Date of Award

2018

Document Type

Open Access Dissertation

Department

Marine Science

First Advisor

Claudia Benitez-Nelson

Abstract

Particle fluxes are an integral part of marine biogeochemical cycling and mediate the transfer of material from the surface ocean to depth. Throughout this process, particles are subjected to a suite of biological and physical processes that influence element and compound composition. Understanding these myriad factors is therefore critical for examining an array of marine biogeochemical questions that range from the role of particles in sequestering anthropogenic carbon dioxide, to serving as a food source and as vector for the removal and/or bioaccumulation of toxic chemicals. This thesis examines spatial and temporal changes in particle formation and export of nutrients, carbon, the neurotoxin domoic acid, and mercury using a combination of water column 238U-234Th disequilibria, in situ pumps, and sediment traps. In Chapter 2, water column bulk Pseudo-nitzschia abundance and dissolved and particulate domoic acid (DA) concentrations were measured in the coastal waters of the Santa Barbara Basin (SBB), California from 2009-2013 and compared to bulk Pseudonitzschia abundance and DA concentrations and fluxes in sinking particles collected using moored sediment traps at 147 m and 509 m. Our results indicate that while a variety of Pseudo-nitzschia spp. exist in the SBB, specific species likely drive toxicity in response to a suite of environmental conditions that are complicated by physical processes and bloom stage. This work demonstrates that dissolved DA is a significant component of the water column and should not be ignored when examining potential vi allelopathic impacts on competitors or deterrents to grazers. Water column Pseudonitzschia abundance and pDA concentrations were poorly correlated with sediment trap concentrations and fluxes, with Pseudo-nitzschia trap fluxes decreasing by an order of magnitude with increasing depth and DA fluxes increasing by a factor of three. However, DA toxicity is likely associated with senescent blooms that rapidly sink to the seafloor, adding another potential source of DA to benthic organisms. In Chapter 3, the downward fluxes of particulate carbon (PC), nitrogen (PN) and total mercury (PHg) were measured at Station ALOHA in the North Pacific Subtropical Gyre using a combination of sediment traps and in situ pumps coupled with 238U -234Th disequilibria. While highest absolute PC fluxes were observed during the summer, our findings indicate that zooplankton exert more influence over both small and large PC (and PN) fluxes in February than in September or May, when heterotrophic bacteria play a proportionally larger role in particle remineralization and cycling. PHg fluxes were also seasonally variable and highest in May and September yet were influenced by different biogeochemical cycling processes than PC in the subsurface. PHg fluxes in May and September were higher than those previously measured in the equatorial Pacific and continue to be high (> 250 pmol Hg m-2 d-1) down to 400 m, thereby providing a significant source of Hg that may be incorporated into the mesopelagic food web. In Chapter 4, the role of dissolved oxygen in PC, PN, and PHg was explored using a combination of sediment traps and in situ pumps coupled with 238U -234Th disequilibria by sampling a transect within the central North Pacific Ocean along 155oW between 5 and 17oN. Results indicate that particle fluxes at 150 m throughout this region are very low, among the lowest measured. Water column profiles of fluorescence across vii both oxygenated waters (5oN) and across the oxygen minimum zone (8oN) were similar, yet the magnitude of particle export was significantly higher in oxygenated waters, by a factor of 3 to 7 for PC, PN, and PHg. Furthermore, the peak in PHg fluxes occurred below the depth of maximum PC export. Combined, our results suggest that PC and PN production may have been influenced by a lack of small grazers, while PHg fluxes were additionally influenced by enhanced solubility in low oxygen waters. Thus, while oxygenation may influence particle production and remineralization rates in the upper water column, fluxes across 150 m remain low regardless of oxygen concentration

Available for download on Tuesday, February 11, 2020

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