Publication Date

2013-10-30

Availability

Open access

Embargo Period

2013-10-30

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Marine Biology and Fisheries (Marine)

Date of Defense

2013-10-04

First Committee Member

Robert K. Cowen

Second Committee Member

Su Sponaugle

Third Committee Member

Peter B. Ortner

Fourth Committee Member

Mei-Ling Shyu

Fifth Committee Member

Jonathan A. Hare

Abstract

Describing the distributions of organisms on scales relevant to individuals (1-100 m) is critical to understanding predator-prey interactions within the plankton. This has driven the development of plankton imaging technology with synoptic physical parameters (temperature, salinity, depth), which facilitates high-resolution taxonomic and spatial descriptions. We utilized a novel In Situ Ichthyoplankton Imaging System (ISIIS) that addressed some shortcomings of other imaging systems by allowing for the simultaneous sampling of both abundant (e.g., copepods, appendicularians, marine snow) and rare (e.g., fish larvae, medusae, ctenophores) members of the plankton community. The main objectives of this study were 1) to describe the physical and biological characteristics of the fine-scale environments near ubiquitous coastal features (fronts, thin layers, and internal waves), and 2) how these descriptions related to trophic interactions potentially affecting the early life stages of fishes. ISIIS was deployed in three separate environments with characteristic hydrographic regimes favorable to the formation of thin layers, internal waves, and fronts. In northern Monterey Bay, thermal stratification led to the development of thin layers of diatom aggregates dominated by Pseudo-nitzschia spp. A variety of gelatinous taxa tended to aggregate within or below thin layers, while copepods seemed to avoid the thin layers and were often found near the surface. The vertical separation of predators and prey showed support for predation avoidance by copepods, with thin layers creating a strong gradient in light levels facilitating contact predation by gelatinous zooplankton at depth. The physical environment near Stellwagen Bank was dominated by a tidally driven oscillation between high stratification and internal wave activity. Copepods were found near the surface, sometimes aggregating in a thin layer several meters shallower than the chlorophyll-a maximum. Larval fishes were found to strongly correlate with the copepods, suggesting they feed on concentrations of prey much higher than average. After the passage of internal waves, larval fish correlation with prey was reduced, while predators, which were abundant at depth, had higher correlation with larval fishes. Internal waves reduced patchiness for a variety of taxa, potentially creating less favorable planktonic habitat for larval fishes. At the shelf-slope front near Georges Bank, we investigated the impact of horizontal gradients on the distribution of plankton. Almost all plankton taxa were found in high abundance on the shelf side of the front. A particle solidity metric showed distinct habitat partitioning of different plankton taxa around the front, with copepods and appendicularians forming a near surface layer just above the convergence of isopycnals defining the front. These grazers were spatially separate from diatom aggregates, which were abundant in zones of high chlorophyll-a fluorescence. The distributions of gelatinous zooplankton and fish tended to follow isopycnals that converged at the front. Taken together, this body of work shows common 5-10 meter scale vertical extent of many planktonic taxa despite the dramatic differences in hydrographic properties.

Keywords

plankton imaging; thin layers; fish larvae; fronts; internal waves; gelatinous zooplankton

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