The sensitivity of primary productivity to intra-seasonal mixed layer variability in the sub-Antarctic Zone of the Atlantic Ocean

Abstract

<p><strong>Abstract.</strong> The seasonal cycle of primary productivity is impacted by seasonal and intra-seasonal dynamics of the mixed layer through the changing balance between mixing and buoyancy forcing, which regulates nutrient supply and light availability. Of particular recent interest is the role of synoptic scale events in supplying nutrients, particularly iron, to the euphotic zone in the Sub Antarctic Zone (SAZ), where phytoplankton blooms occur throughout summer. In this study, we present high resolution measurements of net community production (NCP) constrained by ΔO<sub>2</sub>/Ar ratios, and mixed layer depth (MLD) in the Atlantic SAZ. We found a non-linear relationship between NCP and MLD, with the highest and most variable NCP observed in shallow MLDs (< 45 m). We propose that NCP variability in the SAZ may be driven by alternating states of synoptic-scale deepening of the mixed layer, leading to the entrainment of iron (dFe), followed by restratification, allowing rapid growth in an iron replete, high light environment. Synoptic iron fluxes into the euphotic zone based on water column dFe profiles and high resolution glider MLD data, reveal a potentially significant contribution of "new iron" which could sustain NCP throughout summer. Future process studies will help elaborate these findings further.</p>

Autonomous profiling float observations of the high-biomass plume downstream of the Kerguelen Plateau in the Southern Ocean

“…Our initial research goals included looking for oxygen supersaturations in deep chlorophyll maxima to estimate net community production (Spitzer and Jenkins, 1989), but this could not be achieved owing to confounding effects on supersaturations from strong mixing with higher productivity overlying waters, and on aliasing of daily cycles by internal waves (Park et al, 2008a). Thus our results cannot address the issues of whether productivity in subsurface layers may partly explain offsets between satellite and in situ estimates of the Southern Ocean biological pump (Schlitzer, 2002) or whether the phytoplankton that grow in deep chlorophyll maxima are preferential contributors to carbon export (Kemp et al, 2000;Queguiner, 2013). We were able to make a first simple assessment of subsurface autumn oxygen consumption during the portion of the bio-profiler #4 trajectory that delivered a quasi-Lagrangian time series, and this provided the very useful result that approximately 35 % of the biomass respiration in that period occurred beneath the mixed layer, and thus at depths favouring CO 2 export toward the ocean interior.…”

Sub‐Seasonal Forcing Drives Year‐To‐Year Variations of Southern Ocean Primary Productivity

“…Second, relating non‐seasonal SChl variations to low‐frequency climate modes presumes that high‐frequency variability averages to near‐zero on annual and longer timescales. However, many studies have documented large amplitude sub‐seasonal SChl fluctuations throughout the global ocean (Bonhomme et al., 2007; Resplandy et al., 2009), and particularly in the Southern Ocean (Fauchereau et al., 2011; Joubert et al., 2014; Little et al., 2018). Therefore, here we investigate whether these transient processes imprint on the annual mean and year‐to‐year variations of Southern Ocean primary production (Little et al., 2018).…”

Using Optical Sensors on Gliders to Estimate Phytoplankton Carbon Concentrations and Chlorophyll-to-Carbon Ratios in the Southern Ocean

“…If researchers are to accurately reflect the seasonal cycle of phytoplankton production in predictive climate models and thereby improve our understanding of the sensitivities of the biological carbon pump to changes in climate forcing factors (both needed for predicting long term trends), the Southern Ocean ecosystem has to be investigated at the appropriate scales that link the physical drivers to the biogeochemistry (Lévy et al, 2001;Le Quéré et al, 2007;Klein et al, 2008;Doney et al, 2009;Thomalla et al, 2011;Racault et al, 2012;Joubert et al, 2014;Carranza and Gille, 2015;Swart et al, 2015). There is increasing evidence in the Southern Ocean that seasonal to sub-seasonal temporal scales and meso-to submeso-spatial scales play an important role in determining the response of primary producers to physical forcing (Boyd, 2002;Fauchereau et al, 2011;Thomalla et al, 2011Thomalla et al, , 2015Lévy et al, 2012;Swart et al, 2015), which may in turn affect their sensitivity to climate change.…”