Influence of oceanic mixing on sea ice net community production in the Canadian Arctic

Turbulence-driven nutrient supply has the potential to shape bottom sea ice microbial communities, affecting the balance of O2 uptake and release which defines the net community production (NCP) of the ice. This study contrasts sea ice NCP at two sites with comparatively high and low under-ice turbulent conditions. These locations were sampled over a 4-week spring bloom period in the Dease Strait region of the Kitikmeot Sea near the community of Cambridge Bay and the Canadian High Arctic Research Station (CHARS). We found that strong under-ice tidal currents promoted nutrient replenishment into the bottom ice, often supporting increased sea ice algal growth and a net autotrophic (i.e., net release of O2) signal of NCP throughout the study period. However, the stronger currents also accelerated bottom-ice melt and led to periodic reductions in ice algal chlorophyll a and times of net heterotrophy (net uptake of O2). In contrast, the location of lower turbulence and more stable currents presented more limited chlorophyll a accumulation and a persistent heterotrophic signal of NCP, potentially driven by a greater proportion of heterotrophic bacterial activity within the ice. These observations of contrasting NCP demonstrate the spatio-temporal complexity of bottom-ice algal blooms in the Arctic and challenge the use of a singular autotrophic bloom phenology to describe their seasonal growth. Rather, we show that the phenology of bottom-ice algal bloom development and the balance between autotrophic and heterotrophic activity vary with the sub-ice turbulent regime.