Winter diatom assemblage composition to track shrinking seasonal sea ice cover in a coastal subarctic bay (Quebec, Canada)

Current climate change is affecting sea ice cover on a global scale, with fundamental consequences for associated ecosystems. To better understand these changes, it is necessary to characterize modern ecosystems and document past variations in ice cover. For the latter, identifying sea ice-associated diatoms in sediments and using a paleoceanographic approach is proving to be an effective method. Although Arctic regions are well documented, information on sub-Arctic, coastal and anthropized ecosystems remains limited, as in the Bay of Sept-Îles (BSI, Québec, Canada). The aim of this study was to characterize the sympagic and underlying water diatom communities during a winter day (February 15th, 2023) in the BSI and assess their deposition and preservation in the underlying surface sediments, a first step in developing a sea ice diatom index that can be applied to paleoenvironmental analysis. Light microscopy was used to characterize diatom assemblages in sea ice, surface water, and sediment samples at six sites in the BSI. Centric diatoms of the genus <i>Thalassiosira</i> dominated the ice and water assemblages in terms of relative abundance, while the proportion occupied by this genus in surface sediments was lower. <i>Fossulaphycus arcticus</i>, a species associated with sea ice, was identified in all surface sediment samples. Some species can thrive in both sea ice and the cold surface layer of the water column, making identification of ice-indicator species complex. Analyses of the C:N ratios and δ¹³C using an elemental analyser coupled to an isotope ratio mass spectrometer revealed that marine and sympagic unicellular algae significantly contribute to the organic matter composition of surface sediments. This study characterizes the diatom assemblages of sea ice, surface water, and surface sediments in the BSI in winter and lays the foundation for the development of a diatom-based approach for long-term monitoring of the rapidly disappearing seasonal ice cover in the region of the Gulf of St. Lawrence.