Characterizing light limited growth and orientation of elongate colonial diatoms under laminar and turbulent Couette flow regimes

Ranjoy Barua1,2, Malcolm McFarland2, James Michael Sullivan2, Jan Rines3, Melissa Omand4 and Aditya R Nayak1,2, (1)Florida Atlantic University, Ocean and Mechanical Engineering, Boca Raton, FL, United States, (2)Florida Atlantic University, Harbor Branch Oceanographic Institute, Fort Pierce, FL, United States, (3)University of Rhode Island, Graduate School of Oceanography, Narragansett, RI, United States, (4)University of Rhode Island, Graduate School of Oceanography, Narragansett, United States
Abstract:
Orientation of oceanic particles in the water column can play a significant role in marine ecology, potentially influencing primary productivity in marine ecosystems. Previous field experiments by our group and others have shown preferential particle alignment in oceanic shear flows is not uncommon, especially under favorable flow conditions. To complement the field experiments, we performed laboratory studies in a temperature and light controlled environment to characterize the growth and orientation of colonial diatoms under different flow conditions. The growth and orientation of phytoplankton were observed in a circular Couette flow apparatus under different shear rates representative of laminar and turbulent regimes using digital holography. The Couette apparatus consists of an internal (rotating) and external (fixed) cylinder, 40 cm in height, capable of generating a range of shear rates. The annular gap between the two cylinders (2.23 cm) was filled with a diatom culture of the species Stephanopyxis turris. The gap Reynolds number, defined as Reg=ΩR2(R2-R1)/ν, was used to characterize the laminar/turbulent regimes, where Ω is rotation rate, R2 is radius of outer cylinder, R1 is radius of inner cylinder, and ν is the kinematic viscosity. The laminar and turbulent cases corresponded to 0.5 rpm (Reg=164) and 13 rpm (Reg=4252) respectively. Holograms were recorded and analyzed to characterize counts, size, growth rate and orientation of colonies. The growth rate increased exponentially over three week periods for both laminar and turbulent conditions, with a higher growth rate (~50%) in turbulent as opposed to the laminar regime. In the laminar case, strong preferential orientation was observed, with 60% of chain forming diatoms oriented in the flow direction (horizontal), in agreement with previous field studies showing similar trends. In the turbulent case, orientation was more random, with only 10-15% of the diatoms aligned in the flow direction.
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