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Title: The impact of mixing over topography on plankton dynamics
Author: Smith, C. E.
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2018
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The subtropical oceans make up ~60% of the global ocean. They are stratified, nutrient limited environments characterised by low phytoplankton biomass dominated by picophytoplankton, and rapid recycling of carbon and nutrients in surface waters. Nutrient supply to support phytoplankton export production in these regions has been questioned for the last two decades. Recently, the RidgeMix project (JR15007) and others have observed that internal tide dissipation over the sharp topography of mid-ocean ridges enhances vertical turbulent mixing and nitrate fluxes into the photic zone compared to adjacent basins. In the ocean, higher turbulence and nutrient regimes are often associated with the dominance of larger phytoplankton. This project was based on the main hypothesis that, over the Mid-Atlantic Ridge (MAR) in the subtropical North Atlantic, there is enhanced primary production and a shift towards larger phytoplankton as a result of the enhanced supply of nutrients to the photic zone. During RidgeMix, the size-distribution and species composition of phytoplankton were analysed using size-fractionated chlorophyll-a (0.2-2 µm, 2-20 µm, > 20 µm), flow cytometry (< 10 µm) and light microscopy (> 10 µm). Higher concentrations of chlorophyll-a in the two larger size-fractions (2-20 µm and > 20 µm) and higher abundances of picoeukaryotes (~2.9 µm) and nanoeukaryotes (~4.6 µm) were observed over the ridge. There were no changes in the abundance of > 10 µm cells (diatoms and dinoflagellates) at the chlorophyll-a maximum or the standing stock of particulate organic carbon (POC), over the top 500 m of the water column, which might suggest enhanced zooplankton grazing over the ridge. Zooplankton abundance, composition and biomass from 0-200 m were analysed using light microscopy, size-fractionated into 200-500 µm, 500-1000 µm, 1000-2000 µm and > 2000 µm, and analysis of wet and dry weight. Higher abundances of zooplankton were observed over the ridge compared to the adjacent basin. Larger phytoplankton cells and enhanced zooplankton grazing may ultimately result in higher POC export fluxes over the ridge with implications for the higher trophic levels of the benthic and pelagic community in these regions and global biogeochemical cycling. Finally, three techniques for marine particle collection were compared, namely the Stand Alone Pumps, Marine Snow Catcher and Niskin bottle, during RidgeMix and in the Celtic Shelf Sea (DY026, DY029, DY033). Determination of POC, PN and lipid biomarkers showed that each method collected different quantities (concentrations) and compositions of particles, the latter varying significantly with physical conditions of stations. Consequently, how representative of the water column each sample collected is remains unresolved, but it is important that oceanographers are aware of these issues.
Supervisor: Mahaffey, Claire ; Wolff, George ; Sharples, Jonathan ; Kiriakoulakis, Kostas Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral