Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Differences in zooplankton community structure and diet within the mesopelagic zone (base of euphotic zone to 1000 m) play a key role in affecting the efficiency by which organic matter is exported to depth, but how the structure of mesopelagic food webs change with depth or location is poorly known. I examined how mesopelagic zooplankton affect particle export in an oligotrophic (Hawaii Ocean Time-series site ALOHA) compared to a mesotrophic (Japanese time series site K2) open-ocean system. In the first part of the study, I investigated how fecal pellet characteristics change with depth in order to quantify the extent of particle repackaging by mesopelagic zooplankton. There was significant evidence of mesozooplankton repackaging of sinking particles in the mesopelagic zone, as indicated by presence of new fecal pellet types (as different size, color, or shape) occurring at different depths. Fecal pellets reflected the disparate zooplankton community structure at the two sites, and larger pellets at K2 likely increased transfer efficiency of POC there compared to ALOHA. In the second portion of this study, I analyzed fatty acids (FA) in zooplankton and particles to characterize zooplankton diet and large (>51mum) particles in the mesopelagic zone of these two contrasting regions. Total FA concentration was higher in zooplankton tissue at K2, largely due to FA storage by ontogenetic vertical migrating species there. FA biomarkers that were indicative of particle feeding were also evident at both sites. Finally, I quantified cyanobacteria and small eukaryotic phytoplankton in the guts of mesopelagic zooplankton using light and epifluorescence microscopy to determine if mesopelagic zooplankton mediate the export of small phytoplankton to the deep sea. Cyanobacteria and small, eukaryotic phytoplankton occurred in the guts of nearly all target species sampled from the surface to 1000 m, indicating mesozooplankton grazing on aggregates is a pathway by which export of carbon and picoplankton can be enhanced. Guts of diel vertical migrators still contained picoplankton at their deep, daytime residence depths, indicating active export of these cells. In all three studies it was apparent that carnivory becomes an increasingly important component of mesopelagic zooplankton diet with depth. Evidence included the presence in sediment traps of red fecal pellets produced by carnivores, fatty acids in zooplankton tissue indicative of an increasingly carnivorous diet with depth, and the presence of chitin, gastropod shells, and microzooplankton in mesopelagic zooplankton gut contents. Changes in zooplankton feeding ecology from the surface through the mesopelagic zone, and between contrasting environments, have important consequences for the quality and quantity of organic material transported to the deep sea.



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