Abstract
Our study addresses how environmental variables, such as macronutrients concentrations, snow cover, carbonate chemistry and salinity affect the photophysiology and biomass of Antarctic sea-ice algae. We have measured vertical profiles of inorganic macronutrients (phosphate, nitrite + nitrate and silicic acid) in summer sea ice and photophysiology of ice algal assemblages in the poorly studied Amundsen and Ross Seas sectors of the Southern Ocean. Brine-scaled bacterial abundance, chl a and macronutrient concentrations were often high in the ice and positively correlated with each other. Analysis of photosystem II rapid light curves showed that microalgal cells in samples with high phosphate and nitrite + nitrate concentrations had reduced maximum relative electron transport rate and photosynthetic efficiency. We also observed strong couplings of PSII parameters to snow depth, ice thickness and brine salinity, which highlights a wide range of photoacclimation in Antarctic pack-ice algae. It is likely that the pack ice was in a post-bloom situation during the late sea-ice season, with low photosynthetic efficiency and a high degree of nutrient accumulation occurring in the ice. In order to predict how key biogeochemical processes are affected by future changes in sea ice cover, such as in situ photosynthesis and nutrient cycling, we need to understand how physicochemical properties of sea ice affect the microbial community. Our results support existing hypothesis about sea-ice algal photophysiology, and provide additional observations on high nutrient concentrations in sea ice that could influence the planktonic communities as the ice is retreating.
Highlights
Sea ice plays an important role in Antarctic marine biogeochemical cycles, both in terms of physical, chemical and biological processes [1,2,3,4]
We have investigated macronutrient variability and photophysiology of sea-ice algae in Antarctic pack ice, in a relatively under-studied sector of the Southern Ocean
By using fluorescence kinetics as estimates of photosynthetic acclimation and performance in 237 sea-ice samples, we have identified snow depth, ice thickness and brine salinity to be strong environmental parameters affecting the photosystem II (PSII) activity of sea-ice algae in the Amundsen and Ross Seas during summer
Summary
Sea ice plays an important role in Antarctic marine biogeochemical cycles, both in terms of physical, chemical and biological processes [1,2,3,4]. Organic matter derived from algae plays an important role in sea-ice microbial ecology. Strong bacteria-algae interactions have been reported in many other marine systems, and it has been suggested that the microbial loop is important for recycling of organic matter in sea ice [10,11,12]. Sea ice acts as a source of organic and inorganic nutrients for the planktonic community as the ice is retreating [13]. Nutrient dynamics in pack ice have been identified as overlooked, and relatively little data is available from the Amundsen Sea sector [2]
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