How Are Under Ice Phytoplankton Related to Sea Ice in the Southern Ocean?

Abstract

Earth's polar regions are extreme ecosystems, marked by perennial darkness and seasonal mosaics of sea ice that modify the salinity, temperature, and incoming light of subsurface waters. Recent work in the Arctic has shown that phytoplankton can thrive underneath sea ice, dwarfing previous estimates for phytoplankton productivity across the annual cycle (Arrigo et al., 2012(Arrigo et al., , 2014Assmy et al., 2017), and raising questions of how sea ice influences under ice phytoplankton.The effects of sea ice on phytoplankton in the Southern Ocean remains largely unknown as much research has focused on the Arctic Ocean, although more recent studies have expressed the possibility of widespread microbial life under Antarctic sea ice from observations (Arteaga et al., 2020;Cimoli et al., 2020;Hague & Vichi, 2021). Phytoplankton in the Southern Ocean are primarily limited by light and iron, and massive blooms under ice sea are generally not suspected in this region. However, nutrient replenishment from deeper Winter mixed layer depths combined with light at the onset of Spring may enable phytoplankton growth under ice. Still needed are assessments of how ice characteristics affect the under ice environment. On one hand, the thicker snow cover of Southern Ocean sea ice compared to the Arctic may prohibit the transmission of light to the waters below because the snow has a higher albedo than sea ice. On the other hand, most Antarctic sea ice melts in the Austral Spring and Summer (Pfirman et al., 1990), which may create a stable mixed layer and enhance growth of an already active under ice phytoplankton population previously living in deeper mixed layers (Hague & Vichi, 2021;Petty et al., 2014).

Constraining the Particle Size Distribution of Large Marine Particles in the Global Ocean With In Situ Optical Observations and Supervised Learning

“…Similar to previous work (DeVries & Weber, 2017; Siegel et al., 2014), limiting our reconstruction to regions with more complete satellite coverage leads to gaps in polar regions at times of the year characterized by significant sea ice coverage and pervasive light limitation (Siegel et al., 2002). Although based on in situ measurements some level of particle production is likely to occur in these regions and times of the year (e.g., Bisson & Cael, 2021; Lowry et al., 2018; Hague & Vichi, 2021), we lack both remote sensing and UVP5 observations that would allow robust estimates of particle abundance and size under such conditions. Future work should be devoted to closing these gaps.…”

Evidence of phytoplankton blooms under Antarctic sea ice

“…GCMs used here are too coarse to resolve the complex boundary layer dynamics that result from surface melt processes of sea ice (Holland, 2003;Horvat et al, 2016;Pellichero et al, 2017), and thus, they are not suited for determining the convective state of the upper ocean in the presence of sea ice leads. Instead, we considered the ocean to be non-convecting if sea ice was melting at its base, which would lead to stratification of the upper ocean, consistent with Argo observations of high negative covariance between a shoaling MLD and increasing phytoplankton biomass under ice (Bisson and Cael, 2021).…”

“…In the Southern Ocean, annual sea ice coverage has changed less than in the Arctic over the satellite period (Parkinson, 2019), and sea ice is typically thinner, more seasonal, and more fragmented. Yet, studies have not yet described or quantified the potential for widespread UIBs under Antarctic sea ice, although observations from under-ice biogeochemical (BGC)-Argo floats (Arteaga et al, 2020;Hague and Vichi, 2021;Bisson and Cael, 2021) demonstrate that primary production may initiate before seasonal sea ice retreat, and even before the restratification of surface waters.…”

“…Our primary in situ evidence for sub-ice phytoplankton growth is particulate backscatter-derived phytoplankton carbon (PC) and chlorophyll-a (Chl-a) fluorescence. Chl-a is a pigment common to all phytoplankton, which is historically favored as a proxy for phytoplankton biomass, including in under-ice studies (Arrigo et al, 2010;Arrigo et al, 2014;Briggs et al, 2018;Ardyna et al, 2020;Bisson and Cael, 2021). Biomass and Chl-a may not always be directly connected because of mechanistic (i.e., photoacclimation, nutrient conditions, growth stage) and methodological biases (i.e., lamp source, target volume, or calibration standard) (Haëntjens et al, 2017;Johnson et al, 2017;Roesler et al, 2017).…”

Floes, the marginal ice zone and coupled wave-sea-ice feedbacks

“…Indeed, fractured, thick sea ice was found to support phytoplankton blooms [ 101 ] in May–June, months before seasonal sea ice retreat, and lateral variability in light conditions plays an important role in driving the availability of sunlight in the upper ocean [ 6 , 102 , 103 ]. In the Southern Ocean, the floe-like mosaic that extends across the sea ice zone has a lower concentration than the Arctic, which may permit phytoplankton growth under sea ice throughout the spring and summer that is challenging to observe remotely [ 75 , 104 ].…”