Fe availability drives phytoplankton photosynthesis rates during spring bloom in the Amundsen Sea Polynya, Antarctica

Abstract

To evaluate what drives phytoplankton photosynthesis rates in the Amundsen Sea Polynya (ASP), Antarctica, during the spring bloom, we studied phytoplankton biomass, photosynthesis rates, and water column productivity during a bloom of Phaeocystis antarctica (Haptophyceae) and tested effects of iron (Fe) and light availability on these parameters in bioassay experiments in deck incubators. Phytoplankton biomass and productivity were highest (20 µg chlorophyll a L −1 and 6.5 g C m −2 d −1 ) in the central ASP where sea ice melt water and surface warming enhanced stratification, reducing mixed layer depth and increasing light availability. In contrast, maximum photosynthesis rate (P * max ), initial light-limited slope of the photosynthesis-irradiance curve (a *), and maximum photochemical efficiency of photosystem II (F v / F m ) were highest in the southern ASP near the potential Fe sources of the Dotson and Getz ice shelves. In the central ASP, P * max , a *, and F v / F m were all lower. Fe addition increased phytoplankton growth rates in three of twelve incubations, and at a significant level when all experiments were analyzed together, indicating Fe availability may be rate-limiting for phytoplankton growth in several regions of the ASP early in the season during build-up of the spring bloom. Moreover, Fe addition increased P * max , a*, and F v / F m in almost all experiments when compared to unamended controls. Incubation under high light also increased P * max , but decreased F v / F m and a * when compared to low light incubation. These results indicate that the lower values for P * max , a*, and F v / F m in the central ASP, compared to regions close to the ice shelves, are constrained by lower Fe availability rather than light availability. Our study suggests that higher Fe availability (e.g., from higher melt rates of ice shelves) would increase photosynthesis rates in the central ASP and potentially increase water column productivity 1.7-fold, making the ASP even more productive than it is today.

Particle flux on the continental shelf in the Amundsen Sea Polynya and Western Antarctic Peninsula

“…Large diatom frustules were not apparent, consistent with cruise observations that the phytoplankton bloom in 2010-2011 was heavily dominated by Phaeocystis colonies (Alderkamp et al, 2015). Some fecal "strings" from the ice krill E. crystallarophias and large calanoid copepod C. acutus were apparent during the early part of the first peak, and prior to the second.…”

In situ phytoplankton distributions in the Amundsen Sea Polynya measured by autonomous gliders

“…Thus, given the high biomass and depth of the upper mixed layer, the majority of the phytoplankton biomass resided well below the 1% light level at any given time. Photosynthesis-irradiance curves indicated the light saturation intensity for photosynthesis (E k ) ranged from 37 to 67 μmol photons m −2 s −1 (Alderkamp et al, 2015), which was consistent with low-light-acclimated phytoplankton populations when compared to average water column measurements of E k in other pelagic and coastal locations in the Southern Ocean (Moline et al, 1998). While low salinity AASW was associated with regions of high phytoplankton biomass, it was not a guarantee of high phytoplankton biomass.…”

“…Given the minor shifts in water properties and transport, if we assume biomass accumulation represents the net growth rate of the phytoplankton, this observed increase translates to a net growth rate of ∼ 0.10 d −1 . During the ASPIRE cruise deckboard incubations of natural phytoplankton populations over a range of modified light levels and micronutrient conditions exhibited growth rates that ranged from 0.07 to 0.28 d −1 (Alderkamp et al, 2015). Lowest growth rates were observed for low light (1% light incubation levels) and low iron (Fe) conditions consistent with the ambient conditions of the AASW (Alderkamp et al, 2015).…”

Seasonal sea ice changes in the Amundsen Sea, Antarctica, over the period of 1979–2014

“…Variability in the import of thick multi-year sea ice from the southern Bellingshausen Sea into the eastern Amundsen Sea, together with variability in the number of icebergs on the outer continental shelf that retain summer sea ice, were other factors contributing to PIP/ASP variability. The sPIP and eASP sea ice changes are important, as they affect the timing, duration and size of these two polynyas and thus impact this biologically productive area (Arrigo and van Dijken, 2003;Arrigo et al, 2012;Alderkamp et al, 2015;Mu et al, 2015), as well as possibly indicating a response to changes in heat and freshwater inputs (Randall-Goodwin et al, 2015;Sherrell et al, 2015).…”