Archive for estuary

Impacts of atmospheric nitrogen deposition and coastal nitrogen fluxes on oxygen concentrations in Chesapeake Bay

Posted by mmaheigan 
· Tuesday, April 30th, 2019 

How do atmospheric and oceanic nutrients impact oxygen concentrations in the Chesapeake Bay? Generally, researchers focus on how terrestrial nutrients impact hypoxia. The relative importance of river, atmosphere, and ocean inputs have not been quantified, largely because estimates of nitrogen fluxes from the atmosphere and ocean are limited.

A recent study in Journal of Geophysical Research: Oceans quantified the relative impacts of atmospheric and oceanic nitrogen inputs on dissolved oxygen (DO) in the Chesapeake Bay. The authors combined a 3-D biogeochemical model and estimates of atmospheric deposition from the Community Multiscale Air Quality model and interpolations of nitrogen concentrations along the continental shelf from the Ocean Acidification Data Stewardship Project. Atmospheric nitrogen deposition and coastal nitrogen fluxes most impact Chesapeake Bay DO concentrations during the summer when surface waters are depleted in nitrogen. Overall, atmospheric nitrogen deposition has about the same gram-for-gram impact on Chesapeake Bay DO as riverine loading. Although all three nutrient sources vary spatially and temporally, in the central bay, where summer hypoxia is most prevalent, coastal nitrogen fluxes and atmospheric nitrogen fluxes have roughly the same impact on bottom oxygen as a ~10% change in riverine nitrogen loading (Figure 1).

Figure caption: (Left) Four-year (2002–2005) average increase in DO in the summer by removing the atmospheric nitrogen deposition (AtmN), reducing the riverine loading (ΔRiverN) by ~10% (roughly equivalent to turning off the atmospheric deposition), and removing the nitrogen fluxes from the continental shelf (CoastalN). (Right) Relative impacts of the three nitrogen modification scenarios on summertime bottom DO.

These results indicate that two often-neglected sources of nitrogen—direct atmospheric deposition and fluxes of nitrogen from the continental shelf—substantially impact Chesapeake Bay DO, especially in the summer. Future study is needed to investigate the long-term trend of these relative impacts by continued coordination between modeling and observational work, such as applying higher-resolution atmospheric deposition products and integrating more in situ data along the model ocean boundary when they are available. These efforts will improve our understanding of the impacts of different nutrient sources on biogeochemical cycles in coastal water bodies.

 

Authors:
Fei Da (VIMS, College of William & Mary)
Marjorie A. M. Friedrichs (VIMS, College of William & Mary)
Pierre St-Laurent (VIMS, College of William & Mary)

Biological and physical controls on estuarine nitrous oxide emissions

Posted by mmaheigan 
· Tuesday, February 5th, 2019 

Nitrous oxide (N2O) is a potent greenhouse gas with rising atmospheric concentrations. Atmospheric emissions of N2O are predicted to increase with continued anthropogenic perturbation of the nitrogen cycle, yet the magnitude of these emissions is uncertain, particularly in coastal systems where N2O fluxes are poorly constrained. How do N2O emissions from a eutrophic estuary vary in space and time?

Figure 1: Depth profiles of nitrous oxide (N2O) (circles), salinity (dashed line), and dissolved oxygen (solid line) in the Chesapeake Bay at three stations. Solid circles indicate oversaturation of N2O with respect to equilibrium with the atmosphere, and open circles indicate undersaturation.

In a recent publication in Estuaries and Coasts, Laperriere et al. (2018) examined how physical and biological processes influence the distribution of N2O in Chesapeake Bay using dissolved gas measurements (N2O and N2/Ar) and stable isotope tracer incubations. During stratified summer conditions, the mesohaline region of the Chesapeake Bay was always a source of N2O to the atmosphere. The highest N2O concentrations occurred in the pycnocline at the interface between reducing bottom waters and oxygenated surface waters (Figure 1). Vertical mixing of surface waters across the pycnocline caused elevated rates of ammonia oxidation, a biological source of N2O, and resulted in the accumulation of nitrite (NO2–) below the pycnocline. During periods of weak mixing, ammonia oxidation rates and N2O concentrations were lower, and low dissolved oxygen concentrations below the pycnocline set the stage for N2O consumption via denitrification (Figure 1). The interplay between biological and physical processes controlling fluctuations in N2O concentration was examined using a mass balance approach. Mass balance estimates indicated that both biological processes and physical transport contribute to local changes in N2O concentration. The authors suggest that the fate of N2O during stratified summer conditions is governed by vertical mixing across the pycnocline, controlling whether N2O is released to the atmosphere or consumed at depth.

 

Authors:
Sarah M. Laperriere (University of California, Santa Barbara)
Nicholas J. Nidzieko (University of California, Santa Barbara)
Rebecca J. Fox (Washington College)
Alexander W. Fisher (University of California, Santa Barbara)
Alyson E. Santoro (University of California, Santa Barbara)

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