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Ross Sea Project
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 UV-B 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 incorporation and DNA damage. Some important issues remain to be resolved, though.
Little is known about responses in systems dominated by the colonial alga Phaeocystis antarctica. This species dominates spring blooms in a polynya (ice-free region in an otherwise ice-covered ocean) that develops in the southern Ross Sea in October and November. This polynya has particular interest because it results in open water at a far southerly location in the spring, well within the "ozone hole" exposing plankton to elevated UV-B. The continuous daylight characteristic of this time of year has implications for the regulation of DNA repair most of which normally occurs at "night".
Pat Neale on-site in the Ross Sea with a few friends.
There are a number of studies suggesting that vertical mixing can significantly modify the impact of UV in the S. 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 plankton. To better understand how UV affects planktonic processes in the Ross Sea polynya, particularly in the context of vertical mixing, we will perform measurements of UV effects on phytoplankton photosynthesis, bacterial production and DNA damage, and make physical measurements to characterize vertical mixing processes. The measurements will focus on developing P. antarctica populations in the early spring period of maximum increase in UVB due to ozone depletion.
The primary topics we will address are:
- Sensitivity of photosynthesis and bacterial production to solar UV and how effects are enhanced by ozone depletion (spectral dependence)
- The cumulative impact of daily UV exposures on phytoplankton and bacteria, both in terms of phytoplankton and bacterial production, changes in the patterns of carbon assimilation within phytoplankton and DNA damage (temporal dependence)
- The potential for repair of UV damage and time required for repair
- The residence times of organisms in the near-surface zone where UV exposure occurs
- The potential impact on the quality of material settling out of the euphotic zone.
We will use this information to refine estimates of the overall impact of UV in the Ross Sea using models that integrate mixing and UV responses. The results will enhance our understanding of vertical mixing processes, trophic interactions and biogeochemical cycling in the Ross Sea.
Field Work Objectives:
We will work aboard the R/V Nathaniel B. Palmer in the southern Ross Sea (NBP04-09).
The research icebreaker Nathaniel B. Palmer, amongst the icebergs of the Ross Sea.
The main target area is around the longitude 175° W to 175°E, 76° to 75°S latitude, adjusting as necessary for changing ice conditions. The cruise track will also be guided by satellite imagery to find locations with maximum development of the Phaeocystis bloom in the polynya . This is expected to be in the western part of the indicated area where there is typically more open water and deeper mixing.
The majority of the work is planned for the Phaeocystis dominated area. Comparative stations will be made in the eastern part nearer to the ice edge and in which shallower stratification and a greater dominance by diatom species is expected. The secondary objective is to compare stations with contrasting degrees of vertical density stratification. At each station, the spectral and temporal responses of phytoplankton and bacterioplankton to UV will be characterized in both laboratory and solar incubations over a two-day period. We will measure fine-scale (1 cm) vertical changes in density in series of profiles made with the ship Conductivity-Temperature-Depth sensor. At four of the stations, a second two-day period will be used to conduct diel sampling series to measure depth-dependent profiles of DNA damage, bacterial incorporation, photosynthesis and fluorescence parameters over a 24 h cycle.
The phytoplankton component of the project will measure photosynthetic response to the visible and ultraviolet light spectrum using a special laboratory incubator ("photoinhibitron") as well as visible-light incubators ("photosynthetrons"). We will also set up UV-transparent and UV-opaque outdoor incubators to measure photosynthetic response for samples of phytoplankton in the presence and absence of UV and for samples that are initially exposed to UV and then shielded to observe the degree of recovery. Our studies on photosynthesis sensitivity to UV will involve incubation experiments with 14C as tracer. We will measure total assimilation as well as assimilation into different macro molecular fractions.
Phytoplankton will be characterized though measurements of chlorophyll content, light absorption, fluorescence and microscopic enumeration. Active fluorescence measurements with special pulsed excitation (including measurements of individual Phaeocystis colonies in a microscope) will be used to monitor the dynamic responses of photosynthesis to changes in visible and ultraviolet radiation exposures as occurs during vertical mixing. Incident ultraviolet and visible irradiance will be measured using several types of radiometers including a specially designed Smithsonian filter wheel radiometer. Penetration of UV into the Ross Sea will be measured using a submersible multichannel radiometer.
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