Intellectual merit: In much of the Southern Ocean, iron and irradiance are thought to exert major controls on the timing and location of blooms of the two major phytoplankton groups, Phaeocystis antarctica and diatoms. Recent numerical modeling studies have simulated the broad-scale features of algal biomass and species composition in the southern Ross Sea, by prescribing a higher iron requirement for the growth of diatoms relative to P antarctica and by allowing higher growth rates by P. antarctica under low irradiance. However, there are large uncertainties associated with the parameterizations used in these models, particularly with regard to phytoplankton iron parameters, which are typically assigned static values. Results of recent laboratory experiments with colonial P. antarctica suggest that the iron requirement of this species can be relatively high, and appears to vary strongly as a function of irradiance. In this project, the investigators will use laboratory experiments to quantify the iron requirements of diatom species that are representative of open-water, ice-edge and sea-ice environments of the southern Ross Sea, including Thalassiosira gracilis, Pseudo-nitzschia subcurvata, Fragilariopsis cylindrus and Navicula glaciei. The experimental data will be used to test a number of hypotheses concerning the effects of iron availability, irradiance and UV-B exposure on growth rates, production of dimethylsulfoniopropionate and dimethylsulfide, cellular pigment composition, and the relative uptake of macronutrients. In addition, the investigators will conduct preliminary "algal competition experiments", in which the growth of mixed assemblages containing both diatoms and colonial Phaeocystis antarctica will be examined, as a function of iron and irradiance, with and without UV-B exposure.
Broader impacts: The research will provide quantitative information that is required to construct robust, mechanistically-accurate numerical models of Southern Ocean ecosystems and biogeochemistry. This project also will make educational contributions at several levels, through the direct involvement of graduate and undergraduate students in laboratory research, and through planned outreach activities aimed at K-12 students and the general public, including lecture presentations in Bermuda and South Carolina, and the development of web materials in consultation with a group of K-12 science teachers.
