Patterns and drivers of marine phytoplankton change over the past century
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Marine phytoplankton produce the vast majority of primary production in the world’s oceans and sustain virtually all marine ecosystems. Despite this importance, it is currently unclear how global marine phytoplankton concentrations have been changing over the available oceanographic record, and what the causes and consequences of any such changes may be. In this thesis I use observational datasets, statistical modeling, theory, and experiments, to estimate how the global standing stock of marine phytoplankton (referenced by chlorophyll) has changed over the past century, and what the causes and consequences of any changes may be. I inter-calibrated shipboard measurements of upper ocean chlorophyll, transparency, and colour to generate a publicly-available global chlorophyll database spanning from 1890 to 2010. Generalized additive models and multi-model inference were used to estimate the magnitude and nature of changes over the available record, and to explore the effects of multiple oceanographic and climatic variables on these changes. Finally, I worked collaboratively to design and run a mesocosm experiment to test the mechanisms by which rising ocean temperatures influence phytoplankton and plankton community structure. I observed declining trends in upper ocean chlorophyll concentrations at local, regional, and global scales over the past century. Increasing trends were observed closer to coastlines, and were possibly related to increased land-based nutrient deposition there. I also observed inter-annual to multi-decadal fluctuations overlying the longterm trends, which were partly related to climate variability. Sea surface temperature was a consistently strong driver of observed chlorophyll trends. Strong negative effects of rising ocean temperatures on chlorophyll concentration were observed at mid, and low latitudes, and positive effects were observed at high latitudes. The overall effect of increasing temperature on chlorophyll was negative, yet the mesocosm experiment revealed that the primary mechanisms explaining this effect depend on the nature of the ecosystem. Under nutrient limitation, the physically-mediated effects (stratification) of increasing SST were dominant, while under nutrient saturation, the biologically-mediated effects (trophic) were dominant. This thesis provides new evidence that sustained declines in marine phytoplankton over the past century have occurred across multiple spatial scales and that rising ocean temperatures have contributed to this trend. The possible implications of this sustained decline are wide-ranging, with likely impacts on climate, geochemical cycling, fisheries, and ecosystem structure.