A study of the production and measurement of dissolved organic matter in seawater

摘要

Using $sp{14}$C labelling methods to circumvent the currently controversial bulk dissolved organic carbon (DOC) methods, I investigated the extent to which DOC produced by marine algae is resistant to wet oxidation, the resistance of different molecular weight (MW) size fractions, and the effects of biotic and abiotic ageing on algal DOC to its resistance to wet oxidation procedures.;Batch culture experiments with a variety of marine phytoplankton showed that the amount of algal DOC resisting persulphate oxidation (DOC$sb{rm res}$) ranged from 5-20% of the initial DOC in the samples. Phytoplankton produced mainly low MW material, as on average $sim$80% was $$10,000 nominal MW) and low MW fractions generally showed similar reactivities to persulphate oxidation. Ageing of algal DOC solutions in the presence of bacteria for a period of about 1 year resulted in decreased proportions of DOC$sb{rm res}$ in the aged material, suggesting that bacterial alteration of algal DOC does not increase its resistance to persulphate oxidation over the long term. No significant changes to the resistance of DOC solutions were found from 250 hours exposure to sunlight.;A second aspect of the research into DOM resistant to wet oxidation methods involved a study of dissolved organic phosphorus (DOP) in seawater. I questioned whether the levels of DOP in seawater were underestimated by 'standard' methods, since organic and inorganic N:P ratios (the Redfield ratio) often show only a small variation in seawater. Size fractionation experiments indicated that the bulk (74 $pm$ 14%) of DOP in coastal and open ocean seawater was $<$10,000 nominal MW. The material most resistant to standard methods was found in the low MW fraction.;A mesocosm study examined DOP produced over the course of a mixed algal bloom. Most of the DOP recovered by ultrafiltration ($>$65%) was $<$10,000 MW, which is similar to the proportion of DOP with low MW in surface seawater.;These investigations of DOC and DOP do not support results which indicate the presence of a large pool of previously undetected marine DOM that is characterised by high molecular weight and high resistance to traditional wet oxidation methods. (Abstract shortened by UMI.).