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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.