High-resolution net and gross biological production during a Celtic Sea spring bloom

Isabel Seguro,Alina D Marca,Suzanne J Painting,Jamie D Shutler,David J Suggett,Jan Kaiser

doi:10.1016/j.pocean.2017.12.003

Abstract

Shelf seas represent only 10% of the ocean area, but support up to 30% of all oceanic primary production. There are few measurements of shelf-sea biological production at high spatial and temporal resolution in such heterogeneous and physically dynamic systems. Here, we use dissolved oxygen-to-argon (O2/Ar) ratios and oxygen triple isotopes (16O, 17O, 18O) to estimate net and gross biological production in the Celtic Sea during spring 2015. O2/Ar ratios were measured continuously using a shipboard membrane inlet mass spectrometer (MIMS). Additional discrete water samples from CTD hydrocasts were used to measure O2/Ar depth profiles and the δ(17O) and δ(18O) values of dissolved O2. These high-resolution data were combined with wind-speed based gas exchange parameterisations to calculate biologically driven air-sea oxygen fluxes. After correction for disequilibrium terms and diapycnal diffusion, these fluxes yielded estimates of net community (N(O2/Ar)) and gross O2 production (G(17O)). N(O2/Ar) was spatially heterogeneous and showed predominantly autotrophic conditions, with an average of (33 ± 41) mmol m−2 d−1. G(17O) showed high variability between 0 and 424 mmol m−2 d−1. The ratio of N(O2/Ar) to G(17O), ƒ(O2), was (0.18 ± 0.03) corresponding to 0.34 ± 0.06 in carbon equivalents. We also observed rapid temporal changes in N(O2/Ar), e.g. an increase of 80 mmol m−2 d−1 in <6 h during the spring bloom, highlighting the importance of high-resolution biological production measurements. Such measurements will help reconcile the differences between satellite and in situ productivity observations, and improve our understanding of the biological carbon pump.

Highlights

Phytoplankton primary production is the main mechanism of oceanic carbon fixation
We find net community production rates based on continuous membrane-inlet mass spectrometry measurements of oxygen-to-argon ratios, N(O2/Ar), of up to 144 mmol m-2 d-1 in April 2015, with an average of (33±41) mmol m–2 d-1
When integrating over larger areas, Fbio may present a faithful representation of the metabolic balance of the Celtic Sea as a whole

Summary

Introduction

Phytoplankton primary production is the main mechanism of oceanic carbon fixation. shelf seas comprise only 10 % of the world's oceans by area, they contribute 15 to 30 % of ocean primary production (Hickman et al, 2012). Using the measured O2/Ar supersaturation and wind-speed based air-sea gas exchange parameterisations, we calculate biological O2 air-sea fluxes (Fbio), correct them for diapycnal diffusion and disequilibrium terms to estimate mixed layer net community production rates These measurements are combined with shore-based analyses of discrete samples for oxygen triple isotope ratios to derive gross O2 production rates (Luz et al, 1999). Such measurements can serve to validate satellite ocean colour productivity estimates, and feed models of the carbon pump to predict the impact of climate change This will improve our understanding of primary production variability and potential impacts of human activities in the temperate shelf seas

Methods

Results

Discussion

Conclusion