Date of Award

Fall 2011

Project Type

Dissertation

Program or Major

Earth and Environmental Science: Oceanography

Degree Name

Doctor of Philosophy

First Advisor

John Ruairidh Morrison

Abstract

Understanding and quantifying phytoplankton physiological variability is essential for analyses of biogeochemical cycling, climate change and ecosystem processes. Satellite measurements of chlorophyll indicate phytoplankton biomass but hold minimal direct information about the organisms' photosynthetic capabilities: however, remote sensing-derived fluorescence quantum yields have the potential for monitoring phytoplankton photophysiological states on a global scale. Recent research has demonstrated the effects of both nutrient stress and photoacclimation on fluorescence yield. Here, a novel satellite product comprising seven years of fluorescence quantum yields, derived from MODIS/Aqua normalized fluorescence line heights and corrected for pigment packaging effects, is evaluated and explored. Fluorescence yields are highest during winter, decline to mid-summer minima and increase again during autumn; winter-spring and summer-autumn transitions exhibit different slopes at most locations. Seasonal patterns respond primarily to changes in growth irradiance, forming a closed annual cycle. Monthly climatologies show spatial patterns associated with the annual cycle and the timing of seasonal transitions are modulated by latitude. Across the northwest and northeast Atlantic shelf sea regions, fluorescence yields are lower throughout the year than in the open ocean. Interannual variability is greatest during late autumn and winter months, aligned with times of dynamic water column mixing. An empirical orthogonal function analysis of the seven year time series finds strong seasonally-variable influences with a high degree of geographic complexity. Beyond large spatial and temporal patterns, this new satellite product effectively captures short-term photoacclimation events. Increases in fluorescence quantum yield coincident with the sharp increase in water column stratification but prior to the spring chlorophyll peak were ubiquitous throughout the northern North Atlantic. During such events, phytoplankton are first acclimating to increased growth irradiance before the population bloom. At all scales, fluorescence yield trends compare well with an established conceptual model and are most strongly associated with growth irradiance dynamics rather than other forcing factors (i.e., macro- and micro-nutrient stress and phytoplankton community composition). This thesis provides the first synoptic quantification of seasonal and interannual phytoplankton photophysiological variability in the northern North Atlantic and demonstrates the potential of this new product for global investigations.

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