Controls of primary production in two phytoplankton blooms in the Antarctic Circumpolar Current

Abstract

The Antarctic Circumpolar Current has a high potential for primary production and carbon sequestration through the biological pump. In the current study, two large-scale blooms observed in 2012 during a cruise with R.V. Polarstern were investigated with respect to phytoplankton standing stocks, primary productivity and nutrient budgets. While net primary productivity was similar in both blooms, chlorophyll a –specific photosynthesis was more efficient in the bloom closer to the island of South Georgia (39 °W, 50 °S) compared to the open ocean bloom further east (12 °W, 51 °S). We did not find evidence for light being the driver of bloom dynamics as chlorophyll standing stocks up to 165 mg m−2 developed despite mixed layers as deep as 90 m. Since the two bloom regions differ in their distance to shelf areas, potential sources of iron vary. Nutrient (nitrate, phosphate, silicate) deficits were similar in both areas despite different bloom ages, but their ratios indicated more pronounced iron limitation at 12 °W compared to 39 °W. While primarily the supply of iron and not the availability of light seemed to control onset and duration of the blooms, higher grazing pressure could have exerted a stronger control toward the declining phase of the blooms.

High particulate organic carbon export during the decline of a vast diatom bloom in the Atlantic sector of the Southern Ocean

“…Hence, the fluxes were high and quite similar over the entire study area, despite the differences observed in Chl-a SW inventories (see Section 3.5) and production rates (Hoppe et al, 2017). However, satellite images show widespread high Chl-a concentrations in January 2012 (data not shown).…”

“…comm.). Concentrations of dissolved iron (o0.2 μm) were on average 0.12 70.03 nM within the top 100 m of the water column (Hoppe et al, 2017), suggesting that dissolved iron was already depleted by phytoplankton activity during our sampling.…”

“…The euphotic zone depth ranged between 36 and 66 m (average: 457 7 m, n¼ 40, Cheah et al, 2017). Nutrient concentrations and deficits are described in Hoppe et al (2017). On average, macronutrient concentrations within the euphotic zone were $ 21 mmol NO 3 m À 3 , $ 1.4 mmol PO 4 m À 3 and $ 6.6 mmol Si(OH) 4 m À 3 .…”

“…1), whereas at the peak of the bloom, during the first half of January, the Chl-a concentrations were $ 3 mg m À 3 (Hoppe et al, 2017). The euphotic zone depth ranged between 36 and 66 m (average: 457 7 m, n¼ 40, Cheah et al, 2017).…”

“…Export fluxes were related to the evolution of chlorophyll a (Chl-a) and POC concentrations in the water column, as well as to pigments in sinking particles, following the decline of the bloom during three weeks. Net primary production (NPP) measured during the same cruise (Hoppe et al, 2017) was used to assess the export efficiency.…”