Autonomous oceanic primary production measurements through novel chlorophyll fluorometry

Abstract

Phytoplankton Primary Production (PP) is a key index of marine ecosystem function, setting the constraint on energy flux to the marine biosphere. Extreme under-sampling limits confidence in our best estimates of the magnitude, variability, and drivers of such variability in PP. The current work introduces, assesses, and demonstrates a new active chlorophyll fluorescence measurement sensor, developed by Chelsea Technologies Group (CTG), the STAFES (Single Turnover Active Fluorometry of Enclosed Samples). These sensors measure a range of primary production parameters, based on Electron Transport Rate (ETR), and contain 7 measurement LEDS, compared to 2 on earlier versions of single turnover active fluorometers. They are designed to operate continuously and autonomously making them Marine Autonomous System (MAS) deployable. Such deployments have the potential to greatly improve confidence and crucially, spatio-temporal scales of ETR derived parameters, and PP. As a precursor to in situ deployment, the residual error inherent in STAFES, post spectral correction was investigated over a range of assumed and measured phytoplankton absorption spectra. These analyses were performed using a series of hypothetical calculations using idealised underwater light fields generated with the HYDROLIGHT radiative transfer model, with calculations subsequently used to produce global scale residual error estimates. Overall residual errors in the upper water column (i.e. at or above the 50% light depth) remained modest even in the most productive regions where they were highest. Hence using a 7-wavelength instrument such as STAFES and then performing spectral corrections, results in the residual error already being reduced to a level where other sources of measurement error and uncertainties in algorithms will likely dominate. The STAFES sensor potential was demonstrated by performing a series of experiments in the South East (SE) Pacific sector of the Southern Ocean. Results provided new insight into the effects of iron (Fe) and light availability on the coupling between CO₂ assimilation and photosynthetic electron transport in natural phytoplankton assemblages as quantified using the electron requirement for carbon fixation (Fe:C). Furthermore, Fe:C reduced by around 35-50% after the relief of Fe limitation, both within the light limited and saturated regions of the photosynthesis irradiance curve, expanding the overall range of such observed Fe driven variability. A first demonstration of autonomous capability was then achieved by deploying STAFES on the ALR MAS platform for four successful data gathering deployments at sea, which were validated by a range of additional sensors. The results represent the first known demonstration and analysis of spectrally resolved PEP (Photo Excitation Profile) variable fluorescence by wavelength (Fv (λ)) data using the variable fluorescence technique deployed on a fully autonomous platform. Finally, placing the results chapters in context within the final chapter allowed remaining key uncertainties to be highlighted. The current thesis thus presented a number of completely unique data sets with a new instrument / platform interface, while further highlighting future work which would be required in order to fully realise the potential of STAFES.

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Identifiers

ORCID for Mark Moore: orcid.org/0000-0002-9541-6046

ORCID for Dr Matt Mowlem: orcid.org/0000-0001-7613-6121

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