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  • 1
    Publication Date: 2022-06-28
    Description: Author Posting. © American Geophysical Union, 2022. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 127(1), (2022): e2021JC017715, https://doi.org/10.1029/2021JC017715.
    Description: 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.
    Description: This research was supported by the National Science Foundation (OCE-1657803, OCE-1657855, and OCE-1655686) and the Dalio Explorer Fund. Support for H. Oliver was provided by the WHOI Postdoctoral Scholar program.
    Description: 2022-06-28
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 2
    Publication Date: 2022-05-26
    Description: Author Posting. © American Geophysical Union, 2010. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 33(2), (2019): 181-199. doi:10.1029/2018GB005983.
    Description: One pathway of the biological pump that remains largely unquantified in many export models is the active transport of carbon from the surface ocean to the mesopelagic by zooplankton diel vertical migration (DVM). Here, we develop a simple representation of zooplankton DVM and implement it in a global export model as a thought experiment to illustrate the effects of DVM on carbon export and mesopelagic biogeochemistry. The model is driven by diagnostic satellite measurements of net primary production, algal biomass, and phytoplankton size structure. Due to constraints on available satellite data, the results are restricted to the latitude range from 60°N to 60°S. The modeled global export flux from the base of the euphotic zone was 6.5 PgC/year, which represents a 14% increase over the export flux in model runs without DVM. The mean (± standard deviation, SD) proportional contribution of the DVM‐mediated export flux to total carbon export, averaged over the global domain and the climatological seasonal cycle, was 0.16 ± 0.04 and the proportional contribution of DVM activity to total respiration within the twilight zone was 0.16 ± 0.06. Adding DVM activity to the model also resulted in a deep local maximum in the oxygen utilization profile. The model results were most sensitive to the assumptions for the fraction of individuals participating in DVM, the fraction of fecal pellets produced in the euphotic zone, and the fraction of grazed carbon that is metabolized.
    Description: Support for this work came from the National Science Foundation (OCE‐1434000 and OCE‐1657803) and the National Aeronautics and Space Administration (NASA) as part of the EXport Processes in the global Ocean from RemoTe Sensing (EXPORTS) field campaign (grant 80NSSC17K0692) and the North Atlantic Aerosol and Marine Ecosystems Study (NAAMES, grants NNX15AE72G and 80NSSC18K0018). The satellite data used as drivers for the model are available as supporting information. The World Ocean Atlas data (World Ocean Atlas 2009, Annual Climatology, 1 degree, Temperature, Salinity, Oxygen) containing temperature and oxygen measurements can be downloaded from the ERDAPP data server (https://coastwatch.pfeg.noaa.gov/erddap/index.html).
    Description: 2019-07-19
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 3
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    Massachusetts Institute of Technology and Woods Hole Oceanographic Institution
    Publication Date: 2022-05-26
    Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2021.
    Description: In this thesis, I explore two topics in plankton ecology with a combination of models and observations. First, I investigate the contribution of zooplankton diel vertical migration (DVM) to the vertical flux of carbon as part of the biological pump. I do this by constructing and analyzing a global model that includes DVM and is driven by satellite-based estimates of primary productivity. There has long been speculation about the significance of DVM to the biological pump, but quantitative estimates of its impact are rare. I estimate that DVM constitutes approximately 16% of the global carbon export flux associated with the biological pump and that the relative contribution of DVM is higher in subtropical latitudes. In later chapters, I build two nutrient-hytoplankton-zooplankton (NPZ) models with different levels of complexity to evaluate the role of nutrient supply and grazing in promoting phytoplankton diversity. Zooplankton switching plays a significant role in promoting diversity because it allows competing phytoplankton types to coexist in situations that would otherwise lead to competitive exclusion. When implemented in a size-structured NPZ model, stronger switching increases the evenness of the distribution of biomass between coexisting size classes, which is used as a proxy for taxonomic diversity. I also describe a particular characteristic of the Kill-the-Winner functional response (used in the NPZ models), which I have termed synergistic grazing. Synergistic grazing occurs when the grazing rate on one phytoplankton type increases as the biomass of an alternative phytoplankton type increases. This characteristic can result in unintuitive model dynamics. Finally, I describe patterns in phytoplankton community size structure in the shelfbreak region of the Northeast U.S. Shelf using high-resolution flow-cytometry measurements. I find that enhancement of phytoplankton biovolume at the shelfbreak front is common during the springtime, but these enhancement events are not associated with consistent changes in community size structure. I evaluate these results in the context of hypotheses generated based on my analysis of the NPZ models.
    Description: The research in this thesis was supported by the National Science Foundation (NSF, OCE-1434000 and OCE-1657803) and the National Aeronautics and Space Administration (NASA) as part of the EXport Processes in the global Ocean from RemoTe Sensing (EXPORTS) field campaign (grant 80NSSC17K0692) and the North Atlantic Aerosol and MarineEcosystems Study (NAAMES, grantsNNX15AE72G and 80NSSC18K0018). Additional support came from NSF (OCE-1655686 and OCE-1657803) and the Simons Foundation (561126).
    Keywords: zooplankton ; grazing ; diversity
    Repository Name: Woods Hole Open Access Server
    Type: Thesis
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  • 4
    Publication Date: 2019-02-01
    Print ISSN: 0886-6236
    Electronic ISSN: 1944-9224
    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
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