Hirzel
Andrew J.
Hirzel
Andrew J.
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ArticleEphemeral surface chlorophyll enhancement at the New England shelf break driven by Ekman restratification(American Geophysical Union, 2021-12-28) Oliver, Hilde ; Zhang, Weifeng G. ; Archibald, Kevin M. ; Hirzel, Andrew ; Smith, Walker O. ; Sosik, Heidi M. ; Stanley, Rachel H. R. ; McGillicuddy, Dennis J.The Mid-Atlantic Bight (MAB) hosts a large and productive marine ecosystem supported by high phytoplankton concentrations. Enhanced surface chlorophyll concentrations at the MAB shelf-break front have been detected in synoptic measurements, yet this feature is not present in seasonal means. To understand why, we assess the conditions associated with enhanced surface chlorophyll at the shelf break. We employ in-situ and remote sensing data, and a 2-dimensional model to show that Ekman restratification driven by upfront winds drives ephemerally enhanced chlorophyll concentrations at the shelf-break front in spring. Using 8-day composite satellite-measured surface chlorophyll concentration data from 2003–2020, we constructed a daily running mean (DRM) climatology of the cross-shelf chlorophyll distribution for the northern MAB region. While the frontal enhancement of chlorophyll is apparent in the DRM climatology, it is not captured in the seasonal climatology due to its short duration of less than a week. In-situ measurements of the frontal chlorophyll enhancement reveal that chlorophyll is highest in spring when the shelf-break front slumps offshore from its steep wintertime position causing restratification in the upper part of the water column. Several restratification mechanisms are possible, but the first day of enhanced chlorophyll at the shelf break corresponds to increasing upfront winds, suggesting that the frontal restratification is driven by offshore Ekman transport of the shelf water over the denser slope water. The 2-dimensional model shows that upfront winds can indeed drive Ekman restratification and alleviate light limitation of phytoplankton growth at the shelf-break front.
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ArticleA regional, early spring bloom of Phaeocystis pouchetii on the New England continental shelf(American Geophysical Union, 2021-01-15) Smith, Walker O. ; Zhang, Weifeng G. ; Hirzel, Andrew ; Stanley, Rachel M. ; Meyer, Meredith G. ; Sosik, Heidi M. ; Alatalo, Philip ; Oliver, Hilde ; Sandwith, Zoe O. ; Crockford, E. Taylor ; Peacock, Emily E. ; Mehta, Arshia ; McGillicuddy, Dennis J.The genus Phaeocystis is distributed globally and has considerable ecological, biogeochemical, and societal impacts. Understanding its distribution, growth and ecological impacts has been limited by lack of extensive observations on appropriate scales. In 2018, we investigated the biological dynamics of the New England continental shelf and encountered a substantial bloom of Phaeocystis pouchetii. Based on satellite imagery during January through April, the bloom extended over broad expanses of the shelf; furthermore, our observations demonstrated that it reached high biomass levels, with maximum chlorophyll concentrations exceeding 16 µg L−1 and particulate organic carbon levels > 95 µmol L−1. Initially, the bloom was largely confined to waters with temperatures <6°C, which in turn were mostly restricted to shallow areas near the coast. As the bloom progressed, it appeared to sink into the bottom boundary layer; however, enough light and nutrients were available for growth. The bloom was highly productive (net community production integrated through the mixed layer from stations within the bloom averaged 1.16 g C m−2 d−1) and reduced nutrient concentrations considerably. Long‐term coastal observations suggest that Phaeocystis blooms occur sporadically in spring on Nantucket Shoals and presumably expand onto the continental shelf. Based on the distribution of Phaeocystis during our study, we suggest that it can have a significant impact on the overall productivity and ecology of the New England shelf during the winter/spring transition.
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ArticleDiatom hotspots driven by western boundary current instability(American Geophysical Union, 2021-05-11) Oliver, Hilde ; Zhang, Weifeng G. ; Smith, Walker O. ; Alatalo, Philip ; Chappell, Phoebe Dreux ; Hirzel, Andrew ; Selden, Corday ; Sosik, Heidi M. ; Stanley, Rachel H. R. ; Zhu, Yifan ; McGillicuddy, Dennis J.Climatic changes have decreased the stability of the Gulf Stream (GS), increasing the frequency at which its meanders interact with the Mid-Atlantic Bight (MAB) continental shelf and slope region. These intrusions are thought to suppress biological productivity by transporting low-nutrient water to the otherwise productive shelf edge region. Here we present evidence of widespread, anomalously intense subsurface diatom hotspots in the MAB slope sea that likely resulted from a GS intrusion in July 2019. The hotspots (at ∼50 m) were associated with water mass properties characteristic of GS water (∼100 m); it is probable that the hotspots resulted from the upwelling of GS water during its transport into the slope sea, likely by a GS meander directly intruding onto the continental slope east of where the hotspots were observed. Further work is required to unravel how increasingly frequent direct GS intrusions could influence MAB marine ecosystems.
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ThesisPhysical and biological processes at the Middle Atlantic Bight shelf-break front(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2023-02) Hirzel, Andrew ; McGillicuddy, Dennis J.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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ArticleHigh resolution analysis of plankton distributions at the Middle Atlantic Bight shelf-break front(Elsevier, 2023-09-07) Hirzel, Andrew J. ; Alatalo, Philip ; Oliver, Hilde ; Petitpas, Christian M. ; Turner, Jefferson T. ; Zhang, Weifeng Gordon ; McGillicuddy, Dennis J.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 several studies suggesting persistent upwelling at the front. High resolution cross-frontal transects were conducted during three two-week cruises in April 2018, May 2019, and July 2019. Mesoplankton distributions at the front were measured with a Video Plankton Recorder equipped with hydrographic and bio-optical sensors. Zooplankton were also sampled with a Multiple Opening/Closing Net and Environment Sensing System. Each of the three cruises had distinctly different frontal characteristics, with lower variability in frontal position in April 2018 and higher variability in May and July 2019, primarily due to frontal eddies and a Gulf Stream warm core ring, respectively. Eulerian means of all transect crossings within each cruise did not show mean frontal chlorophyll enhancement in April 2018 or July 2019, despite individual crossings showing chlorophyll enhancement in April 2018. Transformation of the April 2018 data into a cross-frontal coordinate system revealed a weak enhancement of chlorophyll and copepods at the front. Mean frontal chlorophyll enhancement was observed in May and was associated with enhancement in the periphery of a frontal eddy rather than the front itself. None of the planktonic categories observed were enhanced at the front in the cross-shelf mean distribution, though diatom chains and copepods were more abundant inshore of the front, particularly in May and July 2019, as well as within the center of a frontal eddy in May. The high variability of the MAB frontal region obscured the impact of ephemeral frontal enhancement in mean observations of April 2018, while frontal eddies contributed to chlorophyll enhancement in mean observations of May 2019. The influence of both argues for the necessity for 3-D models rather than idealized 2-D models to explain frontal behavior and its effects on biological responses.