Light Availability and Phytoplankton Growth Beneath Arctic Sea Ice: Integrating Observations and Modeling

Abstract

Observations of the seasonal light field in the upper Arctic Ocean are critical to understanding the impacts of changing Arctic ice conditions on phytoplankton growth in the water column. Here we discuss data from a new sensor system, deployed in seasonal ice cover north‐east of Utqiaġvik, Alaska in March 2014. The system was designed to provide observations of light and phytoplankton biomass in the water column during the formation of surface melt ponds and the transition from ice to open water. Hourly observations of downwelling irradiance beneath the ice (at 2.9, 6.9, and 17.9 m depths) and phytoplankton biomass (at 2.9 m depth) were transmitted via Iridium satellite from 9 March to 10 November 2014. Evidence of an under‐ice phytoplankton bloom (Chl a ∼8 mg m−3) was seen in June and July. Increases in light intensity observed by the buoy likely resulted from the loss of snow cover and development of surface melt ponds. A bio‐optical model of phytoplankton production supported this probable trigger for the rapid onset of under‐ice phytoplankton growth. Once under‐ice light was no longer a limiting factor for photosynthesis, open water exposure almost marginally increased daily phytoplankton production compared to populations that remained under the adjacent ice. As strong effects of climate change continue to be documented in the Arctic, the insight derived from autonomous buoys will play an increasing role in understanding the dynamics of primary productivity where ice and cloud cover limit the utility of ocean color satellite observations.

Higher sensitivity towards light stress and ocean acidification in an Arctic sea‐ice‐associated diatom compared to a pelagic diatom

“…Hence, we hypothesized that pelagic algae are highly resilient towards photophysiological stress, while being responsive to OA. Within sea‐ice, light intensities are extremely low, and fluctuations therein are small and typically change slowly as snow and ice melt (Hill et al , 2018). However, sympagic ice‐algal communities need to tolerate subzero temperatures, sustain net growth under such low irradiances (Hancke et al , 2018), and tolerate high salinities and extremely variable nutrient levels as well as distorted carbonate chemistry (Weeks & Ackley, 1986; McMinn et al , 2014; Hill et al , 2018).…”

“…Within sea‐ice, light intensities are extremely low, and fluctuations therein are small and typically change slowly as snow and ice melt (Hill et al , 2018). However, sympagic ice‐algal communities need to tolerate subzero temperatures, sustain net growth under such low irradiances (Hancke et al , 2018), and tolerate high salinities and extremely variable nutrient levels as well as distorted carbonate chemistry (Weeks & Ackley, 1986; McMinn et al , 2014; Hill et al , 2018). Despite the extreme physico‐chemical properties of this habitat, ice‐algae are widespread and often thrive remarkably well as both their productivity and biomass can reach very high levels in spring (Aletsee & Jahnke, 1992; Gradinger, 2009).…”

Photoacclimation State of an Arctic Underice Phytoplankton Bloom

“…In contrast to phytoplankton blooms in the open ocean, underice blooms cannot be monitored from space by means of ocean color remote sensing, and therefore, information about them is sparse. Efforts toward increasing the use of autonomous sampling platforms in ice‐covered waters are underway, demonstrated by adding bio‐optical sensors to, for example, ice‐tethered buoys (Berge et al, ; Hill et al, ; Laney et al, , ) or to autonomous underwater vehicles (AUVs; Johnsen et al, ). On a global scale, efforts to monitor water column biological properties are emerging, like the biogeochemical Argo float program (Johnson & Claustre, ), where the floats carry various biogeochemical and optical instruments to study biological processes.…”

Algal Export in the Arctic Ocean in Times of Global Warming

“…Rapid sea ice retreat, sea ice thinning, and increase in incident light (Comiso, ; Hill et al, ; Nicolaus et al, ; Serreze et al, ) have the potential to modify the magnitude, timing, and duration of the algal growth phase, and these modifications are likely already ongoing in the Eurasian Arctic Ocean where sea ice retreat has recently been considerable (Arrigo et al, ). Our results indicated that nearly all algal export occurred after snowmelt and before ice melt, further suggesting that snow cover regulates light transmission, algal growth, and ice algae release.…”