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Physical drivers of plankton variability in a shelf sea


Project Description

Introduction:
The phytoplankton, microscopic single-celled plants that drift with the ocean currents, play two key roles in the ocean. They provide the organic fuel that supports the ocean’s food chains, and by absorbing carbon as they grow they also affect atmospheric CO2 and Earth’s climate. The life of a single-celled plant in the ocean is controlled by the physics of turbulence and stratification, determining where and when it can grow, and its fate. Variability in the physical forcing (e.g. solar energy, wind, tidal currents, river supplies of freshwater) is well-established as driving variability in the growth of the phytoplankton. For instance the development of a warm surface layer of water in spring causes a sudden outburst of phytoplankton growth, because the stratification inhibits vertical turbulent mixing and so holds the phytoplankton up near the sea surface where there is plenty of light for photosynthesis.

During a recent major research project in the NW European shelf and coastal seas we acquired an unprecedented amount of data spanning 17 months, using ship-based surveys, moored instruments, autonomous ocean gliders and satellites. A parallel project has developed high-resolution numerical models of the ocean, simulating the physics, chemistry and biology that underpin the phytoplankton growth and distribution. These data are suggesting previously unrecognised components of the ocean physics that affect the phytoplankton, such as a new importance for the weak horizontal salinity gradients set up by river inputs at the coast and the role of tidal turbulence in periodically mixing phytoplankton out of the surface water and into the deeper, darker ocean.

Project Summary:
The student will have access to all of the data in order to identify and quantify key new processes governing phytoplankton growth and distributions. Key objectives of the research will be:
• Data from a profiling mooring will be used to assess the relative importance of tides in mixing phytoplankton into deeper water, compared to simple sinking of phytoplankton cells.
• Time series of surface phytoplankton concentration and the vertical structure of stratification will allow detailed links to be made between stratification caused by temperature or salt and variability in phytoplankton growth.
• Information on the different species of the phytoplankton will allow novel links between phytoplankton community changes and ocean physics to be investigated.
• The data from numerical simulations will be used to assess the physical processes and biological responses in more detail, and also to test the model capabilities against the real-world data.

The student will become a part of an ongoing collaboration of ocean scientists from the University of Liverpool and from the National Oceanography Centre. The University of Liverpool offers courses in numerical analyses, ocean physics and ocean biogeochemistry that can augment undergraduate training towards application to this challenging multidisciplinary problem. There are regular opportunities for students to gain experience at sea on ocean research expeditions.

Funding Notes

Full funding (fees, stipend, research support budget) is provided by the University of Liverpool. Formal training is offered through partnership between the Universities of Liverpool and Manchester in both subject specific and transferable skills to the entire PhD cohort and at each University through local Faculty training programmes.

References

Hickman, A.E., P. M. Holligan, C. M. Moore, J. Sharples, V. Krivtsov, M. R. Palmer. 2009. Distribution and chromatic adaptation of phytoplankton within a shelf sea thermocline. Limnology and Oceanography, 54(2), 525-536.
Ruiz-Castillo, E., J. Sharples, J. Hopkins & M. Woodward, 2018. Seasonality in the cross-shelf physical structure of a temperate shelf sea and the implications for nitrate supply. Progress in Oceanography, doi.org/10.1016/j.pocean.2018.07.006.
Sharples, J., et al., 2001. Phytoplankton distribution and survival in the thermocline. Limnology and Oceanography, 46(3), 486-496.
Sharples, J., et al., 2007. Spring-neap modulation of internal tide mixing and vertical nitrate fluxes at a shelf edge in summer. Limnology and Oceanography 52(5): 1735-1747.
Williams, C., J. Sharples, C. Mahaffey & T. Rippeth, 2013. Wind-driven nutrient pulses to the subsurface chlorophyll maximum in seasonally stratified shelf seas. Geophys. Res. Lett., 40, 5467-5472, doi:10.1002/2013GL058171.

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