Terrestrial runoff is the dominant source of a new type of biological INPs observed in Arctic fjords

The excellerated warming of the Arctic manifests in e.g., the thawing of permafrost, the loss of sea ice and the melting of glaciers. Glacial run-off increases the input of freshwater, sediments, and nutrients into fjords and coastal waters. The decrease in sea-ice cover and the loss of albedo results in firstly, increased radiation reaching through the water column and secondly, increased temperatures which both significantly alters the dynamics of the marine biota, e.g., leading to increased gross primary production. Previous studies indicate that marine areas are sources of biological material with ice nucleating properties and it has been suggested that these particles stem from algal exudates released during phytoplanktonic blooms. Rising temperatures may result in increased microbial activity, thereby influencing the concentration and characteristics of ice nucleating particles (INPs) in seawater. However, various microorganisms, which inhabit marine and terrestrial environments, are shown to produce ice nucleating active material. Thus, terrestrial environments can also be a source of INPs transported to the sea water by e.g. rivers and streams. Additionally, outwash of glacial sediment and permafrost soil contain highly active biological INPs that may be released in increasing numbers into the coastal ecosystem. The biological INPs can then be transferred from the marine areas into the atmosphere by wave breaking and bubble bursting and can subsequently be transported to higher altitudes where they may trigger the freezing of cloud droplets. Thereby, they would impact the lifetime of clouds as well as their radiative properties. Despite our knowledge of the presence of INPs in marine environments and possible impact on atmospheric processes, our understanding of both their characteristics and sources remains limited. To address these knowledge gaps, we sampled sea bulk water and sea surface microlayer between June and September 2018 in two southwest Greenlandic fjords and investigated the ice-nucleating particles along with the microbial community composition of both algae and bacteria present in these two marine compartments. Our study reveals a clear seasonal variation in the number and types of INPs. We found the highest concentration in June during the late stage of a phytoplanktonic bloom. Using filtration and heat treatments, we found that the highly active INPs found in June are smaller in size, and less heat sensitive compared to the INPs present in July and September, thus indicating different INP types and origin. A strong negative correlation between the number of INPs and salinity suggests inputs from either glacial melt water or sea ice melt as a potential source of INPs or a combination of both. However, stable oxygen isotope analysis pointed to meteoric water such as river runoff as the major source of the observed freshwater input indicating that the highly active INPs stem from a terrestrial source.