Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Deborah A. Bronk


The chemical, physical, and biological factors affecting the bioavailability of humic nitrogen (N) to coastal phytoplankton were examined. Historically, humic substances have largely been considered biologically refractory, and humic-N is though to be unavailable biologically without prior oxidation via photochemical cleavage or remineralization by bacteria. This is due in part to the high aromaticity and low N content of humic substances. This dissertation investigates whether these assumptions are valid, and whether humic substances may be a more important source of N to the coastal phytoplankton community than previously believed. The research consisted of four main parts. First, changes in the structure and N content of humic substances were monitored by forming humics in the laboratory and following the changes in structure and chemical composition as they aged. It was found that as humics age, they become more aliphatic and fulvic-like. It was also determined that the commonly used XAD-8 extraction technique may underestimate the N content of aquatic humics by stripping ammonium (NH4+) from the humic structure. Second, whether or not this underestimation of humic-N has an effect on previously reported rates of photochemical N liberation from humic compounds was investigated. It was found that while the potential for the underestimation of photochemical release exists, previously reported rates are close to correct, largely due to physical and chemical interactions of humic substances with the surrounding environment. Third, the bioavailability of humic-N was examined using a suite of coastal phytoplankton strains. In short, all coastal strains exposed to humic substances could take up humic-N in short-term incubations; an open ocean strain tested did not take up humic-N. Furthermore, younger humics appeared to be more labile than those aged for time periods greater than three months. Finally, the significance of the salinity-mediated release was investigated and found to be a potentially important transport mechanism of NH4 + to the mid-saline regions of the estuary. In conclusion, the combination of chemical, physical, and biological processes occurring in the estuarine and coastal ecosystems suggests that humic substances are highly dynamic, biologically active compounds and not the biologically recalcitrant molecules portrayed in current literature.



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