| Summary: | Much interest has been focused on the oceans as an ameliorating influence on the "Greenhouse Effect" due to their capacity as a sink for atmospheric CO 2 . Results presented in this thesis focus on the effects that marine phytoplankton have on the ability of the oceans to take up carbon dioxide from the atmosphere. Experiments involving laboratory based analyses of phytoplankton cultures as well as ship based experiments using natural populations of non-calcifying and calcifying species were conducted using the techniques of gas chromatography, infra red gas analysis and for the first time, an ion selective electrode to measure the changes in carbon dioxide concentration. A pulsed electrode system in conjunction with a precise Winkler assay was used for the determination of dissolved oxygen. Results indicate that phytoplankton occurring in dense blooms gradually reduce surface pCO 2 levels and, hence, have a sizeable effect on the capacity of the oceans to act as a sink for atmospheric CO 2 . During the Spring Phytoplankton Bloom in 1990 in the N.E. Atlantic, phytoplankton were responsible for a decrease in sea surface pCO 2 of 175ppm. During a bloom of coccolithophorids in the northern North Atlantic during Summer 1991 the mean air sea difference in pCO 2 was reduced by a mean of 15ppm. Measurements of dissolved oxygen showed an increase in saturation values for the period before and after the bloom and indicated that phytoplankton were acting as a source of oxygen to the surface waters of the N.E. Atlantic Ocean. Measurements were carried out for the first time using gas chromatography to determine the changes in pCO 2 in incubated water samples used to derive rates of primary production in the surface waters of the North Atlantic.
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