Importance of deep mixing and silicic acid in regulating phytoplankton biomass and community in the iron-limited Antarctic Polar Front region in summer

Abstract

a b s t r a c tPhytoplankton community structure and their physiological response in the vicinity of the Antarctic Polar Front (APF; 44°S to 53°S, centred at 10°E) were investigated as part of the ANT-XXVIII/3 Eddy-Pump cruise conducted in austral summer 2012. Our results show that under iron-limited ð o 0:3 μmol m À 3 Þ conditions, high total chlorophyll-a (TChl-a) concentrations ð 40:6 mg m À 3 Þ can be observed at stations with deep mixed layer ð 4 60 mÞ across the APF. In contrast, light was excessive at stations with shallower mixed layer and phytoplankton were producing higher amounts of photoprotective pigments, diadinoxanthin (DD) and diatoxanthin (DT), at the expense of TChl-a, resulting in higher ratios of (DD þDT)/ TChl-a. North of the APF, significantly lower silicic acid (Si(OH) 4 ) concentrations ð o 2 mmol m À 3 Þ lead to the domination of nanophytoplankton consisting mostly of haptophytes, which produced higher ratios of (DD þDT)/TChl-a under relatively low irradiance conditions. The Si(OH) 4 replete ð 4 5 mmol m À 3 Þ region south of the APF, on the contrary, was dominated by microphytoplankton (diatoms and dinoflagellates) with lower ratios of (DD þDT)/TChl-a, despite having been exposed to higher levels of irradiance. The significant correlation between nanophytoplankton and (DD þDT)/TChl-a indicates that differences in taxon-specific response to light are also influencing TChl-a concentration in the APF during summer. Our results reveal that provided mixing is deep and Si(OH) 4 is replete, TChl-a concentrations higher than 0:6 mg m À 3 are achievable in the iron-limited APF waters during summer.

Eddy‐induced carbon transport across the Antarctic Circumpolar Current

“…[] emphasized the importance of deep mixing for supplying iron to surface waters of the Southern Ocean, but Cheah et al . [] suggested that deep mixing of silicic acid as well as iron may regulate phytoplankton productivity between the SAF and the Polar Front. The surface seawater silicate concentrations we measured in the eddy were as low as 3 μmol L −1 despite doming isopycnals (Figure b) and nitrate concentrations above 20 μmol L −1 (Figure a).…”

The Weddell Gyre, Southern Ocean: Present Knowledge and Future Challenges

“…However, the relationship between primary productivity and mixed layer depths is far from linear. Cells mixed up to the surface may experience photoinhibition, for example, a physiological response contributing to decreased photosynthetic efficiency (Alderkamp et al, ; Cheah et al, ), as a result of excessive light availability. On the other hand, a deepening of the mixed layer can also entrain Fe‐rich waters from depth and therefore increase productivity during Fe‐limited conditions (de Jong et al, ; Tagliabue et al, ).…”

Effects of iron and light availability on phytoplankton photosynthetic properties in the Ross Sea

“…Consequently, phytoplankton growing in polar waters have some of the highest per cell concentrations of chl a and a high degree of pigment packaging . However, this enhanced capacity for light absorption makes them vulnerable to photodamage when light levels rapidly increase , Cheah et al 2017, such as when they are mixed vertically to the surface on cloud-free days or when sea ice cover suddenly disappears, exposing the surface ocean to light levels an order of magnitude higher than those at depth or beneath the ice (Perovich & Polashenski 2012). Phytoplankton have a number of strategies for dealing with transient (hours) increases in potentially damaging light levels, including heat dissipation via the xanthophyll pigment cycle, alteration of PS II to PS I ratios, adjustment of RUBISCO activity, and photorepair of the D1 reaction center protein (MacIntyre et al 2000, Lavaud et al 2004, Pfannschmidt 2005, Kropu enske et al 2010.…”