Cross-shelf exchange associated with a shelf-water streamer at the Mid-Atlantic Bight shelf edge

Zhang, Weifeng; Alatalo, Philip; Crockford, Taylor; Hirzel, Andrew J.; Meyer, Meredith G.; Oliver, H. R.; Peacock, Emily E.; Petitpas, Christian M.; Sandwith, Zoe O.; Smith, Walker O; Sosik, Heidi M.; Stanley, Rachel H. R.; Stevens, Bethany L. F.; Turner, Jefferson T.; McGillicuddy, Dennis J.

doi:10.1016/j.pocean.2022.102931

Cited by 16 publications

(25 citation statements)

References 53 publications

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“…May 2019 mean frontal position and structure were most similar to spring climatological means (Zhang et al, 2011) (Figure 3.3 middle column). Mean May chlorophyll and nutrient distributions (Figure 3.2, top middle) were similar to climatological distributions (Zhang et al, 2013), indicating that the features observed in this study reflect typical conditions.…”

Section: Climatological Comparisonmentioning

confidence: 63%

“…At other times of year, when nutrients are depleted in the euphotic zone, chlorophyll enhancement can potentially result from several mechanisms of upwelling at the front. Multiple upwelling mechanisms can potentially support frontal chlorophyll enhancement, including bottom boundary layer convergence and detachment (e.g., Gawarkiewicz and Chapman, 1992;Houghton and Visbeck, 1998;Houghton et al, 2006), upwelling of waters from the offshore interior (Zhang et al, 2011), and upwelling driven by instabilities within the shelf-break front (Zhang and Gawarkiewicz, 2015). Upwelling provided by these mechanisms ranges from tens of cm d -1 (Zhang et al, 2011) to tens of m d -1 (Zhang and Gawarkiewicz, 2015) depending on the mechanism.…”

Section: Introductionmentioning

confidence: 99%

“…The high variability associated with the shelf-break front may also obfuscate trends of chlorophyll enhancement. The dynamics of the frontal region have been described by multiple climatologies (Linder and Gawarkiewicz, 1998;Loder et al, 2001;Zhang et al, 2011), all of which show that the physical, biological, and chemical attributes of the shelf-break front vary significantly in both time and space. These variabilities can be partially attributed to the plethora of both internal and external forcings acting upon the shelf-break front, ranging from frontal meanders (Pickart, 1999) and frontally generated submesoscale eddies (Gawarkiewicz et al, 2001) to Gulf Stream warm core rings (Ryan et al, 2001) and, in rare instances, meanders of the Gulf Stream (Gawarkiewicz et al, 2018).…”

Section: Introductionmentioning

confidence: 99%

“…Within the MAB, cold and fresh water on the shelf is separated from offshore warm and salty water by a shelf-break front. Climatological conditions of the frontal region have been described by multiple studies (Linder and Gawarkiewicz, 1998;Loder et al, 2001;Zhang et al, 2011), all of which show that the physical, biological, and chemical attributes of the shelf-break front vary significantly in both time and space. Such variabilities may come from internal or external forcing and occur over broad and fine spatial scales.…”

Section: Introductionmentioning

confidence: 99%

“…A second proposed mechanism is upwelling of waters from the offshore interior. From a climatologically based 2-D model, Zhang et al (2011) showed that mean flow within the surface and bottom boundary layers is predominantly offshore, due to the influence of mean wind stress and bottom Ekman layer dynamics respectively. These offshore flows are balanced by interior onshore flow parallel to the seafloor, causing mean upward motion of offshore waters with speeds of tens of cm d -1 to a few m d -1 depending on the season (Zhang et al, 2011).…”

Section: Introductionmentioning

confidence: 99%

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Physical and biological processes at the Middle Atlantic Bight shelf-break front

Hirzel¹

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The Middle Atlantic Bight (MAB) is a highly productive ecosystem, supporting several economically important commercial fisheries. Chlorophyll enhancement at the MAB shelf-break front has been observed only intermittently, despite numerous studies that suggest persistent upwelling at the front. High resolution cross-frontal transect crossings were collected from three two-week cruises in April 2018, May 2019, and July 2019. Chapter 2 focused on applying a novel method of classifying planktonic images taken by a Video Plankton Recorder to enable processing of the large volumes of data collected with the instrument. Chapter 3 investigated cross-frontal trends by temporally averaging in both Eulerian and frontally-aligned coordinates. For April 2018, transient chlorophyll enhancement was seen at the front in individual transects and within the frontally-aligned mean transect, but not within the Eulerian mean transect. The Eulerian mean for May 2019 showed chlorophyll enhancement as a result of frontal eddies, which were further explored in chapter 4. No frontal enhancement was observed in July 2019. The frontal eddies observed in May 2019 were simulated using an idealized model, which showed that upwelling occurred within both of the frontal eddies, despite having opposite rotational directions. This result was consistent with nutrient enhancement observed within the centers of both eddies. Biological enhancement within each eddy was observed, which may have been a result of advection from source waters and/or a local response to upwelled nutrients. The influence of frontal variability and frontal eddies on nutrients and plankton at the front argues for the necessity for 3-D models to fully explain frontal behavior and its effects on biological responses.

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Section: Climatological Comparisonmentioning

confidence: 63%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Physical and biological processes at the Middle Atlantic Bight shelf-break front

Hirzel¹

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Contrasting nitrogen dynamics across the Mid‐Atlantic Bight shelfbreak front: Insights from nitrate dual isotopes and nitrifier gene abundance

Zhu,

Mulholland,

Selden

et al. 2024

Limnology & Oceanography

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Observations and model studies suggest that front dynamics can enhance phytoplankton productivity. This study tested whether frontal systems also increase the abundance of nitrifying microbes and nitrogen recycling during repeat sampling transects across the Mid‐Atlantic Bight shelfbreak in July 2019. We measured ammonium concentrations, nitrate dual isotopes (δ15N, δ18O), and ammonia monooxygenase subunit A (amoA) genes of ammonia‐oxidizing archaea (AOA) and bacteria (AOB). In subsurface shelf waters, ammonium concentrations exceeded 2 μmol L−1, due to a temporary imbalance in regeneration from sinking particles and subsequent nitrification. The inverse correlation between nitrate δ15N values and ammonium concentrations confirmed nitrate was partially or entirely from local nitrification on the shelf. In contrast, the shelfbreak frontal zone and slope sea subsurface waters had much lower ammonium concentrations (0.1–0.2 μmol L−1) due to tight coupling between ammonium regeneration and nitrification. The deviation of nitrate δ15N and δ18O from algal uptake‐driven 1 : 1 ratio suggests concurrent nitrification in the euphotic zone. The shelfbreak front acted as an ecological boundary where AOA and AOB amoA gene numbers were partitioned, with AOAs abounding in slope waters and AOBs in shelf waters, likely due to ammonium availability. At certain slope stations, deep‐water nutrient inputs via isopycnal lifting induced by Gulf Stream intrusions caused unexpectedly high phytoplankton biomass, which doubled nitrifier abundance and potentially stimulated both ammonium regeneration and nitrification. These findings demonstrate distinct distributions of nitrifying microbes along the salinity gradient from shelf to slope and highlight the significant influence of coastal ocean‐western boundary current interactions on nitrogen biogeochemistry.

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Vertical nitrate flux fuels new production over summertime Northeast U.S. Shelf

Zheng,

Zhang,

et al. 2024

Limnology & Oceanography

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In aquatic ecosystems, allochthonous nutrient transport to the euphotic zone is an important process that fuels new production. Here, we use high‐resolution physical and biogeochemical observations from five summers to estimate the mean vertical nitrate flux, and thus new production over the Northeast U.S. Shelf (NES). We find that the summertime nitrate field is primarily controlled by biological uptake and physical advection–diffusion processes, above and below the 1% light level depth, respectively. We estimate the vertical nitrate flux to be 8.2 ± 5.3 × 10−6 mmol N m−2 s−1 for the mid‐shelf and 12.6 ± 8.6 × 10−6 mmol N m−2 s−1 for the outer shelf. Furthermore, we show that the new production to total primary production ratio (i.e., the f‐ratio), consistently ranges between 10% and 15% under summer conditions on the NES. Two independent approaches—nitrate flux‐based new production and O2/Ar‐based net community production—corroborate the robustness of the f‐ratio estimation. Since ~ 85% of the total primary production is fueled by recycled nutrients over sufficiently broad spatial and temporal scales, less than 15% of the organic matter produced in summer is available for export from the NES euphotic zone. Our direct quantification of new production not only provides more precise details about key processes for NES food webs and ecosystem function, but also demonstrates the potential of this approach to be applied to other similar datasets to understand nutrient and carbon cycling in the global ocean.

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Cross-shelf exchange associated with a shelf-water streamer at the Mid-Atlantic Bight shelf edge

Cited by 16 publications

References 53 publications

Physical and biological processes at the Middle Atlantic Bight shelf-break front

Physical and biological processes at the Middle Atlantic Bight shelf-break front

Contrasting nitrogen dynamics across the Mid‐Atlantic Bight shelfbreak front: Insights from nitrate dual isotopes and nitrifier gene abundance

Vertical nitrate flux fuels new production over summertime Northeast U.S. Shelf

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