Light availability rather than Fe controls the magnitude of massive phytoplankton bloom in the Amundsen Sea polynyas, Antarctica

Abstract

Amundsen Sea polynyas are among the most productive, yet climate-sensitive ecosystems in the Southern Ocean and host massive annual phytoplankton blooms. These blooms are believed to be controlled by iron fluxes from melting ice and icebergs and by intrusion of nutrient-rich Circumpolar Deep Water, however the interplay between iron effects and other controls, such as light availability, has not yet been quantified. Here, we examine phytoplankton photophysiology in relation to Fe stress and physical forcing in two largest polynyas, Amundsen Sea Polynya (ASP) and Pine Island Polynya (PIP), using the combination of highresolution variable fluorescence measurements, fluorescence lifetime analysis, photosynthetic rates, and Feenrichment incubations. These analyses revealed strong Fe stress in the ASP, whereas the PIP showed virtually no signatures of Fe limitation. In spite of enhanced iron availability in the PIP, chlorophyll biomass remained $ 30-50% lower than in the Fe-stressed ASP. This apparent paradox would not have been observed if iron were the main control of phytoplankton bloom in the Amundsen Sea. Long-term satellite-based climatology records revealed that the ASP is exposed to significantly higher solar irradiance levels throughout the summer season, as compared to the PIP region, suggesting that light availability controls the magnitude of phytoplankton blooms in the Amundsen Sea. Our data suggests that higher Fe availability (e.g., due to higher melting rates of ice sheets) would not necessarily increase primary productivity in this region. Furthermore, stronger wind-driven vertical mixing in expanding ice-free areas may lead to reduction in light availability and productivity in the future.

Light Is the Primary Driver of Early Season Phytoplankton Production Along the Western Antarctic Peninsula

“…The inverse correlation we observed between dFe concentrations and NPP (Figure i) indicates that during spring, phytoplankton were growing on the available dFe, resulting in low dFe concentrations in areas with high NPP. As described above, Park et al () also found that light, instead of dFe, controlled summer phytoplankton biomass in the Pine Island Polynya (Amundsen Sea), consistent with our findings in the WAP. Nearby in Ryder Bay, Annett et al () reported sufficient dFe concentrations for complete macronutrient drawdown, indicating that dFe concentration would not limit phytoplankton growth throughout the growing season.…”

“…Arrigo et al () showed that net phytoplankton growth occurs in stratified regions with low ice cover, where there is sufficient light in the mixed layer, indicating that in the spring light appears to be the most important factor controlling phytoplankton growth. Similarly, in the Amundsen Sea's Pine Island Polynya, Park et al () found that light rather than dFe controls the magnitude of summer phytoplankton blooms. In both of these cases, dFe was sufficient so as to not limit phytoplankton growth: in the springtime WAP, this was because dFe resupplied during the winter had not yet been fully consumed (this study; Arrigo et al, ); in the summertime Pine Island Polynya, continual glacial melt provided sufficient dFe input so as to not limit growth (Park et al, ).…”

“…Similarly, in the Amundsen Sea's Pine Island Polynya, Park et al () found that light rather than dFe controls the magnitude of summer phytoplankton blooms. In both of these cases, dFe was sufficient so as to not limit phytoplankton growth: in the springtime WAP, this was because dFe resupplied during the winter had not yet been fully consumed (this study; Arrigo et al, ); in the summertime Pine Island Polynya, continual glacial melt provided sufficient dFe input so as to not limit growth (Park et al, ). Our results add further support to the conclusion that light is the dominant controlling factor for phytoplankton growth in the WAP in the spring, although there are regions where dFe limits growth in both the spring and summer (see, for example, the Amundsen Sea Polynya in Alderkamp et al () and Park et al ()).…”

“…Despite the negative correlation between heterotrophic respiration and dFe, remineralization of organic matter may provide a small source of regenerated iron throughout the growing season (Boyd et al, ). Depending on the wintertime dFe concentration, NPP may become dFe‐limited as the season progresses (Annett et al, ; Park et al, ). The inverse correlation we observed between dFe concentrations and NPP (Figure i) indicates that during spring, phytoplankton were growing on the available dFe, resulting in low dFe concentrations in areas with high NPP.…”

Testing the Canyon Hypothesis: Evaluating light and nutrient controls of phytoplankton growth in penguin foraging hotspots along the West Antarctic Peninsula

“…All samples were kept under dim light conditions (ca. 5 μ mol quanta m −2 s −1 ) at in situ temperatures for > 30 min to alleviate the effects of nonphotochemical quenching and photoinhibition (Park et al ). Fluorescence response was analyzed using FPRO Software developed by M. Gorbunov.…”