Graduate Thesis Or Dissertation
 

Production and partitioning of organic matter during simulated phytoplankton blooms

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  • Few studies have examined the partitioning of organic matter in upwelling systems, despite the fact that these systems play a key role in carbon and nitrogen budgets in the ocean. We examined the production and partitioning of phytoplankton-derived organic matter in deck incubations off Oregon during the upwelling season. During exponential growth of the phytoplankton, ≥ 78% of total accumulated organic matter was in particulate form. A small accumulation of dissolved organic matter (DOM) was only observed in one incubation and the percentage of daily primary production released extracellularly averaged 12.7 ± 2.75 %, suggesting that DOM is a small fraction of primary production during exponential growth of coasta1 phytoplankton blooms. Following nitrate depletion, carbon-rich (C:N ≥ 16) DOM accumulated and the percentage of daily primary production released as dissolved organic carbon (DOC) averaged 57.7 ± 10.7 %. Abundances and growth rates of bacteria with a high DNA content increased rapidly concomitant with the large DOM release, while little response was observed from bacteria with a low DNA content. Despite the enhanced bacterial growth, a net decrease in DOC was only observed in one incubation and immediately after reaching maximum abundance, high-DNA bacterial abundances declined while heterotrophic nanoflagellate abundances increased. These results indicate that both bottom- up and top- down controls may act to prevent bacterial degradation of phytoplankton DOM, thus allowing the short-term accumulation (several weeks) of C rich DOM in the Oregon upwelling system. In addition to the accumulation of C rich DOM, approximately 70 to 157% more C was fixed than would be predicted by Redfield stoichiometry based on measured concentrations of nitrate and accumulated TOC. Accumulation of C rich DOM and excess carbon fixation suggests that nitrate assimilation (i.e., new production) might not equate to net production of POM in coastal upwelling systems.
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