Date of Award

Summer 8-1-2019

Document Type

Thesis: Open Access

Degree Name

MS Marine Science


Environmental and Ocean Sciences

Committee Chair

Ronald Kaufmann, Ph.D.

Committee Member

Drew Talley, Ph.D.

Committee Member

Elizabeth Hetherington, Ph.D.


Analyses of quantitative data on zooplankton diets are vital for understanding the drivers of zooplankton abundance within an ecosystem. Such analyses also provide insight into trophic pathways within the lower planktonic food web, which support populations of higher trophic level species. This study used carbon and nitrogen stable isotope ratios of size-fractionated plankton in Mission Bay, San Diego, CA to examine the spatial and temporal variation in zooplankton trophic ecology and determine potential environmental drivers of zooplankton community structure. Carbon stable isotopes reflect primary production sources in an organism’s diet, and nitrogen stable isotope ratios can be used to estimate the relative trophic positions of organisms. From April 2017 to April 2018, monthly sampling of environmental parameters and plankton tows were conducted at three sites, which varied in distance from the mouth of the bay. Plankton samples from each tow were divided into four size classes: 53-120 μm, 120-250 μm, 250-475 μm, and 475-1000 μm. Among the size classes, there was no significant variation in δ15N values, suggesting that either the food web at this level is not strongly size-structured or that δ15N values cannot delineate trophic structure in the lower planktonic portion of the food web. There were significant spatial differences in δ13C in the two smallest size classes (53-120 and 120-250 μm). The comparison among sites also revealed a significant difference in δ15N within the second largest size class (250-475 μm), which indicates that this size class may be feeding on organisms at different trophic positions at each site. Additionally, positive correlations were found within each size class between δ15N values and one or more environmental parameters, suggesting that there is an influence of environmental factors on stable isotope values of plankton in Mission Bay.