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Nutrient management improves hypoxia in the Chesapeake Bay despite record-breaking precipitation and warming

Posted by mmaheigan

· Friday, August 26th, 2022

Hypoxia is currently one of the greatest threats to coastal and estuarine ecosystems around the world, and this threat is projected to get worse as waters warm and human populations continue to increase. Over the past 35-years, a massive effort has been underway to decrease hypoxia in the Chesapeake Bay by reducing nutrient input from land. Despite this effort, record-breaking precipitation in 2018-2019 fueled particularly large hypoxic volumes in the Bay, calling into question the efficacy of management actions.

Figure 1. The number of days of additional hypoxia (O2 < 3 mg L^-1) that would have occurred in the Chesapeake Bay if the 35 years of nutrient reductions never occurred, as calculated by differences between a realistic numerical model simulation and one with 1985 nitrogen levels. This management effort has had the greatest impact at the northern and southern edges of the hypoxia in the Bay, where there would have been an additional 60-90 days of O2 < 3 mg L^-1 if nutrient reductions never occurred.

In a recent paper published in Science of the Total Environment, researchers used empirical and numerical modeling to quantify the impact of nutrient management efforts on hypoxia in the Chesapeake Bay. Results suggest that if the nutrient reduction efforts beginning in 1985 had not taken place, hypoxia 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 management impact was most pronounced in regions of the Bay where the hypoxia season would have been 60-90 days longer if nutrient reductions did not occur (Figure 1).

Although these results suggest that management has reduced hypoxic conditions in the Bay, additional analysis revealed that warming temperatures have already offset 6-34% of this improvement. This highlights the importance of factoring in climate change when setting future management goals.

Figure 2. The number of days of additional hypoxia (O2 < 3 mg L^-1) that would have occurred in the Chesapeake Bay if the 35 years of nutrient reductions never occurred, as calculated by differences between a realistic numerical model simulation and one with 1985 nitrogen levels. This management effort has had the greatest impact at the northern and southern edges of the hypoxia in the Bay, where there would have been an additional 60-90 days of O2 < 3 mg L^-1 if nutrient reductions never occurred.

Authors:
Luke T. Frankel (Virginia Institute of Marine Science, William & Mary)
Marjorie A. M. Friedrichs (Virginia Institute of Marine Science, William & Mary)
Pierre St-Laurent (Virginia Institute of Marine Science, William & Mary)
Aaron J. Bever (Anchor QEA)
Romuald N. Lipcius (Virginia Institute of Marine Science, William & Mary)
Gopal Bhatt (Pennsylvania State University; Chesapeake Bay Program)
Gary W. Shenk (USGS; Chesapeake Bay Program)

Carbon fluxes in the coastal ocean: Synthesis, boundary processes and future trends

Posted by mmaheigan

· Friday, August 26th, 2022

A vital part of mitigating climate change is the coastal and open ocean carbon sink, without this, it is not possible to meet the target set by the Paris Agreement. More research is needed to better understand the ocean carbon cycle and its future role in the uptake of anthropogenic carbon. A review provides an analysis of the current qualitative and quantitative understanding of the coastal ocean carbon cycle at regional to global scales, with a focus on the air-sea CO₂ exchange. It includes novel findings obtained using the full breadth of methodological approaches, from observation-based studies and advanced statistical methods to conceptual and theoretical frameworks, and numerical modeling.

Figure 1: Updated sea-air CO₂ flux density (mol C m⁻² year⁻¹) in the global coastal oceans that reveals that the global coastal ocean is an integrated CO₂ sink with the strongest CO₂ uptake at high latitudes. The challenges associated with identifying current and projected responses of the coastal ocean and it source/sink role in the global carbon budget require observational networks that are coordinated and integrated with modeling programs; development of this capability is a priority for the ocean carbon research and management communities.

Based on a new quantitative synthesis of air-sea CO₂ exchange, this study yields an estimate for the globally integrated coastal ocean CO₂ flux of −0.25 ± 0.05 Pg C year⁻¹, with polar and subpolar regions accounting for most of the CO₂ removal (>90%). A framework that classifies river-dominated ocean margin (RiOMar) and ocean-dominated margin (OceMar) systems is used in to conceptualize coastal carbon cycle processes. Ocean carbon models are reviewed in terms of the ability to simulate key processes and project future changes in different continental shelf regions. Concurrent trends and changes in the land-ocean-atmosphere coupled system introduce large uncertainties into projections of ocean carbon fluxes, in particular into defining the role of the coastal carbon sink and its evolution, both of which are of fundamental importance to climate science and climate policies developed before and after achievement of net-zero CO₂ emissions. The major gaps and challenges identified for current coastal ocean carbon research have important implications for climate and sustainability policies. This study is a contribution to the Regional Carbon Cycle Assessment and Processes Phase 2 supported by the Global Carbon Project.

Authors:
M. H. Dai, J. Z. Su, Y. Y. Z., E. E. Hofmann, Z. M. Cao, W.-J. Cai, J. P. Gan, F. Lacroix, G. G. Laruelle, F. F. Meng, J. D. Müller, P. A.G. Regnier, G. Z. Wang, and Z. X. Wang

What drives decadal changes in the Chesapeake Bay carbonate system?

Posted by mmaheigan

· Tuesday, May 3rd, 2022

Understanding decadal changes in the coastal carbonate system (CO₂-system) is essential for predicting how the health of these waters is affected by anthropogenic drivers, such as changing atmospheric conditions and terrestrial inputs. However, studies that quantify the relative impacts of these drivers are lacking.

A recent study in Journal of Geophysical Research: Oceans identified the primary drivers of acidification in the Chesapeake Bay over the past three decades. The authors used a three-dimensional hydrodynamic-biogeochemistry model to quantify the relative impacts on the Bay CO₂-system from increases in atmospheric CO₂, temperature, oceanic dissolved inorganic carbon (DIC) concentrations, terrestrial loadings of total alkalinity (TA) and DIC, as well as decreases in terrestrial nutrient inputs. Decadal changes in the surface CO₂-system in the Chesapeake Bay exhibit large spatial and seasonal variability due to the combination of influences from the land, ocean and atmosphere. In the upper Bay, increased riverine TA and DIC from the Susquehanna River have increased surface pH, with other drivers only contributing to decadal changes that are one to two orders of magnitude smaller. In the mid- and lower Bay, higher atmospheric CO₂ concentrations and reduced nutrient loading are the two most critical drivers and have nearly equally reduced surface pH in the summer. These decadal changes in surface pH show significant seasonal variability with the greatest magnitude generally aligning with the spring and summer shellfish production season (Figure 1).

Figure 1: Overall changes in modeled surface pH (ΔpH_all) due to all global and terrestrial drivers combined over the past 30 years (i.e., 2015–2019 relative to 1985–1989). ΔpH_all includes changes in surface pH due to increased atmospheric CO2, increased atmospheric thermal forcing, increased oceanic dissolved inorganic carbon concentrations, decreased riverine nitrate concentrations, decreased riverine organic nitrogen concentrations, and increased riverine total alkalinity and dissolved inorganic carbon concentrations.

These results indicate that a number of global and terrestrial drivers play crucial roles in coastal acidification. The combined effects of the examined drivers suggest that calcifying organisms in coastal surface waters are likely facing faster decreasing rates of pH than those in open ocean ecosystems. Decreases in surface pH associated with nutrient reductions highlight that the Chesapeake Bay ecosystem is returning to a more natural condition, e.g., a condition when anthropogenic nutrient input from the watershed was lower. However, increased atmospheric CO₂ is simultaneously accelerating the rate of change in pH, exerting increased stress on estuarine calcifying organisms. For ecosystems such as the Chesapeake Bay where nutrient loading is already being managed, controlling the emissions of anthropogenic CO₂ globally becomes increasingly important to decelerate the rate of acidification and to relieve the stress on estuarine calcifying organisms. Future observational and modeling studies are needed to further investigate how the decadal trends in the Chesapeake Bay CO₂-system may vary with depth. These efforts will improve our current understanding of long-term change in coastal carbonate systems and their impacts on the shellfish industry.

Authors:
Fei Da (Virginia Institute of Marine Science, William & Mary, USA)
Marjorie A. M. Friedrichs (Virginia Institute of Marine Science, William & Mary, USA)
Pierre St-Laurent (Virginia Institute of Marine Science, William & Mary, USA)
Elizabeth H. Shadwick (CSIRO Oceans and Atmosphere, Australia)
Raymond G. Najjar (The Pennsylvania State University, USA)
Kyle E. Hinson (Virginia Institute of Marine Science, William & Mary, USA)

Introducing the Coastal Ocean Data Analysis Product in North America (CODAP-NA)

Posted by mmaheigan

· Friday, October 22nd, 2021

Coastal ecosystems are hotspots for commercial and recreational fisheries, and aquaculture industries that are susceptible to change or economic loss due to ocean acidification. These coastal ecosystems support about 90% of the global fisheries yield and 80% of the known marine fish species, and sustain ecosystem services worth $27.7 Trillion globally (a number larger than the U.S. economy). Despite the importance of these areas and economies, internally-consistent data products for water column carbonate and nutrient chemistry data in the coastal ocean—vital to understand and predict changes in these systems—currently do not exist. A recent study published in Earth Syst. Sci. Data compiled and quality controlled discrete sampling-based data—inorganic carbon, oxygen, and nutrient chemistry, and hydrographic parameters collected from the entire North American ocean margins—to create a data product called the Coastal Ocean Data Analysis Product for North America (CODAP-NA) to fill the gap. This effort will promote future OA research, modeling, and data synthesis in critically important coastal regions to help advance the OA adaptation, mitigation, and planning efforts by North American coastal communities; and provides a foothold for future synthesis efforts in the coastal environment.

Figure caption. Sampling stations of the CODAP-NA data product.

Authors:
Li-Qing Jiang (University of Maryland; NOAA NCEI)
Richard A. Feely (NOAA PMEL)
Rik Wanninkhof (NOAA AOML)
Dana Greeley (NOAA PMEL)
Leticia Barbero (University of Miami; NOAA AOML)
Simone Alin (NOAA PMEL)
Brendan R. Carter (University of Washington; NOAA PMEL)
Denis Pierrot (NOAA AOML)
Charles Featherstone (NOAA AOML)
James Hooper (University of Miami; NOAA AOML)
Chris Melrose (NOAA NEFSC)
Natalie Monacci (University of Alaska Fairbanks)
Jonathan Sharp (University of Washington; NOAA PMEL)
Shawn Shellito (University of New Hampshire)
Yuan-Yuan Xu (University of Miami; NOAA AOML)
Alex Kozyr (University of Maryland; NOAA NCEI)
Robert H. Byrne (University of South Florida)
Wei-Jun Cai (University of Delaware)
Jessica Cross (NOAA PMEL)
Gregory C. Johnson (NOAA PMEL)
Burke Hales (Oregon State University)
Chris Langdon (University of Miami)
Jeremy Mathis (Georgetown University)
Joe Salisbury (University of New Hampshire)
David W. Townsend (University of Maine)

Extreme events are accelerating coastal carbon cycling

Posted by mmaheigan

· Monday, March 1st, 2021

The world is getting stormier, and recent evidence shows significant impacts on coastal carbon cycling. The upticks in extreme weather events such as tropical cyclones have resulted in enhanced delivery of nutrients and organic matter across the land-ocean continuum. Lagoonal estuaries such as the Albemarle-Pamlico Sound (APS) in North Carolina and Galveston Bay in Texas are key coastal environments in which we can observe the long-term carbon cycling consequences of these events. Residence times of these coastal environments are on the order of months to over a year, providing ample opportunity for biogeochemical processing. Emerging from studies of Atlantic and Gulf of Mexico hurricanes in 2016 and 2017 is a clear example of the role of terrestrial dissolved organic carbon (DOC) as a key reactant driving the observed carbon cycling and ecosystem effects ( Figure 1).

Figure. 1. The impact of hurricanes on CO₂ fluxes (top) and terrestrial DOC decay constants (bottom) demonstrate the sustained effect on the coastal carbon cycle caused by extreme weather events. Top panel shows results from Hurricane Matthew in 2016, where date is month and day and Km downstream represents observations taken along the main axis of the Neuse River Estuary and lower Pamlico Sound, eastern North Carolina. F_CO2 is the daily sea-to-air flux of CO₂ estimated from measurements of temperature, salinity, dissolved inorganic carbon, and wind speed. The results indicate the Sound existed as a weak yet sustained CO2 source to the atmosphere well after the storm. Outgassing of CO₂ is driven by the rapid mineralization of terrestrial DOC. Bottom panel shows the high bioreactivity of flood-derived terrestrial DOC indicated by elevated microbial decay constants for Galveston Bay and the coastal Gulf of Mexico in 2017 as compared to high and low latitude coastal environments.

In coastal North Carolina, 36 tropical cyclones (TCs), including three floods of historical significance in the past two decades, have occurred in the past 20 years. The lingering effects of these storms include extensive periods of carbon dioxide (CO₂) supersaturation. For example, Hurricane Matthew in 2016 caused the lower Pamlico Sound to emit CO₂ for months after the passage of the storm. With similar results documented for the Pamlico Sound for storms in 2011 and 2012, there is solid evidence that shifts in the ecosystem state of this mesotrophic estuary from net autotrophic to net heterotrophic are a major effect of this process.

Reactive DOC from the landscape appears to be driving the shift in ecosystem state. Large plumes of brown-colored DOC are observable from space in numerous satellite images of the Atlantic and Gulf coasts following these storms. The color is part of a phenomenon known as “coastal darkening"—spectroscopic, stable isotopic, and biomarker evidence show this darkening is related to the flushing of wetlands in the flood-plain adjacent to the rivers draining into these estuaries.

Along the Texas coast, Hurricane Harvey produced the largest rainfall event recorded in US history and caused extensive flooding in 2017. Similar to results from coastal North Carolina, flood-derived terrestrial DOC in Galveston Bay exhibited high bioreactivity, with decay constants exceeding those observed for terrestrial DOC across coastal environments from high and low latitudes by almost three-fold. The rapid processing of terrestrial DOC was linked to an active microbial community capable of decomposing aromatic compounds that are abundant in colored DOC as indicated by genomic analyses. These recent studies clearly demonstrate the impacts of large storm events on coastal carbon cycling via the transport of reactive terrestrial DOC into coastal waters. Climate-driven increases in the frequency and intensity of such storm events warrant more sustained capacity to monitor episodic deliveries of carbon and nutrients and their impacts on coastal marine ecosystems.

Authors:
Chris Osburn (North Carolina State University) @closburn
Hans Paerl (University of North Carolina, Institute of Marine Sciences)
Ge Yan (Institute of Deep-Sea Science and Engineering, Chinese Academy of Sciences)
Karl Kaiser (Texas A&M University, Galveston Campus)

Citations:

Yan, G., Labonté, J. M., Quigg, A., & Kaiser, K. (2020). Hurricanes accelerate dissolved organic carbon cycling in coastal ecosystems. Frontiers in Marine Science, 7, 248.

Osburn, C. L., Rudolph, J. C., Paerl, H. W., Hounshell, A. G., & Van Dam, B. R. (2019). Lingering carbon cycle effects of Hurricane Matthew in North Carolina's coastal waters. Geophysical Research Letters, 46(5), 2654-2661.

Counterintuitive effects of shoreline armoring on estuarine water clarity

Posted by mmaheigan

· Wednesday, February 24th, 2021

Around the world, human-altered shorelines change sediment inputs to estuaries and coastal waters, altering water clarity, especially in areas of dense human population. The Chesapeake Bay estuary is recovering from historically high nutrient and sediment inputs, but water clarity improvement has been ambiguous. Long-term trends show increasing water clarity in terms of deepening light attenuation depth, yet degrading clarity in terms of shallowing Secchi depth over time. High water clarity is needed to support seagrass meadows, which act as nursery habitats for commercially important fish species such as striped bass. How are these opposing water clarity trends possible?

In a recent paper published in Science of the Total Environment, researchers performed experiments with a coupled hydrodynamic-biogeochemical model to test a simulated Chesapeake Bay under realistic conditions, more shoreline erosion, and highly armored shorelines. Comparing the two extreme conditions (Figure 1), there was a striking difference between (a) an estuary experiencing more shoreline erosion and greater resuspension versus (b) a highly armored estuary with decreased resuspension. Reduced erosion yielded improved water clarity in terms of light attenuation depth, but a shallower Secchi depth (reduced visibility). In estuaries, reducing sediment inputs is often proposed as a strategy for improving water quality. This study shows that, under certain conditions in a productive estuary, reduced sediments can have unintended secondary effects on water clarity due to enhanced production of organic particles. This study also highlights the need to consider other sediment sources in addition to rivers, such as seabed resuspension and shoreline erosion, especially at times and locations of low river input.

Figure 1. Schematic of how shoreline armoring causes deepening light attenuation depth (navy) yet shallowing Secchi depth (green) during the spring growing season in the mid-bay central channel.

Authors:
Jessica S. Turner
Pierre St-Laurent
Marjorie A. M. Friedrichs
Carl T. Friedrichs
(all Virginia Institute of Marine Science)

Wildfire impacts on coastal ocean phytoplankton

Posted by mmaheigan

· Wednesday, February 24th, 2021

Wildfire frequency, size, and destructiveness has increased over the last two decades, particularly in coastal regions such as Australia, Brazil, and the western United States. While the impact of fire on land, plants, and people is well documented, very few studies have been able to evaluate the impact of fires on ocean ecosystems. A serendipitously planned research cruise one week after the Thomas Fire broke out in California in December 2017 allowed the authors of this study and their colleagues to sample the adjacent Santa Barbara Channel during this devastating extreme fire event.

In a recent paper published in Journal of Geophysical Research: Oceans, the authors describe the phytoplankton community in the Santa Barbara Channel during the Thomas Fire. Phytoplankton community composition was described using a combination of images of phytoplankton from the Imaging FlowCytobot (McLane Labs) and phytoplankton pigments. Dinoflagellates were the dominant phytoplankton group in the surface ocean during the Thomas Fire, according to both methods (Figure 1).

Figure 1. (A) The fraction of total particle volume imaged by the Imaging FlowCytobot (IFCB) comprised of phytoplankton (green) and detritus (brown). Example IFCB images of ash (counted as part of detritus) particles are outlined in brown. (B) The phytoplankton fraction is then further divided by taxonomy, showing the abundance of nano-sized phytoplankton and especially dinoflagellates during the week of sampling. Example IFCB images of Gonyaulax (outlined in dark green), Prorocentrum (outlined in light green), and Umbilicosphaera (outlined in purple) cells are also shown.

While this study was not able to demonstrate a causal relationship between the Thomas Fire and the presence of dinoflagellates, this result is quite different from previous winters in the Santa Barbara Channel, when picophytoplankton and diatoms typically dominate the winter community. The incidence of dinoflagellates in the Santa Barbara Channel in December 2017 was correlated with the warmer-than-average water temperature during this study, which matched observations from other areas along the Central California coast that winter.

At the time this study was conducted, the Thomas Fire was the largest wildfire in California history. Since then, California fires have increased in danger, destruction, and human mortality; the Mendocino Fire complex (summer 2018) and five separate wildfires in summer 2020 exceeded the impacts of the Thomas Fire. With wildfire severity and frequency increasing not only in California but in coastal regions worldwide, this study gives an important first look at the impact of wildfire smoke and ash on oceanic primary productivity and community composition.

Authors:
Sasha Kramer (University of California Santa Barbara)
Kelsey Bisson (Oregon State University)
Alexis Fischer (University of California Santa Cruz)

Ice sheets mobilize trace elements for export downstream

Posted by mmaheigan

· Thursday, January 7th, 2021

Trace elements are essential micronutrients for life in the ocean and also serve as valuable fingerprints of chemical weathering. The behaviour of trace elements in the ocean has gained interest because some of these elements are found at vanishingly low concentrations that limit ecosystem productivity. Despite delivering >2000 km³ yr^-1 of freshwater to the polar oceans, ice sheets have largely been overlooked as major trace element sources. This is partly due to a lack of data on meltwater endmember chemistry beneath and emerging from the Greenland and Antarctic ice sheets, which cover 10% of Earth’s land surface area, and partly because meltwaters were previously assumed to be dilute compared to most river waters.

In a study published in PNAS, authors analysed the trace element composition of meltwaters from the Mercer Subglacial Lake, a hydrologically active subglacial lake >1000 m below the surface of the Antarctic Ice Sheet, and a meltwater river emerging from beneath a large outlet glacier of the Greenland Ice Sheet (Leverett Glacier). These subglacial meltwaters (i.e., water travelling along the ice-rock interface beneath an ice mass) contained much higher concentrations of trace elements than anticipated. For example, typically immobile elements like iron and aluminium were observed in the dissolved phase (<0.45 µm) at much higher concentrations than in mean river or open ocean waters (up to 20,900 nM for Fe and 69,100 nM for Al), but exhibited large size fractionation between colloidal/nanoparticulate (0.02 – 0.45 µm) and soluble (<0.02 µm) size fractions (Figure 1). Subglacial physical and biogeochemical weathering processes are thought to mobilize many of these trace elements from the bedrock and sediments beneath ice sheets and export them downstream. Antarctic subglacial meltwaters were more enriched in dissolved trace elements than Greenland Ice Sheet outflow, which is likely due to longer subglacial residence times, lack of dilution from surface meltwater inputs, and differences in underlying sediment geology.

These results indicate that ice sheet systems can mobilize large quantities of trace elements from the land to the ocean and serve as major contributors to regional elemental cycles (e.g., coastal Southern Ocean). In a warming climate with increasing ice sheet runoff, subglacial meltwaters will become an increasingly dynamic source of micronutrients to coastal oceanic ecosystems in the polar regions.

Figure caption: Leverett Glacier (Greenland Ice Sheet) and Mercer Subglacial Lake (Antarctic Ice Sheet) dissolved elemental concentrations (<0.45 µm) normalized to mean non-glacial riverine trace element concentrations (Gaillardet et al., 2014) and major element concentrations (Martin and Meybeck, 1979). Grey regions indicate ±50 % of the riverine mean. Although major elements can be significantly depleted compared to non-glacial rivers, trace elements are commonly similar to or enriched.

Authors:
Jon R. Hawkings (Florida State Univ and German Research Centre for Geosciences)
Mark L. Skidmore (Montana State Univ)
Jemma L. Wadham (Univ of Bristol, UK)
John C. Priscu (Montana State Univ)
Peter L. Morton (Florida State Univ)
Jade E. Hatton (Univ of Bristol, UK)
Christopher B. Gardner (Ohio State Univ)
Tyler J. Kohler (École Polytechnique Fédérale de Lausanne, Switzerland)
Marek Stibal (Charles University, Prague, Czech Republic)
Elizabeth A. Bagshaw (Cardiff Univ, UK)
August Steigmeyer (Montana State Univ)
Joel Barker (Univ of Minnesota)
John E. Dore (Montana State Univ)
W. Berry Lyons (Ohio State Univ)
Martyn Tranter (Univ of Bristol, UK)
Robert G. M. Spencer (Florida State Univ)
SALSA Science Team

Chesapeake Bay acidification partially offset by submerged aquatic vegetation

Posted by mmaheigan

· Wednesday, September 30th, 2020

Ocean acidification is often enhanced by eutrophication and subsequent hypoxia and anoxia in coastal waters, which collectively threaten marine organisms and ecosystems. Acidification is particularly of concern for organisms that form shells and skeletons from calcium carbonate (CaCO₃) such as commercially important shellfish species. Given that CaCO₃ mineral dissolution can increase the total alkalinity of water and neutralize anthropogenic and metabolic CO₂, it is important to include CaCO₃ cycle in the coastal water acidification study. However, very few studies have linked CaCO₃ dissolution to the timing and location of its formation in coastal waters. A recent study from the Chesapeake Bay published in Nature Geoscience reveals the occurrence of a bay-wide pH-buffering mechanism resulting from spatially decoupled CaCO₃ mineral cycling (Figure 1). Photosynthesis by submerged aquatic vegetation at the head of the Bay and in other shallow, nearshore waters can remove nutrient pollution from the Bay, generate very high pH, and elevate the carbonate mineral saturation state (Figure 1). This facilitates the formation of CaCO₃ minerals, which are then transported downstream along with other biologically produced carbonate shells into acidic subsurface waters, where they dissolve. This dissolution of carbonate minerals helps “buffer” the water against pH decreases and even drive pH increases. This finding has great ecological and natural resource management significance, in that coastal nutrient management and reduction via the resurgence of submerged aquatic vegetation can help mitigate low oxygen and acidification stress for these environments and organisms.

Figure 1: Conceptual model of the self-regulated pH-buffering mechanism in the Chesapeake Bay. Calcium carbonate is formed within the high-pH submerged aquatic vegetation beds in shallow waters (top left and upper part of diagram, all Shoals with SAV), where it could be subsequently transported longitudinally and/or laterally into the deep main channel of the mid and lower bay (center) and upon dissolution, increase pH buffering capacity and alleviate coastal acidification (lower section).

Authors:
Jianzhong Su (University of Delaware, Xiamen University)
Wei-Jun Cai (University of Delaware)
Jean Brodeur (University of Delaware)
Baoshan Chen (University of Delaware)
Najid Hussain (University of Delaware)
Yichen Yao (University of Delaware)
Chaoying Ni (University of Delaware)
Jeremy Testa (University of Maryland Center for Environmental Science)
Ming Li (University of Maryland Center for Environmental Science)
Xiaohui Xie (University of Maryland Center for Environmental Science, Second Institute of Oceanography)
Wenfei Ni (University of Maryland Center for Environmental Science)
K. Michael Scaboo (University of Delaware)
Yuanyuan Xu (University of Delaware)
Jeffrey Cornwell (University of Maryland Center for Environmental Science)
Cassie Gurbisz (St. Mary’s College of Maryland)
Michael S. Owens (University of Maryland Center for Environmental Science)
George G. Waldbusser (Oregon State University)
Minhan Dai (Xiamen University)
W. Michael Kemp (University of Maryland Center for Environmental Science)

Estuarine sediment resuspension drives non-local impacts on biogeochemistry

Posted by mmaheigan

· Friday, September 18th, 2020

Sediment processes, including resuspension and transport, affect water quality in estuaries by altering light attenuation, primary productivity, and organic matter remineralization, which then influence oxygen and nitrogen dynamics. In a recent paper published in Estuaries and Coasts, the authors quantified the degree to which sediment resuspension and transport affected estuarine biogeochemistry by implementing a coupled hydrodynamic-sediment transport-biogeochemical model of the Chesapeake Bay. By comparing summertime model runs that either included or neglected seabed resuspension, the study revealed that resuspension increased light attenuation, especially in the northernmost portion of the Bay, which subsequently shifted primary production downstream (Figure 1). Resuspension also increased remineralization in the central Bay, which experienced higher organic matter concentrations due to the downstream shift in primary productivity. When combined with estuarine circulation, these resuspension-induced shifts caused oxygen to increase and ammonium to increase throughout the Bay in the bottom portion of the water column. Averaged over the channel, resuspension decreased oxygen by ~25% and increased ammonium by ~50% for the bottom water column. Changes due to resuspension were of the same order of magnitude as, and generally exceeded, short-term variations within individual summers, as well as interannual variability between wet and dry years. This work highlights the importance of a localized process like sediment resuspension and its capacity to drive biogeochemical variations on larger spatial scales. Documenting the spatiotemporal footprint of these processes is critical for understanding and predicting the response of estuarine and coastal systems to environmental changes, and for informing management efforts.

Figure 1: Schematic of how resuspension affects biogeochemical processes based on HydroBioSed model estimates for Chesapeake Bay.

Authors:
Julia M. Moriarty (University of Colorado Boulder)
Marjorie A. M. Friedrichs (Virginia Institute of Marine Science)
Courtney K. Harris (Virginia Institute of Marine Science)

Also see the Geobites piece “Muddy waters lead to decreased oxygen in Chesapeake Bay” on this publication, by Hadley McIntosh Marcek

Funding for the Ocean Carbon & Biogeochemistry Project Office is provided by the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA). The OCB Project Office is housed at the Woods Hole Oceanographic Institution.

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