Shelf-sea gross and net production estimates from triple oxygen isotopes and oxygen-argon ratios in relation with phytoplankton physiology

Seguro Requejo, Maria Isabel (2017) Shelf-sea gross and net production estimates from triple oxygen isotopes and oxygen-argon ratios in relation with phytoplankton physiology. Doctoral thesis, University of East Anglia.

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Shelf seas represent only 10 % of the ocean area, but support 30 % of oceanic primary production. There are few measurements of biological production at high spatial and temporal resolution in such physically dynamic systems. Here, I use dissolved oxygento- argon (O2/Ar) ratios and triple oxygen isotopes (δ(17O), δ(18O)) to estimate net and gross biological production seasonally in the Celtic Sea between summer 2014 and summer 2015, as part of the NERC Shelf-Sea Biogeochemistry programme. O2/Ar was measured continuously using a shipboard membrane inlet mass spectrometer. Discrete water samples from hydrocasts were used to measure O2/Ar, δ(17O) and δ(18O) depth profiles. The data were combined with wind-speed based gas exchange parameterisations to calculate biological air-sea oxygen fluxes. These fluxes were corrected for non-steady state and diapycnal diffusion to give net community production (N(O2/Ar)) and gross O2 production (G(17O)). N(O2/Ar) was highest in spring at (33±41) mmol m-2 d-1, and G(17O) was highest in summer at (494±370) mmol m-2 d-1, while autumn was net heterotrophic with N(O2/Ar) = (–14±28) mmol m-2 d-1. During spring, biological production was spatially heterogeneous, highlighting the importance of high resolution biological production measurements. The ratio of N(O2/Ar) to G(17O), ƒ(O2), was highest in spring at 0.18±0.03 corresponding to 0.34±0.06 in carbon equivalents; about 0.05 in summer and < 0 in autumn/winter. Statistical measurement uncertainties increase when terms other than air-sea exchange fluxes are included in the calculations.
Additionally, electron transfer rate derived from fast repetition rate fluorometry measurements was compared with G(17O), but no simple relationship was found. This study characterised the seasonal biological patterns in production rates and shows that the Celtic Sea is a net carbon sink in spring and summer. Such measurements can help reconcile the differences between satellite and in situ productivity estimates, and improve our understanding of the biological carbon pump.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Environmental Sciences
Depositing User: Megan Ruddock
Date Deposited: 19 Dec 2018 13:44
Last Modified: 20 Dec 2021 01:38


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