Publication Date

2015-12-09

Availability

Embargoed

Embargo Period

2016-12-08

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Marine Biology and Fisheries (Marine)

Date of Defense

2015-10-30

First Committee Member

Robert K. Cowen

Second Committee Member

Su Sponaugle

Third Committee Member

Gary Hitchcock

Fourth Committee Member

Jean-Olivier Irisson

Fifth Committee Member

Robert H. Condon

Abstract

The gelatinous zooplankton (GZ), comprised of cnidarians, ctenophores, and pelagic tunicates, are an abundant, diverse group of organisms that often exhibit boom-and-bust population dynamics. Most of the work on GZ has been focused on the large scyphozoans, while the smaller members of this group (hydrozoans, ctenophores, and pelagic tunicates) are relatively understudied due to their size, fragility, and transparency. Furthermore, spatially and temporally variable GZ populations can have important ecosystem impacts, e.g. to the biological pump via sinking carcasses and fecal pellet production. This dissertation investigates gelatinous zooplankton and their role in marine communities and ecosystems, with respect to 1) their fine-scale distribution and aggregation at a frontal feature, 2) their fine-scale vertical migration patterns and trophic role in driving distributions of the zooplankton community, and 3) their contribution to global carbon cycling. The In Situ Ichthyoplankton Imaging System (ISIIS), which is a towed, underwater camera system capable of capturing images of mesozooplankton in their natural environment, was deployed first in the Southern California Bight (SCB) between October 15-17, 2010 for the frontal feature study, and then in the northern Gulf of Mexico (GOM) between July-August, 2011 for the diel vertical migration (DVM) study. In 5,450 m-3 of water at a strong salinity-defined front in the SCB, we found over 129,000 medusae and ctenophores, and nearly 650,000 pelagic tunicates, in over 50 taxa. One species (Solmaris rhodoloma) was highly aggregated at the front, five other taxa were somewhat aggregated, but the rest were not aggregated at the front. As a whole, the community was highly structured, but the four major assemblages were separated primarily by depth-related factors. Results suggested that the front was not highly influential in driving GZ distributions from a community perspective; temperature, oxygen and fluorometry were more important. In the northern GOM, the fine-scale vertical distribution and migration patterns for the zooplankton community were described. Most taxa exhibited diel vertical migration (DVM), though for the GZ, migration direction and strength were variable: strongest in the hydromedusae and Pelagia noctiluca (mostly showing a ‘normal’ pattern), but weaker for ctenophores and siphonophores. Most of the larval fish only showed significant migrations in inshore sites; in those cases, four taxa exhibited reverse DVM patterns, suggesting predator-avoidant migration behavior. Boosted regression tree (BRT) models constructed on ten target taxa, with a range of biotic, abiotic, and site explanatory variables, suggest that predator concentration (mainly comprised of GZ and chaetognaths) was the dominant variable driving distributions. These results add to our understanding of the role of gelatinous and invertebrate predators in driving fine-scale vertical structure in a plankton community. A spatially-explicit carbon-cycle model was constructed using a single, annual time-step, using inputs from the Jellyfish Database Initiative, to evaluate the relative contributions of jelly-mediated carbon within the upper ocean and to the seafloor. Biomass export flux was the largest flux in the upper ocean, totaling 2.96 ± 2.82 Pg C y-1, roughly equivalent to 25% of all mesozooplankton production flux. Egestion flux from the upper ocean totaled 2.56 ± 3.35 Pg C y-1, with over 80% being fast-sinking tunicate fecal pellets. Due to fast sinking rates of carcasses and fecal pellets, 26% of all C export from the upper ocean reached the seafloor. Since GZ biomass export flux is not considered in global POC flux calculations, its addition could increase current global estimates of POC surface flux by 20-25%. This is the first estimate of the role of GZ in the global carbon cycle. This dissertation enhances our understanding of the abundance and fine-scale distribution of gelatinous zooplankton, particularly the small organisms, with emphasis on the various biophysical processes that structure zooplankton populations, and reveals that the biggest global flux of jelly-mediated carbon is through carcass export to depth. This work provides new insights on the large role that gelatinous zooplankton can play in marine communities and ecosystems, and suggests that this is a group of organisms that can no longer be ignored in studies of the global oceans.

Keywords

gelatinous zooplankton; fine-scale distribution; imaging systems; ISIIS; plankton imaging; fronts; diel vertical migration; carbon cycle; carbon pump; biogeochemistry; jellyfish; zooplankton; trophic ecology; marine community ecology

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