Masters Theses

Naomi Parker

Date of Award

5-1997

Degree Type

Thesis

Degree Name

Master of Science

Major

Ecology and Evolutionary Biology

Major Professor

Walker O. Smith

Committee Members

Dewey Bunting, Jackie Grebmeier

Abstract

Stratospheric ozone depletion in the austral spring results in increased fluxes of Ultraviolet B (UV-B) radiation (280-320 nanometers) to the earth's surface which is biologically harmful to phytoplankton. The potential effects of ultraviolet-B radiation on the Ross Sea polynya spring phytoplankton blooms were investigated through measurements of UV-B penetration (along with other wavelengths) into the water column and through field experiments on phytoplankton responses to variations in UV-B levels. This work was carried out during two research cruises, the first from November 10 to December 8, 1994 under significantly ozone depleted conditions, and the second from December 16 to January 14, 1995/96 when 'normal' ozone levels were present. UV-B was found to penetrate to biologically significant depths, extending through almost the entire mixed layer at the longer wavelengths. The vertical attenuation coefficients (K) for all wavelengths measured varied greatly throughout the polynya and were positively correlated with integrated chlorophyll concentrations, reflecting the importance of phytoplankton as attenuators of light. The dominant alga of the early spring in the polynya is Phaeocystis sp., while later in the season diatoms, particularly Nitzschia subcurvata, become numerically dominant. Primary production in surface waters increased under UV-B exclusion during both cruises, suggesting the possibility that current measurements of productivity for the area using UV-opaque incubation vessels may be overestimated. Experiments in which UV-B exposure of phytoplankton was varied indicated that in the Ross Sea Phaeocystis is better able to tolerate enhanced UV-B conditions. To my knowledge this has not been observed previously, since in other environments diatoms have been found to be more tolerant to increased UV-B fluxes. It is suggested that the cellular geometry of the dominant Ross Sea polynya diatom (Nitzschia subcurvata) which is small and solitary, may be one reason for this discrepancy, as all diatoms studied for their UV-B tolerance have been either larger or occurred in chains. The higher UV-B tolerance of Phaeocystis may increase its competitiveness over diatoms in the early spring, although lack of pre-ozone data make it impossible to quantify the effect. The poorer nutritional value of Phaeocystis compared to diatoms implies that any benefit of ozone depletion for Phaeocystis may have a negative impact on the ecosystem as a whole by decreasing the amount of energy transferred to higher trophic levels, both pelagic and benthic. More work needs to be done to determine the impact of this effect and the implications for the area of continued ozone depletion.

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