16S rRNA amplicon sequencing in the Hudson Bay estuaries

There is a clear divide between typical freshwater and marine microbial communities, but there is less consensus on the source or uniqueness of the microbes found in small estuarine transition zones between rivers and the sea. Most studies to date have focused on anthropogenically impacted temperate estuaries and there is less data on the biodiversity and biogeography of microbes in Arctic or sub-Arctic fresh to salt water transition zones. To identify microbial communities in two river to sea systems in the sub-Arctic to Arctic Hudson Bay, we used the V4 region of the 16S rRNA gene to characterize bacterioplankton communities collected in early summer 2018. We identified distinct riverine bacterial communities and found significant differences between the Nelson and Churchill estuaries and offshore communities. The Churchill estuary showed a marked influence of freshwater microbial species, but limited exchange with the offshore marine system. In contrast in the more regulated higher flow Nelson River, riverine bacterioplankton were less evident in the estuary, with a much stronger marine signal. The more marine waters of both systems also showed a greater tendency for differences between the attached and free-living bacterial communities. We conclude that the marine bacterioplankton were retained in the Nelson maximum turbidity zone, giving rise to a more marine assemblage compared to the Churchill estuary.