of Chesapeake Bay because the impact of local anthropogenic stressors on the system, once isolated, may be mitigated by appropriate environmental policy implemented at the regional scale. Two of the PIs have a strong history of proven relationships with Chesapeake Bay managers and policy makers, which will insure direct infusion of these scientific results into ongoing management decisions.

In this project researchers will study the diurnal, seasonal, and interannual variability of the CO2 system in the Chesapeake Bay, a non-pristine estuary, using a combination of conventional shipboard sampling (of dissolved inorganic carbon, and alkalinity) and new high-frequency autonomous instrumentation (for observations of pH and CO2 partial pressure) to assess the impact of extreme events, like tropical storms and nor?easters on carbonate chemistry. These high-quality observations will afford a rigorous assessment of the uncertainty associated with a 30-year water-quality monitoring time series of pH and alkalinity. The team will use an estuarine-carbon-biogeochemical model evaluated and calibrated with the new and long-term observations. Sensitivity experiments will be applied to disentangle multiple impacts on the CO2 system in the estuary over the last 30 years, including increased atmospheric temperature and CO2, sea-level rise, eutrophication due to increases in nutrient run-off, and changing carbonate characteristics of riverine end-members.