Ice-algal and under-ice phytoplankton bloom dynamics in a changing Arctic icescape.

The main aim of the Boom or Bust project was to increase our understanding of the role of ice-algal and under-ice phytoplankton blooms in the changing Arctic Ocean. We have therefore applied a combination of methods to 1) identify the environmental and biological 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 was largely based on a unique time-series dataset from the high-Arctic obtained during the multidisciplinary Norwegian young sea ICE (N-ICE2015) drift expedition carried out in the Arctic Ocean north of Svalbard (between 80 and 83°N) from 11 January to 24 June 2015. During N-ICE2015, we studied ice-algal and phytoplankton blooms under the new ice regime, consisting of thin ice in refrozen leads, first- and second year ice. The most pertinent emerging physical properties observed during N-ICE2015 were the thinner and more dynamic ice pack, relatively thick snow cover, and the formation of leads and ridges caused by frequent winter storms. This regime shift allowed phytoplankton blooms to develop in early spring below the sea ice despite the thick snow cover because light could penetrate into the underlying water column through open or refrozen leads. Previous under-ice phytoplankton blooms have been reported from the summer melt season when the sea ice is much more transparent to light, thus our findings add a new aspect to phytoplankton production below the Arctic ice pack. Our in situ observations are particularly important because under-ice blooms cannot be detected from space. We also studied the ice algal communities in the three different ice types outlined above. The ice algal communities in first and second-year ice were dominated by ice-associated diatoms throughout the study period while the community in the thin ice was initially dominated by pelagic algae and transitioned to dominance by typical ice-associated diatoms. Our observations indicate that the thin ice was colonized by sea ice diatoms from the adjacent thicker ice, highlighting the importance of the older ice for ice algal recruitment and indicating that the observed loss of multi-year ice could have negative consequences for ice algal blooms. When colonizing the thin ice, the highly shade-adapted sea ice diatoms had to invest energy in photoprotective pigments (sunscreens) to cope with the high light levels, both in the visible and the UV range. These observations also highlight that more light might not necessarily translate into more ice algal production. Furthermore, we identified pressure ridges as algal hotspots that could contribute a significant if not a major share of ice algal stocks in a more dynamic ice pack. The heavy snow load on the first- and second year ice led to flooding of the sea ice in early June and growth of phytoplankton at the snow-ice interface. These snow-infiltration communities are common in the Antarctic but have rarely been reported from the Arctic and could be another harbinger of a new Arctic sea ice regime. We 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. The model results, supported by observations, suggest that maximum growth rates of ice algae will increase whilst vertically integrated NPP and biomass will decrease under the thinner ice regime. Results from the Boom or Bust project contribute to a better understanding of the ecosystem consequences of the rapidly changing sea ice regime in the high-Arctic. Our findings have important management implications as the observed changes in ice algal and phytoplankton bloom dynamics will potentially have cascading effects on the entire Arctic food web and may alter 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. We will therefore apply a combination of methods to characterize and quantify under-ice phytoplankton and ice algal communities in combination with in situ and controlled experiments on the photosynthetic performance of phytoplankton and ice algae. The Norwegian Young sea ICE (N-ICE) campaign in 2015 with R/V Lance to the pack-ice north of Svalbard and the MicroPolar cruise in 2016 to the same area provide an unique opportunity to study both under-ice phytoplankton and ice algal communities in drifting sea ice. 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 modeling. 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 modelers, and those studying natural and human systems impacted by changes in Arctic sea ice.