Controls on Carbonate System Dynamics in a Coastal Plain Estuary: A Modeling Study

Abstract

The study of acidification in Chesapeake Bay is challenged by the complex spatial and temporal patterns of estuarine carbonate chemistry driven by highly variable freshwater and nutrient inputs. A new module was developed within an existing coupled hydrodynamic‐biogeochemical model to understand the underlying processes controlling variations in the carbonate system. We present a validation of the model against a diversity of field observations, which demonstrated the model's ability to reproduce large‐scale carbonate chemistry dynamics of Chesapeake Bay. Analysis of model results revealed that hypoxia and acidification were observed to cooccur in midbay bottom waters and seasonal cycles in these metrics were regulated by aerobic respiration and vertical mixing. Calcium carbonate dissolution was an important buffering mechanism for pH changes in late summer, leading to stable or slightly higher pH values in this season despite persistent hypoxic conditions. Model results indicate a strong spatial gradient in air‐sea CO2 fluxes, where the heterotrophic upper bay was a strong CO2 source to atmosphere, the mid bay was a net sink with much higher rates of net photosynthesis, and the lower bay was in a balanced condition. Scenario analysis revealed that reductions in riverine nutrient loading will decrease the acid water volume (pH < 7.5) as a consequence of reduced organic matter generation and subsequent respiration, while bay‐wide dissolved inorganic carbon (DIC) increased and pH declined under scenarios of continuous anthropogenic CO2 emission. This analysis underscores the complexity of carbonate system dynamics in a productive coastal plain estuary with large salinity gradients.

Ecosystem Metabolism and Carbon Balance in Chesapeake Bay: A 30‐Year Analysis Using a Coupled Hydrodynamic‐Biogeochemical Model

“…ROMS‐RCA‐CC has been validated against a wide range of observational data in Chesapeake Bay at multiple time and space scales (Li et al, ; Li et al, ; Shen et al, ; Testa et al, ; Zhong & Li, ). We compared 30‐year (1986–2015) simulation results with field observations in the upper, middle, and lower bay for further validation.…”

“…A newly developed carbonate chemistry model was coupled to RCA and directly tracked dissolved inorganic carbon (DIC), total alkalinity (TA), and aragonite CaCO 3 (Shen et al, ). The effects on DIC and TA from photosynthesis and respiration, SR, sulfide oxidation, nitrification and denitrification, and CaCO 3 precipitation and dissolution are all explicitly modeled in ROMS‐RCA‐CC (Shen et al, ). Other components of the carbonate chemistry system including pH, [CO 2 ], [HCO 3 − ], [CO 3 2− ], and p CO 2 were calculated with the CO2SYS program (Lewis & Wallace, ) diagnostically based upon the RCA‐CC output (DIC, TA, temperature, salinity, etc.).…”

“…Other components of the carbonate chemistry system including pH, [CO 2 ], [HCO 3 − ], [CO 3 2− ], and p CO 2 were calculated with the CO2SYS program (Lewis & Wallace, ) diagnostically based upon the RCA‐CC output (DIC, TA, temperature, salinity, etc.). A detailed description of the CC model and its validation is presented in Shen et al ().…”

Seasonal Variability of the CO2 System in a Large Coastal Plain Estuary

“…Recurring hypoxia in the deep channel of the Chesapeake Bay (CB) mainstem is stimulated by eutrophication and the resulting production and respiration of excess organic matter (Hagy et al, ; Harding et al, ; Harding et al, ; Zimmerman & Canuel, ). If the organic matter generated by phytoplankton growth is not consumed locally or laterally exported from the region, it sinks and subsequent remineralization consumes dissolved oxygen (O 2 ) and leads to elevated concentrations of dissolved inorganic carbon (DIC) at depth (e.g., Cai et al, , ; Shen, Testa, Li, et al, ; Shen, Testa, Ni, et al, ). Reduced vertical exchange during periods of seasonal stratification prevents the ventilation of high‐DIC and low‐O 2 waters at depth and the accumulation of DIC at depth may exacerbate coastal acidification in eutrophic estuarine systems.…”

“…Here we present new observations spanning four seasons (Autumn 2016 to Summer 2017) to diagnose the spatiotemporal variability of the CO 2 system in the main stem of the CB, the largest estuary in the continental United States. This study builds on a growing body of work focused on the seasonality of the CO 2 system in the region (e.g., Brodeur et al, ; Shadwick, Friedrichs, et al, ; Shen, Testa, Li, et al, ). New shipboard observations are used to partition the seasonality of DIC into physical and biological drivers.…”