Archive for oxygen – Page 2

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

Turning a spotlight on grazing

Posted by mmaheigan 
· Thursday, July 23rd, 2020 

Microscopic plankton in the surface ocean make planet Earth habitable by generating oxygen and forming the basis of marine food webs, yielding harvestable protein. For over 100 years, oceanographers have tried to ascertain the physical, chemical, and biological processes governing phytoplankton blooms. Zooplankton grazing of phytoplankton is the single largest loss process for primary production, but empirical grazing data are sparse and thus poorly constrained in modeling frameworks, including assessments of global elemental cycles, cross-ecosystem comparisons, and predictive efforts anticipating future ocean ecosystem function. As sunlight decays exponentially with depth, upper-ocean mixing creates dynamic light environments with predictable effects on phytoplankton growth but unknown consequences for grazing.

Figure caption: Rates (d−1) of phytoplankton growth (μ), grazing mortality (g), and biomass accumulation (r) under four mixed layer scenarios simulated using light as a proxy of (a) sustained deep mixing, (b) rapid shoaling, (c) sustained shallow mixing, and (d) rapid mixed layer deepening. Error bars represent one standard deviation of the mean of duplicate experiments. Grazing was measured but not detected in the sustained deep mixing and rapid shoaling conditions, denoted with x.

Using data from a spring cruise in the North Atlantic, authors of a recent study published in Limnology & Oceanography compared the influences of microzooplankton predation and fluctuations in light availability—representative of a mixing water column—on phytoplankton standing stock. Data from at-sea incubations and light manipulation experiments provide evidence that phytoplankton’s instantaneous and zooplankton’s delayed responses to light fluctuations are key modulators of the balance between phytoplankton growth and grazing rates (Figure 1). These results suggest that light is a potential, remotely retrievable predictor of when and where in the ocean zooplankton grazing may represent an important loss term of phytoplankton production. If broadly verified, this approach could be used to systematically assess sparsely measured grazing across spatial and temporal gradients in representative regions of the ocean. Such data will be essential for enhancing our predictive capacity of ocean food web function, global biogeochemical cycles and the many derived processes, including fisheries production and the flow of carbon through the oceans.

Authors:
Françoise Morison (University of Rhode Island)
Gayantonia Franzè (University of Rhode Island, currently Institute of Marine Research, Norway)
Elizabeth Harvey (University of Georgia, currently University of New Hampshire)
Susanne Menden-Deuer (University of Rhode Island)

 

Modern OMZ copepod dynamics provide analog for future oceans

Posted by mmaheigan 
· Thursday, July 23rd, 2020 

Global warming increases ocean deoxygenation and expands the oxygen minimum zone (OMZ), which has implications for major zooplankton groups like copepods. Reduced oxygen levels may impact individual copepod species abundance, vertical distribution, and life history strategy, which is likely to perturb intricate oceanic food webs and export processes. In a study recently published in Biogeosciences, authors conducted vertically-stratified day and night MOCNESS tows (0-1000 m) during four cruises (2007-2017) in the Eastern Tropical North Pacific, sampling hydrography and copepod distributions in four locations with different water column oxygen profiles and OMZ intensity (i.e. lowest oxygen concentration and its vertical extent in a profile). Each copepod species exhibited a different vertical distribution strategy and physiology associated with oxygen profile variability. The study identified sets of species that (1) changed their vertical distributions and maximum abundance depth associated with the depth and intensity of the OMZ and its oxycline inflection points, (2) shifted their diapause depth, (3) adjusted their diel vertical migration, especially the nighttime upper depth, or (4) expanded or contracted their depth range within the mixed layer and upper part of the thermocline in association with the thickness of the aerobic epipelagic zone (habitat compression concept) (Figure 1). Distribution depths for some species shifted by 10’s to 100’s of meters in different situations, which also had metabolic (and carbon flow) implications because temperature decreased with depth.  This observed present-day variability may provide an important window into how future marine ecosystems will respond to deoxygenation.

Figure caption: Schematic diagram showing how future OMZ expansion may affect zooplankton distributions, based on present-day responses to OMZ variability. The dashed line indicates diel vertical migration (DVM) and highlights the shoaling of the nighttime depth as the aerobic habitat is compressed. The lower oxycline community and the diapause layer for some species, associated with a specific oxygen concentration, may deepen as the OMZ expands.

 

Authors:
Karen F. Wishner (University of Rhode Island)
Brad Seibel (University of South Florida)
Dawn Outram (University of Rhode Island)

The competing impacts of climate change and nutrient reductions on dissolved oxygen in Chesapeake Bay

Posted by mmaheigan 
· Wednesday, June 12th, 2019 

The Chesapeake Bay is a 200-mile-long estuary with both economic and ecological importance to the mid-Atlantic region. Runoff, pollution, and algae blooms resulting in hypoxia have been major issues over the past 50 years, and much work has been done to improve the water quality and health of the Bay. Dissolved oxygen concentrations will be altered in response to climate change, but whether this will counteract the benefits of reduced nutrient loading is an important scientific and management question. Specifically, what are the impacts of climate change on future Chesapeake Bay hypoxia and on progress towards meeting water quality standards associated with the Chesapeake Bay Total Maximum Daily Load (TMDL)?

(Left) Latitudinal along-bay dissolved oxygen (DO) transects for the Base scenario (Base+noCC) and TMDL scenario (TMDL+noCC) without climate change; transects for the absolute and percent changes in DO due to climate change (TMDL+CC). (Right) Cumulative hypoxic volume for six ranges of DO concentrations for each of the study years and each of the scenarios (colored circles).

A recent study in Biogeosciences quantified the competing impacts of climate change and nutrient reductions on Chesapeake Bay hypoxia. The authors used a 3-D modeling system along with projected mid-21st century changes in temperature, freshwater flow, and sea level, assuming fully achieved goals of TMDL nutrient reductions. Of these three climate change factors, increased temperature most strongly impacts future hypoxia, primarily due to decreased solubility year-round and increased respiration and remineralization in the spring. Sea level rise is expected to exhibit a small positive impact resulting from increased estuarine circulation and reduced residence time. Increased river flow is anticipated to exert a small negative impact due to increased nutrient loading.

These results demonstrate that climate change may limit the effectiveness of future management actions aimed at reducing nutrient inputs to the Chesapeake Bay. However, the positive impacts of mandated nutrient reductions still outweigh the negative impacts of climate change. Given that climate impacts are expected to intensify with time and large uncertainties remain among different climate projections, it is critical to continue examining how the Bay may evolve in the future by assessing the sensitivity of oxygen concentrations to different climate change scenarios.

 

Authors:
Isaac D. Irby (VIMS, William & Mary)
Marjorie A. M. Friedrichs (VIMS, William & Mary)
Fei Da (VIMS, William & Mary)
Kyle E. Hinson (VIMS, William & Mary)

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)

Long-term coastal data sets reveal unifying relationship between oxygen and pH fluctuations

Posted by mmaheigan 
· Thursday, June 7th, 2018 

Coastal habitats are critically important to humans, but without consistent and reliable observations we cannot understand the direction and magnitude of unfolding changes in these habitats. Environmental monitoring is therefore a prescient—yet still undervalued—societal service, and no effort better exemplifies this than the work conducted within the National Estuarine Research Reserve System (NERRS). NERRS is a network of 29 U.S. estuarine sites operated as a partnership between NOAA and the coastal states. NERRS has established a system-wide monitoring program with standardized instrumentation, protocols, and data reporting to guide consistent and comparable data collection across all NERRS sites. This has resulted in high-quality, comparable data on short- to long-term changes in water quality and biological systems to inform effective coastal zone management.

Figure 1: Using dissolved oxygen and salinity, monthly mean pH can be predicted within and across coastal systems due to the unifying metabolic coupling of oxygen and pH.

 

In a recent study published in Estuaries and Coasts, Baumann and Smith (2017) used a subset of this unique data set to analyze short- and long-term variability in pH and dissolved oxygen (DO) at 16 NERRS sites across the U.S. Atlantic, Caribbean, Gulf of Mexico, and Pacific coasts (> 5 million data points). They observed that large, metabolically driven fluctuations of pH and DO are indeed a unifying feature of nearshore habitats. Furthermore, mean pH or mean diel pH fluctuations can be predicted across habitats simply from salinity and oxygen levels/fluctuations (Fig.1). These results provide strong empirical evidence that common metabolic principles drive diel to seasonal pH and DO variations within and across a diversity of estuarine environments. As expected, the study did not yield interannual, monotonic trends in nearshore pH conditions; rather, interannual fluctuations were of similar magnitude to the pH decrease predicted for the average surface ocean over the next three centuries (Fig.2). Correlations of weekly anomalies of pH, oxygen, and temperature yielded strong empirical support for the hypothesis that coastal acidification—in addition to being driven by eutrophication and atmospheric CO2 increases—is exacerbated by warming, likely via increased community respiration.

Figure 2: Interannual variations in temperature, pH, and dissolved oxygen (DO) anomalies in 16 NERRS sites across the US Atlantic, Gulf of Mexico, Caribbean, and Pacific coasts.

Analyses of these long-term data sets have provided important insights on biogeochemical variability and underlying drivers in nearshore environments, highlighting the value and utility of long-term monitoring efforts like NERRS. Sustained, high-quality data sets in these nearshore environments are essential for the study of environmental change and should be prioritized by funding agencies. The observed metabolically driven pH and DO fluctuations suggest that local measures to reduce nutrient pollution can be an effective management tool in support of healthy coastal environments, a boon for both the habitats and humans.

 

Authors:
Hannes Baumann (University of Connecticut)
Erik M. Smith (North Inlet-Winyah Bay National Estuarine Research Reserve, University of South Carolina)

Hotspots of biological production: Submesoscale changes in respiration and production

Posted by mmaheigan 
· Thursday, April 26th, 2018 

The biological pump is complex and variable. To better understand it, scientists have often focused on variations in biological parameters such as fluorescence and community structure, and have less often observed variations in rates of production. Production rates can be estimated using oxygen as a tracer, since photosynthesis produces oxygen and respiration consumes it. In a recent article in Deep Sea Research Part I, the authors presented high-resolution maps of oxygen in the upper 140 m of the ocean in the subtropical and tropical Atlantic, produced from a towed undulating instrument. This provides a synoptic, high-resolution view of oxygen anomalies in the surface ocean. These data reveal remarkable hotspots of biological production and respiration co-located with areas of elevated fluorescence. These hotspots are often several kilometers wide (horizontal) and ~10 m long (vertical). They are preferentially associated with edges of eddies, but not all edges sampled contained hotspots. Although this study captures only two-dimensional glimpses of these hotspots, precluding formal calculations of production rates, likely estimates of source water suggest that many of these hotspots may actually be areas of enhanced respiration rather than enhanced photosynthesis. The paper describes a conceptual model of nutrients, new production, respiration, fluorescence, and oxygen during the formation and decline of these hotspots. These data raise intriguing questions–if the hotspots do indeed have substantially different rates of production and respiration than surrounding waters, then they could lead to significant changes in estimates of production in the upper ocean. Additionally, understanding the mechanisms that produce these hotspots could be critical for predicting the effects of climate change on the magnitude of the biological pump.

(a) Oxygen concentrations and (b) fluorescence at ~1 km resolution over 300 km from 15.13°N, 57.47°W to 12.30°N, 56.42° W, as measured by sensors attached to the (c) Video Plankton Recorder II. Note that no contouring was used for this plot – every pixel represents an actual data point. Figure modified from Stanley et al., 2017. VPR image photograph by Phil Alatalo.

Authors:
Rachel H. R. Stanley (Wellesley College)
Dennis J. McGillicuddy Jr. (WHOI)
Zoe O. Sandwith (WHOI)
Haley Pleskow (Wellesley College)

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