Interactive Effects of UV and Vertical Mixing on Phytoplankton and Bacterial Productivity of Ross Sea Phaeocystis Blooms

Principal Investigator: Wade H. Jeffrey, PhD

Co-principal Investigators: Patrick Neale, PhD. Smithsonian Environmental Research Center, Edgewater, MD and Ann Gargett, PhD. Old Dominion University, Norfolk, VA

Support Agency: National Science Foundation

Location of Study: Ross Sea, Antarctica

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Ultraviolet radiation influences the dynamics of plankton processes in the near-surface waters of most aquatic ecosystems and, in particular, the Southern Ocean in the austral spring period when biologically damaging UVB is enhanced by ozone depletion. Progress has been made in estimating the quantitative impact of UV (and enhanced UVB) in the S. Ocean for such processes as phytoplankton photosynthesis, bacterial production and DNA damage. Some important issues remain to be resolved, though. Little is known about responses in systems dominated by the colonial haptophyte Phaeocystis antarctica. This species dominates spring blooms in a polyna that develops in the southern Ross Sea. The Ross Sea is also of interest because of the occurrence of open water at a far southerly location in the spring, well within the "ozone hole", and continuous daylight, with implications for the regulation of DNA repair.
There are a number of studies suggesting that vertical mixing can significant modify the impact of UV in the Southern Ocean and elsewhere. However, there are limited measurements of turbulence intensity in the surface layer and measurements have not been integrated with parallel studies of UV effects on phytoplankton and bacterioplankton.
To address these issues, we are conducting a collaborative study of vertical mixing and the impact of UV in the Ross Sea. We will conduct a six week oceanographic research cruise in the Ross Sea, Antarctica aboard the RVIB NATHANIEL B. PALMER during October - November, 2003. The spectral and temporal responses of phytoplankton and bacterioplankton to UV will be characterized in both laboratory and solar incubations. These will lead to the definition of biological weighting functions and response models capable of predicting the depth and time distribution of UV impacts on photosynthesis, bacterial incorporation and DNA damage in the surface layer. We will also conduct several diel sampling series to measure depth-dependent profiles of DNA damage, bacterial incorporation, photosynthesis and fluorescence parameters over a 24 h cycle. Sampling will include stations with contrasting wind-driven mixing and stratification as the polyna develops.
Our program of vertical mixing measurements is optimized for the situation in which turbulence of intermediate intensity is insufficient to mix the upper layer thoroughly in the presence of stabilizing influences like solar heating and/or surface freshwater input from melting ice. This is probably typical for the Ross Sea during the spring. We will measure fine-scale vertical density profiles with a free-fall CTD unit. The profiles will be used to directly estimate large-eddy scales by determining Thorpe scales. Eddy scales and estimated turbulent diffusivities will be directly related to surface layer effects, and used to generate lagrangian depth-time trajectories in models of UV responses in the surface mixed layer.
The proposed research is the first in-depth study of UV effects in the Ross Sea and will provide a valuable comparison with previous work in the Weddell-Scotia Confluence and Palmer Station regions. It will also enhance our understanding of vertical mixing processes, trophic interactions and biogeochemical cycling in the Ross Sea.