Ice-algal and under-ice phytoplankton bloom dynamics in a changing Arctic icescape (Boom or Bust)

The main aim of the “Boom or Bust” project is to increase our understanding of the role of ice-algal and under-ice phytoplankton blooms in the changing icescape of the high Arctic Ocean. We have therefore applied a combination of methods to 1) identify the environmental factors that determine ice-algal and phytoplankton bloom dynamics and community composition, 2) increase our understanding of the photobiology of Arctic marine algae under changing environmental conditions and 3) apply a one-dimensional (1D) vertically resolved ice and ocean model to synthesize the project findings.
The project is based on a unique time-series dataset from the high-Arctic obtained during the multidisciplinary Norwegian young sea ICE (N-ICE2015) expedition carried out in the Arctic Ocean north of Svalbard (between 80 and 83°N) from 11 January to 24 June 2015. We provide the first evidence that a large phytoplankton spring bloom was actively growing below high-Arctic sea ice early in the season despite thick snow cover. Our observations extend the spatial and temporal domain of under-ice phytoplankton blooms that have previously only been observed below transparent ice covered by melt ponds during the summer melt season. We identified leads (large openings with open water or covered by thin young ice) in the ice pack as the main conduit of added sunlight necessary to initiate and sustain the bloom. We performed in-depth studies on the bio-optical properties of thin ice covering re-frozen leads and found high concentrations of UV-protecting compounds in ice algae growing in the thin ice. Our results also indicate that algae locked up in the upper layers of multiyear ice (MYI) during winter function as an important seed stock for the ice algal spring bloom. This proposed seeding mechanism of MYI may be seriously compromised due to the disappearance of multiyear ice and the anticipated regime shift towards an ice-free Arctic Ocean in summer. We furthermore used the data collected during the N-ICE2015 expedition to evaluate the performance of the Los Alamos sea ice model (CICE) to predict temporal trends in Arctic sea ice physical and biogeochemical properties at time scales of a few weeks. In addition, results from 2016 field campaigns, including the ARCEx spring cruise (http://www.arcex.no/category/arcex-cruise-2016/), a summer cruise in Svalbard and Greenland waters and a spring bloom study in Kongsfjorden, are currently being analyzed.
Results from the Boom or Bust project will contribute to a better understanding of the ecological processes in first-year sea ice in the high Arctic and add to our understanding of the functioning of Arctic marine ecosystems in an era of rapid change. This will have important management implications as changes in Arctic primary production and algal species composition will have cascading effects on Arctic marine mammals, sea birds and commercial fish species and consequences for the carbon sequestration potential of the Arctic Ocean and thus ultimately our climate. The obtained results will be valuable to sea-ice and plankton ecologists, biologists and modelers, and those studying natural and human systems impacted by changes in Arctic sea ice.

The Arctic icescape is rapidly transforming from a thick multi-year ice cover to a thinner and largely seasonal first-year ice cover with significant consequences for Arctic primary production and algae growing inside the ice (ice algae) and in the underlying Arctic Ocean (phytoplankton). However, the effect of a thinning ice-cover on under-ice phytoplankton and ice algal blooms and primary production is not well constrained because ice algal and under-ice blooms cannot be measured from space and field observations from the high Arctic pack-ice environment are very limited. We also have a very rudimentary understanding of the environmental factors determining species composition and species dominance of high Arctic sea ice and pelagic communities. To address these knowledge gaps we will apply a combination of methods to characterize and quantify under-ice phytoplankton and ice algal communities in combination with in situ experiments on the photosynthetic performance of phytoplankton and ice algae. We will utilize a unique time-series on the ice associated-ecosystem obtained during the Norwegian young sea ICE (N-ICE) ice-drift expedition in 2015 with R/V Lance to the pack-ice north of Svalbard. As a result, we will be able to better link key physical, chemical and biological drivers governing ice algal and under-ice phytoplankton bloom dynamics in the high Arctic over spatial and temporal scales relevant for regional extrapolation and modelling. The proposed work will result in a better understanding of the ecological processes in first-year sea ice in the high Arctic and how the vital ice-associated primary producers in Arctic marine ecosystem will respond to the drastic changes in the Arctic icescape. The obtained results will be valuable to sea-ice and plankton ecologists, biologists and modellers, and those studying natural and human systems impacted by changes in Arctic sea ice.