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 Lagrangian Transport and Transformation Experiment
October 2004 – September 2006
Project Summary:
Urban estuarine plumes represent a major pathway for the transport of nutrients and chemical contaminants from land to the coastal ocean. However, the fate and transport of this material is controlled not only by the physical dynamics of the plume but also by biological and chemical mediated transformation processes. We hypothesize that these biological and chemical processes are impacted by the plume.s structure, and thus transformation rates are in part related to the physical dynamics of the plume. It is our objective to understand how these physical, chemical and biological processes are coupled and interact, and to quantify their rates. By conducting a series of dye studies within an existing research-oriented operational ocean observatory, the proposed work will distinguish mixing processes that merely dilute material in the plume from biological and chemical processes that transform material. The dye tracer will provide a Lagrangian framework from which we will measure the time rate of change of physical, chemical and biological properties of a parcel of fluid in a buoyant plume. The dye study will resolve time scales from hours to days, and will span time scales of biological and chemical processes of interest such as metal uptake (hours), hytoplankton growth (hours to days), photochemical processes (hours), biological metal ligand production (hours to days) and trophic transfer (hours to days).
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Surface diatom chain density in the study area colleted off the R/V Oceanus during the 2005 field season. Highest concentrations were at the north end of the plume and decreased southward as the chains settled out of the water column. |
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The central hypothesis of the project is that the biological and chemical processes that transform nutrients and contaminants in a buoyant plume are impacted by the physical structure of the plume and that the rates associated with these processes change between upwelling and downwelling conditions. More specifically:
- Downwelling leads to a narrow nearshore buoyant plume. The plume is thick (on the order of ~15 meters) and characterized by a single bottom Ekman layer. Enhanced turbulence retains particulate matter throughout the water column and augments light reduction by CDOM and high phytoplankton loads. Low light levels constrain both phytoplankton growth and CDOM photobleaching rates. Phytoplankton have a metal profile similar to plume conditions, and overall metal concentrations within the plume do not change dramatically as material is advected down the coast due to low phytoplankton growth rates.
- Upwelling will lead to a horizontally dispersed and thin surface buoyant plume. The plume is detached from the bottom and highly stratified. The stratification reduces turbulence and particulate matter settles out of the plume. Reduced particulate matter in the thinned plume (~5m) enhances light levels and promotes both phytoplankton growth and CDOM photobleaching rates. Phytoplankton metal profiles are quickly modified as a result of enhanced phytoplankton growth and metal reactions with CDOM and biogenic ligands. The total metal profile within the plume will thus change over time. This trended change in the phytoplankton metal profiles will be the result of enhanced growth.
Specific Project Objectives:
- To quantify the intensity and spatial structure of diapycnal mixing rates in a buoyant plume and its dependence on along shore wind stress.
- To quantify the strength and structure of secondary flows in the plume, their dependence on alongshore wind stress, and to characterize their role in delivering buoyant fluid to regions of enhanced diapycnal mixing.
- To quantify the transformation and loss processes of estuarine contaminants and dissolved organic matter within the plume (i.e. CDOM photodegradation rates, contaminant metal uptake and remineralization rates, contaminant metal redox and speciation changes).
- To assess the growth and species composition of the phytoplankton community in the evolving plume and the response of the zooplankton community.
- To relate variations in the chemical and biological processes of objectives 3 and 4 to changes in the plume.s circulation, structure and mixing processes due to variations in the along-shore wind stress.
- To develop methods to assimilate tracers into coastal circulation models to test and improve current biological and chemical models.
People Involved:
- Dr. Mark Moline, Principal Investigator
- Shelley Blackwell, Research Associate
- Jessica Connolly, Former Graduate Student, Currently a Research Associate
- Jenn Yost, Graduate Student
- Ian Robbins, Former Graduate Student, Currently a Research Associate
- Michael Sauer, Former Graduate Student
Collaborators:
| Rutgers University |
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| Lamont-Doherty Earth Observatory |
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| University of Massachusetts - Boston |
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| Florida Environmental Research Institution |
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| University of Florida-Gainsville |
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Related Information:
Research Funded by:
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