Modelled estimates of spatial variability of iron stress in the Atlantic sector of the Southern Ocean

Abstract

Abstract. The Atlantic sector of the Southern Ocean is characterized by markedly different frontal zones with specific seasonal and sub-seasonal dynamics. Demonstrated here is the effect of iron on the potential maximum productivity rates of the phytoplankton community. A series of iron addition productivity versus irradiance (PE) experiments utilizing a unique experimental design that allowed for 24 h incubations were performed within the austral summer of 2015/16 to determine the photosynthetic parameters α B , P B max and E k . Mean values for each photosynthetic parameter under iron-replete conditions were 1.46 ± 0.55 (µg (µg Chl a) −1 h −1 (µM photons m −2 s −1 ) −1 ) for α B , 72.55 ± 27.97 (µg (µg Chl a) −1 h −1 ) for P B max and 50.84 ± 11.89 (µM photons m −2 s −1 ) for E k , whereas mean values under the control conditions were 1.25 ± 0.92 (µg (µg Chl a) −1 h −1 (µM photons m −2 s −1 ) −1 ) for α B , 62.44 ± 36.96 (µg (µg Chl a) −1 h −1 ) for P B max and 55.81 ± 19.60 (µM photons m −2 s −1 ) for E k . There were no clear spatial patterns in either the absolute values or the absolute differences between the treatments at the experimental locations. When these parameters are integrated into a standard depth-integrated primary production model across a latitudinal transect, the effect of iron addition shows higher levels of primary production south of 50 • S, with very little difference observed in the subantarctic and polar frontal zone. These results emphasize the need for better parameterization of photosynthetic parameters in biogeochemical models around sensitivities in their response to iron supply. Future biogeochemical models will need to consider the combined and individual effects of iron and light to better resolve the natural background in primary production and predict its response under a changing climate.

Seasonal regulation of the coupling between photosynthetic electron transport and carbon fixation in the Southern Ocean

“…This most often occurred within the winter experiments, and as such the values reported here may be underestimations of the true values for the phytoplankton assemblage. Despite this potential underestimation, the values reported here are in good agreement for PE experiments conducted, which employed similar methodology ( 13 C‐uptake, 4–24 h incubation time), within this region (Ryan‐Keogh et al b ; Thomalla et al ). α B (mol C mol Chl a −1 s −1 ( μ mol photons m −2 s −1 ) −1 ) ranged between 0.2 and 0.6 in winter, while in summer the range was 1–2.3; resulting in a mean value that was 4 times higher than winter.…”

“…The potential driver of the observed variability within the 13 C‐Uptake PE parameters was Chl a (Fig. ), with significant positive and negative correlations with α B and E k ( p < 0.05, n = 16), which is surprising considering the lack of relationships found in previous studies in this region (Ryan‐Keogh et al b ; Thomalla et al ). However, no significant relationships are found with α B and Chl a when examining either winter or summer alone, potentially suggesting it's an artefact of the specific seasonal ranges in α B .…”

“…4). One measure of excess energy dissipation can be approximated through non-photochemical quenching (NPQ) (Falkowski and Kiefer 1985;Falkowski et al 1986;Falkowski 1992;Falkowski and Kolber 1995;Krause and Jahns 2004;Sackmann et al 2008), where NPQ NSV (normalized Stern-Volmer quenching coefficient) is calculated as F o 0 /F v 0 (Mckew et al 2013). Increases in NPQ NSV and the induction of alternative electron pathways are linked mechanistically, i.e., alternative electron pathways increase the trans-thylakoid membrane pH gradient triggering NPQ NSV in pigment antenna complexes (Falkowski and Raven 2007;Nawrocki et al 2015;Saroussi et al 2016).…”

“…A study by Schuback et al () in the more comparable Arctic environment determined that under iron‐limited conditions Φ e:C can vary between 13 (mol e − mol C −1 ) and 28 (mol e − mol C −1 ), with iron limitation increasing the potential Φ e:C for phytoplankton. Due to the importance of iron in regulating phytoplankton PP in the Southern Ocean (De Baar et al ; Martin et al ; Hopkinson et al ; Moore et al ; Hiscock et al ; Feng et al ; Smith and Donaldson ; Ryan‐Keogh et al 2017 a , b ) a similar degree of variability in Φ e:C is expected in the Southern Ocean. Furthermore, light availability is also a significant factor determining PP rates in the Southern Ocean (Moore et al ), and Φ e:C has been shown to co‐vary with light (Zhu et al ).…”

Seasonal development of iron limitation in the sub-Antarctic zone

“…Under low-light conditions phytoplankton can maximize photosynthesis in different ways by either increasing the size or number of their photosynthetic units, the latter resulting in an increase in iron requirements under low light (Maldonado et al, 1999;Raven, 1990;Strzepek et al, 2011Strzepek et al, , 2012Sunda and Huntsman, 1997). This close coupling of light and iron, which increases the cellular demand for iron under low-light conditions, can diminish lightdependent photosynthesis when iron concentrations are too low to support growth (Hiscock et al, 2008;Moore et al, 2013;Ryan-Keogh et al, 2017b). Iron is also required in the function of both nitrate and nitrite reductase (de Baar et al, 2005), which function to facilitate the assimilation of nitrate and nitrite and their subsequent intracellular reduction to ammonium.…”