Gelatinous Ice – The Ocean Ice Air Interface in the Arctic

Global warming, among other effects, leads to a decrease in sea-ice in the Arctic Ocean. During the summer, melting ice forms pools of water on the surface of the ice. As the season progresses the ponds become larger and permeate the ice. Melt ponds lower the albedo of the sea-ice, thus further accelerating the melting. Moreover, melt ponds increase the amount of light available to autotrophic organisms under the ice. In addition to increasing temperatures, low-level Arctic clouds influence the amount of energy that reaches the sea-ice surface. Previous studies have suggested the gels of organic compounds excreted by phytoplankton to be one of the major sources of primary organic aerosols (POA) in the summer. Generally, sea spray from the sea surface microlayer (SML) leads to the formation of POA, which in turn induce the condensation of clouds. In their paper Galgani et al. (2016) aim to understand how the decrease of Artic ice influences the composition of the SML and the possible implications for POA emission.

During a cruise to the central Artic Ocean, the authors sampled SML and underlying water (ULW) from seven different stations. Each station was divided into three sites, shallow and open melt ponds as well as open sea. Thin first year ice (FYI) dominated the studied area, while 30-40% of the ice was covered with melting ponds. The SML was collected using borosilicate glass plates and the organic compounds were filtered out of the seawater. Bacterial abundance was measured by staining and a flow cytometer. 

The amount of dissolved organic carbon (DOC) increased from shallow melt ponds to the open ocean, correlated with the increase in salinity. The bacterial abundance was significantly higher in open sea sites than in melt ponds, yet, bacteria were not more abundant in SML than in ULW. Furthermore, bacterial abundance showed a positive relationship with DOC and TEP concentrations.

Additionally, the authors were able to show that artic SML exhibited a high concentration of proteinaceous gels. In general, polymeric particles were more abundant in the SML than TEP was. In contrast to polymeric particles, TEP abundance and size appeared to be correlated with salinity. The authors attribute these findings to the increased formation of TEP with increased salinity. Moreover, the significant abundance of proteinaceous particles in the SML of melt ponds suggested a protein-rich source in the ice. 

The authors propose that the melting of sea-ice during the summer months releases trapped exopolymers into melt ponds. Simultaneously primary production increases, thus leading to an enriched SML. During refreezing, particles and organisms become trapped in the ice. Subsequently, microbial survival metabolism is activated and polymeric DOM is produced, which is released during melting. Additionally, increased UV-stress in melt ponds and viral lysis may also contribute to the increase in proteinaceous particles. The transition to FYI and the increased number of melt ponds will likely further increase the number of proteinaceous gels in the SML, who in turn contribute to POA. At low wind speeds and in absence of breaking waves, POA are formed by bubble rising and bursting. Density gradients, trapped bubbles in the ice and the respiration of organisms lead to rising bubbles. During winter, polymeric gels are incorporated in frost flowers and skim layers on the ice. These may be possible sources for POA in summer and also contribute to CO₂-exchange. 

In conclusion, Artic melt ponds appear to act as dynamic interfaces. However, our current knowledge of the interactions between the ocean, the ice and the atmosphere is still very limited. Further research is necessary to understand these processes. However, it seems as though the authors have described a kind of vicious circle, caused by global warming. 

Reference:

Galgani, L., Piontek, J., & Engel, A. (2016). Biopolymers form a gelatinous microlayer at the air-sea interface when Arctic sea ice melts. Scientific Reports, Link: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4951643/