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.
Highlights
As the euphotic zone starts to deepen until enhanced light attenuation by blooming phytoplankton reverses this process (Figure 5, Oziel et al, 2019), the increasing difference in light transmittance through ponded versus bare ice, combined with the lateral spreading of photons within the ice layer, create large fluctuations of up to 43% in under-ice photosynthetically active radiation (PAR) levels for drifting phytoplankton communities in the upper ocean beneath landfast sea ice (Matthes et al, 2020)
The high nutrient availability further supports large cell sizes, phytoplankton cells are heavily packed with pigments resulting in reduced cross sectional absorption due to self-shading
According to the observations by Palmer et al (2011), communities were able to acclimate to the highlight environment in the surface water after 4–10 days while phytoplankton at the subsurface chlorophyll maximum (SCM) remained shade-acclimated with comparable α and Pmax to those of under-ice communities
Summary
The idea of phytoplankton blooms developing underneath an ice cover, called under-ice blooms (UIBs), had been sporadically reported in the past (see below) but became widely accepted by the scientific community only after a massive UIB was reported in the Chukchi Sea (Arrigo et al, 2012, 2014). The two separate drift stations, Alpha and Bravo, drifting on opposite sides of the Beaufort High Gyre, showed surprisingly similar results (Figure 1) Both sites had a 3 m thick multiyear ice cover with hummocks reaching more than 9 m thick and ∼1% lead coverage (estimates by aircraft; Apollonio, 1959; English, 1961). Apollonio (1959) concluded that light availability controlled phytoplankton production, the nitrogen-depleted upper 150 m water column, with NO3:PO4 ratios averaging 7:1 relative to the Redfield ratio of 16:1 (Redfield et al, 1963), would constrain the central Arctic to have one of the lowest productivities in the world even if light limitation was not a prime factor Since those early studies, UIBs in the Arctic have received a modest amount of attention based on the number of times those features have been described in the literature (Table 1 and Figure 2). As a consequence of a warming Arctic climate, Arctic sea ice melts earlier and more widely, leading to Arctic-wide reductions in sea-ice thickness and age (Kwok, 2018; Stroeve and Notz, 2018)
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