Environmental drivers of under-ice phytoplankton bloom dynamics in the Arctic Ocean

Abstract

The decline of sea-ice thickness, area, and volume due to the transition from multi-year to first-year sea ice has improved the under-ice light environment for pelagic Arctic ecosystems. One unexpected and direct consequence of this transition, the proliferation of under-ice phytoplankton blooms (UIBs), challenges the paradigm that waters beneath the ice pack harbor little planktonic life. Little is known about the diversity and spatial distribution of UIBs in the Arctic Ocean, or the environmental drivers behind their timing, magnitude, and taxonomic composition. Here, we compiled a unique and comprehensive dataset from seven major research projects in the Arctic Ocean (11 expeditions, covering the spring sea-ice-covered period to summer ice-free conditions) to identify the environmental drivers responsible for initiating and shaping the magnitude and assemblage structure of UIBs. The temporal dynamics behind UIB formation are related to the ways that snow and sea-ice conditions impact the under-ice light field. In particular, the onset of snowmelt significantly increased under-ice light availability (>0.1–0.2 mol photons m–2 d–1), marking the concomitant termination of the sea-ice algal bloom and initiation of UIBs. At the pan-Arctic scale, bloom magnitude (expressed as maximum chlorophyll a concentration) was predicted best by winter water Si(OH)4 and PO43– concentrations, as well as Si(OH)4:NO3– and PO43–:NO3– drawdown ratios, but not NO3– concentration. Two main phytoplankton assemblages dominated UIBs (diatoms or Phaeocystis), driven primarily by the winter nitrate:silicate (NO3–:Si(OH)4) ratio and the under-ice light climate. Phaeocystis co-dominated in low Si(OH)4 (i.e., NO3:Si(OH)4 molar ratios >1) waters, while diatoms contributed the bulk of UIB biomass when Si(OH)4 was high (i.e., NO3:Si(OH)4 molar ratios <1). The implications of such differences in UIB composition could have important ramifications for Arctic biogeochemical cycles, and ultimately impact carbon flow to higher trophic levels and the deep ocean.

The influence of Arctic Fe and Atlantic fixed N on summertime primary production in Fram Strait, North Greenland Sea

“…Surface primary production in the Arctic Ocean due to increased summertime light availability has increased during the past few decades 3 , 93 . Second to light limitation, Arctic Ocean net primary production seems foremost limited by the availability of fixed N 15 , 80 and may at some point hinder future increases in productivity, although changes in nutrient supply ratios and/or phytoplankton species composition may result in other nutrients such as Si, PO 4 or dFe to become limiting factors as well 11 , 94 . In the future, the Arctic Ocean is projected to have less ice coverage and become fresher and more stratified, which would reduce winter mixing depth and the associated supply of deep water nutrients 8 , 95 .…”

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

“…Arctic warming also leads to an earlier transition from snow-to melt pond-covered ice, which significantly reduces the albedo of the ice-covered Arctic Ocean (AO) earlier in the year, and closer to peak insolation of the annual solar cycle (Perovich et al, 2007). Reduced surface albedo due to a greater melt pond coverage of FYI compared to MYI (Nicolaus et al, 2012;Perovich and Polashenski, 2012) and reduced attenuation by thinner ice are crucial for the initiation of UIBs (Arrigo et al, 2012(Arrigo et al, , 2014Oziel et al, 2019;Ardyna et al, 2020). Together, these factors lead to a substantial increase in the availability of photosynthetically active radiation (PAR; 400-700 nm) for primary production in the icecovered upper ocean, making widespread UIBs a possibility that was hypothesized by Mundy et al (2009) and recently recognized (Horvat et al, 2017;Kinney et al, 2020).…”

“…Other important taxonomic groups of UIBs such as Phaeocystis are not known to form resting stages, but they have been found in single cell state throughout the winter in surface waters (Vader et al, 2015). While taxonomic composition and maximum biomass of an UIB seems to be strongly correlated with the type and amount of nutrients available (e.g., the winter nitrate:silicate ratio; Ardyna et al, 2020), the environmental cue for bloom initialization is an increase in light intensity, often caused by snow melt onset (Oziel et al, 2019;Ardyna et al, 2020) and sloughing of the ice algal community (Mundy et al, 2014).…”

“…Along the continental margins of the AO, UIBs are generally dominated by pelagic centric diatoms of the genera Chaetoceros and Thalassiosira and/or the colonial stage of the haptophyte alga Phaeocystis, with UIB magnitude decreasing and the role of Phaeocystis increasing toward the Atlantic sector (Ardyna et al, 2020). This overall pattern can be primarily attributed to the larger nutrient inventory and more upwelling-favorable conditions in the Pacific sector and the low silicate relative to nitrate concentrations in the Atlantic sector, respectively (Ardyna et al, 2020).…”

Subglacial upwelling in winter/spring increases under-ice primary production

“…CC BY 4.0 License. snow and sea ice cover (Fortier et al, 2002, Mundy et al, 2009, Ardyna et al, 2020. Some blooms have been observed, mostly under snow-free sea ice, such as after snow melt (Fortier et al, 2002), under melt ponds (Arrigo et al, 2012, Arrigo et al, 2014, after rain events (Fortier et al, 2002), or at the ice edge related to ice edge driven upwelling (Mundy et al, 2009).…”