Transparent Exopolymer Particles and Phytoplankton Nutrient Physiology in the North Pacific and Arctic Oceans
Date
2024
Authors
Livingston, Michael
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Abstract
The export of organic carbon from the surface to the deep ocean is a key process that regulates the level of carbon dioxide (CO2) in the atmosphere, and is known as the biological carbon pump (BCP). The efficiency of the BCP is largely governed by biological influences in the upper layers of the water column, including the magnitude of primary production, and the sinking rate of particulate organic carbon (POC). However, these processes are still poorly understood in large parts of the ocean. A key factor affecting the BCP is the presence of transparent exopolymer particles (TEP) in the surface ocean. These particles originate from the exudation of organic exopolymeric substances by marine phytoplankton and form sticky carbon (C) gels that facilitate the aggregation of organic matter. These particles are less dense than seawater and affect the sinking of POC from the surface to the deep ocean, and therefore play an important role in the ocean’s C cycle. The overall objective of this thesis was to quantify key biological factors that affect surface ocean C cycling and to investigate the effects of variation in environmental conditions on the strengths of these factors. Over a 4-year study, I examined the concentrations and rates of production of TEP, phytoplankton nutrient physiology, and contributions of different sized phytoplankton to nutrient cycling in the Eastern Subarctic North Pacific (ESNP), and Pacific and Central Arctic regions. Measurements of TEP and a variety of biological and environmental variables were made across all regions in the ESNP and Arctic, and total and size-fractionated nutrient uptake rates of C, nitrate (NO3-) and silicic acid (Si(OH)4) were additionally measured across the ESNP. The concentrations of TEP were largely correlated with the amount of phytoplankton biomass and productivity, but also by environmental variables such as temperature, mixed layer depth, and nutrient concentrations. We used multivariate models derived from experimental observations measured during this study to produce novel estimates of surface TEP concentrations in the ESNP from 1998 to 2018. Model results show that the concentration of C in the form of TEP (TEP-C) in the surface of oceanic regions of the ESNP is estimated to be between 5-15μg C L-1 year-round. We further measured TEP concentrations relative to C fixation by phytoplankton and export potential (i.e., new production), providing the first quantitative comparison among these variables across large spatiotemporal gradients in the ESNP. Results show that low productivity regimes are characterized by higher concentrations of TEP-C (and higher estimates of TEP-C turnover) relative to C uptake and new production, which suggests these regions may undergo less efficient C export. This thesis presents the first field-based connection between excess C consumption by phytoplankton and TEP concentrations. Size-fractionated measurements reveal that small-sized phytoplankton (< 5 μm) are responsible for most of the nutrient uptake and TEP production in the oceanic and less-productive regions of the ESNP. The contribution of the small size-class to nutrient uptake rates did not appear to be influenced by environmental variations. Overall, this work provides new perspectives on surface ocean C cycling in the ESNP by shedding light on the main predictors of TEP, predicting TEP-C concentrations, and by relating TEP-C to total primary productivity and new production. It is also the first of its kind to measure total and size-fractionated Si uptake over large spatiotemporal scales.