Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)



First Advisor

Karen Wishner


Oxygen minimum zones (OMZs) are areas in which midwater oxygen concentration can be significantly lower than in non OMZ areas at comparable depths. OMZs are of great interest because of the limits oxygen availability places on life. The potential for OMZ expansion with warming of the oceans has refocused attention on the importance of these areas and their potential impacts on the global carbon cycle. The large open ocean OMZs of the Eastern Tropical Pacific and Arabian Sea are notable for their size, thickness, and the intensity of oxygen depletion (

Here, I use measurements of natural abundance carbon and nitrogen stable isotopes (as δ13C and δ15N, respectively) to investigate zooplankton diets within the Eastern Tropical Pacific OMZ. The use of stable isotopes rests on the premise that the δ13C values of consumers are similar to their ultimate organic carbon source, primary producers, while the δ15N values reflect more proximal sources of organic matter and can be used to evaluate trophic position. To examine the energy sources and trophic interactions of zooplankton in the Eastern Tropical North Pacific OMZ, samples were collected in 2007 and 2008 at two stations, the Tehuantepec Bowl (TB) and Costa Rica Dome (CRD), using vertically stratified MOCNESS net tows between 0-1200 m. Environmental data were collected concurrently with zooplankton samples and particulate organic matter (POM) was collected with McLane large volume in situ pumps. Zooplankton samples were separated into four size fractions and processed at sea to select aliquots for bulk (mixed zooplankton) analysis and individual taxa samples were analyzed for stable carbon and nitrogen isotopes.

Bulk zooplankton and POM δ13C and δ15N values revealed strong depth gradients associated with oxyclines (oxygen gradients) at the upper and lower OMZ boundaries. While the source of low δ13C values at the upper oxycline could not be determined, the sharp gradient in δ15N values at the lower oxycline indicated a depth zone of increased trophic progression. Furthermore, δ15N values were significantly lower at CRD than at TB, suggesting that nitrogen fixation may have been present at CRD. Low δ13C-δ15N correlations in all zones at CRD suggested that the products of nitrogen fixation were transferred to zooplankton food webs throughout the sampled water column.

Stable isotope values of individual zooplankton taxa also suggested that zooplankton collected within and above the OMZ core likely fed on material from the upper 110m of the water column, while lower oxycline zooplankton likely consumed deep POM exiting the OMZ. There were significant differences in the isotope values of zooplankton taxa with different trophic strategies (particle feeders, omnivores, carnivores) in the OMZ core and lower oxycline community, but not for those living in the mixed layer and upper oxycline. Furthermore, calculated trophic levels for lower oxycline taxa were much higher than expected based on a priori trophic level designations. The copepod Eucalanus inermis is thought to diapause in the lower oxycline, but low C:N ratios and δ15N values measured were not consistent with expectations for a diapausing population. However, storage of ammonium by E. inermis fro buoyancy regulation could have resulted in the observed C:N ratios and δ15N values. In the lower oxycline, the trophic isolation of particle feeders the sharp δ15N gradients at that depth, and the large proportion of carnivorous taxa supported the idea that this is a zone of vigorous trophic processing.

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Oceanography Commons