Constructing a model including the cryptic sulfur cycle in Chesapeake Bay requires judicious choices for key processes and parameters.

A new biogeochemical model for Chesapeake Bay has been developed by merging two published models - the ECB model of Da et al. (2018) that has been calibrated for the Bay but only simulates nitrogen, carbon and oxygen and the BioRedoxCNPS model of al Azhar et al. (2014) and Hantsoo et al. (2018) that includes cryptic sulfur cycling. Comparison between these models shows that judicious choices are required for key processes and parameters. This manuscript documents the sources of differences between the two published models in order to select the most realistic configuration for our new model.•This study focuses on three sets of differences-processes only included in ECB (burial and dissolved organic matter), processes only included in BioRedoxCNPS (explicit dynamics for hydrogen sulfide, sulfate and nitrite, light attenuation that does not include CDOM or sediments), and differences in parameters common to the two codes.•Sensitivity studies that highlight particular choices (absorption by dissolved organic matter, nitrification rates, stoichiometric ratios) are also shown.•The new model includes sulfur cycling and has comparable skill in predicting oxygen as ECB, but also has improved simulation of nitrogen species compared with both original codes.

Nitrogen reductions have decreased hypoxia in the Chesapeake Bay: Evidence from empirical and numerical modeling.

Seasonal hypoxia is a characteristic feature of the Chesapeake Bay due to anthropogenic nutrient input from agriculture and urbanization throughout the watershed. Although coordinated management efforts since 1985 have reduced nutrient inputs to the Bay, oxygen concentrations at depth in the summer still frequently fail to meet water quality standards that have been set to protect critical estuarine living resources. To quantify the impact of watershed nitrogen reductions on Bay hypoxia during a recent period including both average discharge and extremely wet years (2016-2019), this study employed both statistical and three-dimensional (3-D) numerical modeling analyses. Numerical model results suggest that if the nitrogen reductions since 1985 had not occurred, annual hypoxic volumes (O2 < 3 mg L-1) would have been ~50-120% greater during the average discharge years of 2016-2017 and ~20-50% greater during the wet years of 2018-2019. The effect was even greater for O2 < 1 mg L-1, where annual volumes would have been ~80-280% greater in 2016-2017 and ~30-100% greater in 2018-2019. These results were supported by statistical analysis of empirical data, though the magnitude of improvement due to nitrogen reductions was greater in the numerical modeling results than in the statistical analysis. This discrepancy is largely accounted for by warming in the Bay that has exacerbated hypoxia and offset roughly 6-34% of the improvement from nitrogen reductions. Although these results may reassure policymakers and stakeholders that their efforts to reduce hypoxia have improved ecosystem health in the Bay, they also indicate that greater reductions are needed to counteract the ever-increasing impacts of climate change.

Effects of reduced shoreline erosion on Chesapeake Bay water clarity.

Shoreline erosion supplies sediments to estuaries and coastal waters, influencing water clarity and primary production. Globally, shoreline erosion sediment inputs are changing with anthropogenic alteration of coastlines in populated regions. Chesapeake Bay, a prime example of such a system where shoreline erosion accounts for a large proportion of sediments entering the estuary, serves here as a case study for investigating the effects of changing sediment inputs on water clarity. Long-term increases in shoreline armoring have contributed to decreased erosional sediment inputs to the estuary, changing the composition of suspended particles in surface waters. This study examined the impact of shoreline erosion on water clarity using a coupled hydrodynamic-biogeochemical model. Experiments were conducted to simulate realistic shoreline conditions representative of the early 2000s, increased shoreline erosion, and highly armored shorelines. Together, reduced shoreline erosion and the corresponding reduced rates of resuspension result in decreased concentrations of inorganic particles, improving water clarity particularly in the lower Bay and in dry years where and when riverine sediment influence is low. This clarity improvement relaxed light limitation, which increased organic matter production. Differences between the two extreme experiments revealed that in the mid-estuary in February to April, surface inorganic suspended sediment concentrations decreased 3-7 mg L-1, while organic suspended solids increased 1-3 mg L-1. The resulting increase in the organic-to-inorganic ratio often had opposite effects on clarity according to different metrics, improving clarity in mid-Bay central channel waters in terms of light attenuation depth, but simultaneously degrading clarity in terms of Secchi depth because the resulting increase in organic suspended solids decreased the water's transparency. This incongruous water clarity effect, the spatial extent of which is defined here as an Organic Fog Zone, was present in February to April in all years studied, but occurred farther south in wet years.

Challenges in Quantifying Air-Water Carbon Dioxide Flux Using Estuarine Water Quality Data: Case Study for Chesapeake Bay.

Estuaries play an uncertain but potentially important role in the global carbon cycle via CO2 outgassing. The uncertainty mainly stems from the paucity of studies that document the full spatial and temporal variability of estuarine surface water partial pressure of carbon dioxide ( pCO2). Here, we explore the potential of utilizing the abundance of pH data from historical water quality monitoring programs to fill the data void via a case study of the mainstem Chesapeake Bay (eastern United States). We calculate pCO2 and the air-water CO2 flux at monthly resolution from 1998 to 2018 from tidal fresh to polyhaline waters, paying special attention to the error estimation. The biggest error is due to the pH measurement error, and errors due to the gas transfer velocity, temporal sampling, the alkalinity mixing model, and the organic alkalinity estimation are 72%, 27%, 15%, and 5%, respectively, of the error due to pH. Seasonal, interannual, and spatial variability in the air-water flux and surface pCO2 is high, and a correlation analysis with oxygen reveals that this variability is driven largely by biological processes. Averaged over 1998-2018, the mainstem bay is a weak net source of CO2 to the atmosphere of 1.2 (1.1, 1.4) mol m-2 yr-1 (best estimate and 95% confidence interval). Our findings suggest that the abundance of historical pH measurements in estuaries around the globe should be mined in order to constrain the large spatial and temporal variability of the CO2 exchange between estuaries and the atmosphere.