Under-Ice Phytoplankton Blooms: Shedding Light on the “Invisible” Part of Arctic Primary Production

Abstract

The growth of phytoplankton at high latitudes was generally thought to begin in open waters of the marginal ice zone once the highly reflective sea ice retreats in spring, solar elevation increases, and surface waters become stratified by the addition of sea-ice melt water. In fact, virtually all recent large-scale estimates of primary production in the Arctic Ocean (AO) assume that phytoplankton production in the water column under sea ice is negligible. However, over the past two decades, an emerging literature showing significant under-ice phytoplankton production on a pan-Arctic scale has challenged our paradigms of Arctic phytoplankton ecology and phenology. This evidence, which builds on previous, but scarce reports, requires the Arctic scientific community to change its perception of traditional AO phenology and urgently revise it. In particular, it is essential to better comprehend, on small and large scales, the changing and variable icescapes, the under-ice light field and biogeochemical cycles during the transition from sea-ice covered to ice-free Arctic waters. Here, we provide a baseline of our current knowledge of under-ice blooms (UIBs), by defining their ecology and their environmental setting, but also their regional peculiarities (in terms of occurrence, magnitude, and assemblages), which is shaped by a complex AO. To this end, a multidisciplinary approach, i.e., combining expeditions and modern autonomous technologies, satellite, and modeling analyses, has been used to provide an overview of this pan-Arctic phenological feature, which will become increasingly important in future marine Arctic biogeochemical cycles.

Changes in Under‐Ice Primary Production in the Chukchi Sea From 1988 to 2018

“…More frequent observations of UIBs in recent years (Ardyna et al, 2020) and model results by Horvat et al (2017) lend credence to the hypothesis that changing ice conditions have led to an increased incidence of UIBs in recent years (Arrigo et al, 2014). However, our model demonstrates that, as early as 1988, conditions in the northern Chukchi Sea were amenable to UIB formation.…”

“…More frequent observations of UIBs in recent years (Ardyna et al., 2020) and model results by Horvat et al. (2017) lend credence to the hypothesis that changing ice conditions have led to an increased incidence of UIBs in recent years (Arrigo et al., 2014).…”

“…Indeed, observations of UIBs dating back to at least 1957 (English, 1961) indicate that UIBs have been generated sporadically for decades. However, as thinner, younger sea ice has come to dominate much of the Arctic Ocean in recent years (Laxon et al., 2013; Serreze & Stroeve, 2015), observations of UIBs have grown to include most of the Arctic Ocean (Ardyna et al., 2020). It is quite likely that continued changes in sea ice cover will only make UIBs more widespread in the Arctic Ocean in the future (Horvat et al., 2017), further demonstrating the critical importance of better understanding NPP under sea ice.…”

Environmental drivers of spring primary production in Hudson Bay

“…Under-ice blooms occur in Hudson Bay as evidenced from our study and the historical record of blooms beneath landfast ice. However, considering the calculated mean integrated TChl a of 35.10 mg m -2 over the icecovered euphotic zone in central Hudson Bay (Table 2), phytoplankton biomass was comparable to the central Arctic Ocean, but much lower than under-ice blooms in the Arctic shelf regions (Ardyna et al, 2020).…”

The Polar Night Shift: Annual Dynamics and Drivers of Microbial Community Structure in the Arctic Ocean

“…Detection of related sequences on ice-algal aggregates 60 supports a presumed niche through utilization of ice-algal carbohydrates. Overall, these light-independent, ice-fueled processes might explain signatures and activities of specific microbial taxa in other Arctic regions under warming [108][109][110] .…”