Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Deborah K. Steinberg


Zooplankton play a key role in the cycling of dissolved organic matter (DOM) and inorganic nutrients. The factors that affect these processes, however, are not fully understood. I measured the effects of various diets on DOM and inorganic nutrient production by the copepod Acartia tonsa and the heterotrophic dinoflagellate Oxyrrhis marina, and explored the mechanisms of nutrient release from copepods. Copepods feeding on a mixed diet, the preferred diet of most copepods, had significantly lower dissolved organic carbon (DOC), ammonium (NH4+), and total dissolved nitrogen (TDN) release rates compared to feeding on a carnivorous or herbivorous diet. Thus, copepod feeding strategy can control the magnitude and composition of regenerated nutrients supplied to bacteria and phytoplankton. Secondly. I determined the effects of non-bloom and bloom concentrations of non-toxic and toxic cultures of harmful algal bloom (HAB) species Prorocentrum minimum and Karlodinium veneficum on grazing and production of DOM and inorganic nutrients by A. tonsa and O. marina. All algal diets deterred grazing, which likely resulted in starvation and subsequent catabolism of grazer body tissue. Additionally, DOM was typically a higher proportion of total dissolved nutrients released by zooplankton while feeding on the toxic algal culture, suggesting algal nutrient quality or direct toxic effects played a role in the differential nutrient release. Low ingestion rates coupled with high nutrient release rates could lead to feedback mechanisms that could intensify HABs. Finally, the various mechanisms of A. tonsa nutrient release, including sloppy feeding, excretion, and fecal pellet leaching, were isolated. Excretion and sloppy feeding were the dominant modes of DOC and NH4+ release, while sloppy feeding and fecal pellet leaching were dominant modes of urea release. A large proportion of ingested PON was lost as dissolved NH4+ and urea from copepods via all release mechanisms. These results have implications for the rapidity and location at which the regenerated nutrients are recycled in the water column. My dissertation results emphasize the importance of diet and release mechanism on the production of nutrients, particularly DOM, by zooplankton, which are important in understanding the recycling and transfer of nutrients and organic matter in marine food webs.



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