Date of Award

2020

Degree Type

Thesis

Degree Name

Master of Science in Oceanography

Specialization

Physical Oceanography

Department

Oceanography

First Advisor

Melissa Omand

Abstract

Stimulated chlorophyll fluorescence (ChlF) is a fairly simple measurement to collect, and is ubiquitous on most biogeochemical autonomous platforms. However, complex physiological phenomenon challenge the interpretation of the data. Nonphotochemical quenching (NPQ) markedly reduces the fluorescence during high light regimes, favoring dissipation of this energy as heat. This study evaluates previously developed NPQ correction methods, typically implemented on floats or gliders, for a fast profiling, autonomous, drifting Wirewalker deployed during the EXPORTS field campaign at Ocean Station Papa (OSP) in the Subpolar North Pacific Ocean. Through validating NPQ-corrected chlorophyll to absorption line height, which is not subject to the same physiological effects as fluorescence, the best performing NPQ correction was developed by Thomalla et al. (2018), which uses a combination of night-time profiles of ChlF and backscatter. However, if it is preferred to correct for fluorescence quenching independently from other optical parameters, this study suggests that a linear relationship to PAR performs similarly well. A comparison between Wirewalker ChlF from OSP to data from coastal California reveals two distinct physiological effects that are not yet well understood. The OSP region shows 1) much stronger quenching, and 2) a night-time enhanced ChlF peaking near midnight. Fitting and correcting the evening ChlF relative with a Gaussian function effectively reduces the maximum fluorescence yield at night to offer a presumably more robust ChlF-based proxy for chlorophyll concentration in iron-limited systems. Overall, this work highlights the continuum of efforts that can improve in situ ChlF observations, and the challenges encountered when teasing apart physiological effects and environmental variability.

Available for download on Saturday, February 13, 2021

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