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

Frankel, Luke T.; Friedrichs, Marjorie A. M.; St‐Laurent, Pierre; Bever, Aaron J.; Lipcius, Romuald N.; Bhatt, G.; Shenk, Gary W.

doi:10.1016/j.scitotenv.2021.152722

Cited by 28 publications

(41 citation statements)

References 70 publications

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“…Recent findings support the hypothesis that nutrient reductions will improve water quality despite projected climate impacts in both freshwater systems (Wade et al, 2022) and other coastal marine systems (Whitney and Vlahos, 2021;Saraiva et al, 2019a;Wåhlström et al, 2020;Meier et al, 2021;Große et al, 2020;Jarvis et al, 2022). In the Chesapeake Bay, reduced nutrient loading (Zhang et al, 2018;Murphy et al, 2022) has already helped mitigate growing climate change pressures (Frankel et al, 2022), despite rapidly increasing Bay temperatures over the past 30 years (Hinson et al, 2021). Like these prior studies, our findings confirm that management actions will likely produce even greater benefits to O2 in coastal zones strongly affected by terrestrial runoff.…”

Section: Hypoxia Lessened By Impacts Of Management Actionsmentioning

confidence: 63%

“…Model simulations are conducted with ChesROMS-ECB, a fully coupled, three-dimensional, hydrodynamic and Estuarine Carbon Biogeochemistry (ECB) implementation of the Regional Ocean Modeling System (ROMS) developed for the Chesapeake Bay with 20 terrain-following vertical levels and an average horizontal resolution of approximately 1.8 kilometers in the estuary's mainstem (Feng et al, 2015;St-Laurent et al, 2020;Frankel et al, 2022). Two parameter changes were recently made to improve the representation of modeled oxygen: (1) a decrease of the maximum growth rate of phytoplankton, which, following Irby et al (2018), preserves the temperature-dependent linear Q10 described in Lomas et al (2002), and (2) a decrease in the critical bottom shear stress from 0.010 Pa to 0.007 Pa, which increases the resuspension of organic matter and is well within the range of observed shear stresses evaluated by Peterson (1999).…”

Section: Estuarine and Watershed Modeling Tools And Evaluationmentioning

confidence: 99%

“…1). Watershed outputs were mapped to estuarine variables as in Frankel et al (2022), except that a more realistic partitioning of terrestrial organic nitrogen loading into labile and refractory pools was implemented such that the percent refractory organic nitrogen loading increases with discharge at high flow volumes (Appendix A).…”

Section: Estuarine and Watershed Modeling Tools And Evaluationmentioning

confidence: 99%

“…Original partitioning of organic nitrogen pools from the DLEM and Phase 6 watershed models was based on fixed fractions previously described in Frankel et al (2022). There, 80% of the refractory organic nitrogen (rorN) loadings from Phase 6 were allocated to the small detritus nitrogen (SDeN) pool and the remainder was applied to the refractory dissolved organic nitrogen (rDON) pool in ChesROMS-ECB.…”

Section: Appendix Amentioning

confidence: 99%

“…Observed lags in water quality responses to nutrient reductions (Murphy et al, 2022) are also evident in recent years (Zahran et al 2022). Despite the difficulties in assessing long-term improvements in water quality due to strong interannual variability, new research has demonstrated that the Chesapeake Bay is more resilient to recent and ongoing climate change impacts that have decreased oxygen levels as a result of decades of nutrient load reductions (Frankel et al, 2022).…”

Section: Introductionmentioning

confidence: 99%

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Impacts and uncertainties of climate-induced changes in watershed inputs on estuarine hypoxia

Hinson

Friedrichs²,

Najjar³

et al. 2022

Preprint

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Abstract. Multiple climate-driven stressors, including warming and increased nutrient delivery, are exacerbating hypoxia in coastal marine environments. Within coastal watersheds, environmental managers are particularly interested in climate impacts on terrestrial processes, which may undermine the efficacy of management actions designed to reduce eutrophication and consequent low-oxygen conditions in receiving coastal waters. However, substantial uncertainty accompanies the application of Earth System Model (ESM) projections to a regional modeling framework when quantifying future changes to estuarine hypoxia due to climate change. In this study, two downscaling methods are applied to multiple ESMs and used to force two independent watershed models for Chesapeake Bay, a large coastal-plain estuary of the eastern United States. The projected watershed changes are then used to force a coupled 3-D hydrodynamic-biogeochemical estuarine model to project climate impacts on hypoxia, with particular emphasis on projection uncertainties. Results indicate that all three factors (ESM, downscaling method, and watershed model) are found to contribute significantly to the uncertainty associated with future hypoxia, with the choice of ESM being the largest contributor. Overall, in the absence of management actions, there is a high likelihood that climate change impacts on the watershed will expand low-oxygen conditions by 2050, relative to a 1990s baseline period; however, the projected increase in hypoxia is quite small (4 %) because only climate-induced changes in watershed inputs are considered and not those on the estuary itself. Results also demonstrate that the attainment of established nutrient reduction targets will reduce annual hypoxia by about 50 % compared to the 1990s. Given these estimates, it is virtually certain that fully implemented management actions reducing excess nutrient loadings will outweigh hypoxia increases driven by climate-induced changes in terrestrial runoff.

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Section: Hypoxia Lessened By Impacts Of Management Actionsmentioning

confidence: 63%

Section: Estuarine and Watershed Modeling Tools And Evaluationmentioning

confidence: 99%

Section: Estuarine and Watershed Modeling Tools And Evaluationmentioning

confidence: 99%

Section: Appendix Amentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Impacts and uncertainties of climate-induced changes in watershed inputs on estuarine hypoxia

Hinson

Friedrichs²,

Najjar³

et al. 2022

Preprint

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Determination of site‐specific nitrogen cycle reaction kinetics allows accurate simulation of in situ nitrogen transformation rates in a large North American estuary

Tang,

Fortin,

Intrator

et al. 2024

Limnology & Oceanography

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Nitrogen (N) bioavailability affects phytoplankton growth and primary production in the aquatic environment. N bioavailability is partly determined by biological N cycling processes that either transform N species or remove fixed N. Reliable estimates of their kinetic parameters can help understand the distribution of N cycling processes. However, available estimates of kinetic parameters are often derived from microbial isolates and may not be representative of the natural environment. Observations are particularly lacking in estuarine and coastal waters. We conducted isotope tracer addition incubations to evaluate substrate affinities of nitrification, denitrification and anammox in the Chesapeake Bay water column. The half‐saturation constant for ammonia oxidation ranged from 0.38 to 0.75 μM ammonium, substantially higher than observed in the open oceans. Half‐saturation constants for denitrification—0.92–1.86 μM nitrite or 1.15 μM nitrate—were within the lower end or less than those reported for other aquatic environments and for denitrifier isolates. Interestingly, water column denitrification potential was comparable to that of sedimentary denitrification, highlighting the contribution of the water column to N removal during anoxia. Mostly undetectable anammox rates prevented us from deriving the half‐saturation constants, suggesting a low affinity of anammox. Using these substrate kinetics, we were able to predict in situ N cycling rates and explain the vertical distribution of N nutrient concentrations. Our newly derived substrate kinetics parameters can be useful for improving model representation of N nutrient dynamics in estuarine and coastal waters, which is critical for assessing the ecosystem productivity and function.

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Disaster avoided: current state of the Baltic Sea without human intervention to reduce nutrient loads

Ehrnsten,

Humborg,

Gustafsson

et al. 2024

Limnol Oceanogr Letters

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Excessive nutrient inputs have caused eutrophication of coastal ecosystems worldwide, triggering extensive algal blooms, oxygen‐depletion, and collapse of local fisheries. In the Baltic Sea, inputs of nitrogen (N) and phosphorus (P) have been significantly reduced since the 1980s, but the environmental state shows little to no signs of recovery. However, a simulation with continued high loads from the mid‐1980s demonstrates that while the state has not improved yet, it would be considerably worse today without the load reductions (e.g., 82% larger oxygen‐free bottom areas and 104% and 58% higher wintertime concentrations of inorganic N and P, respectively, in the Baltic Proper). Additional simulations with current nutrient loads continuing into the future indicate that conditions will likely improve in the coming decades. This study underscores the significance of acting on early warning signs of eutrophication, and furthermore how sustained efforts to decrease nutrient loads can mitigate the severity of eutrophication.

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

Cited by 28 publications

References 70 publications

Impacts and uncertainties of climate-induced changes in watershed inputs on estuarine hypoxia

Impacts and uncertainties of climate-induced changes in watershed inputs on estuarine hypoxia

Determination of site‐specific nitrogen cycle reaction kinetics allows accurate simulation of in situ nitrogen transformation rates in a large North American estuary

Disaster avoided: current state of the Baltic Sea without human intervention to reduce nutrient loads

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