No abstract

). In autumn, the community was dominated by unidentified flagellates, prymnesiophytes and diatoms, in various proportions from early to late fall. From a summer situation characterized by stronger stratification, higher incident irradiance and depleted nutrients in surface waters, it evolved to an autumn situation characterized by decreasing air temperature and irradiance associated with an environmental forcing (e.g. weather) allowing cooling and greater vertical mixing of the water column. Combining our observations with those from the literature, we suggest the following annual succession in the Labrador fjord phytoplankton community: (winter) dinoflagellates and small flagellated cells -(spring) Fragilariopsis spp., Chaetoceros spp., Thalassiosira spp. and Phaeocystis pouchetii -(summer) Chaetoceros spp., P. pouchetii and Chrysochromulina spp. -(fall) Gymnodinium/Gyrodinium spp., Chrysochromulina spp. and other flagellates. Overall, the protist richness was 2 times higher in fall than in summer, the highest richness being observed in early fall, with 201 taxonomic entries, 72 genera and 131 species identified.

This study was conducted in 4 Labrador fjords (Nachvak, Saglek, Okak, and Anaktalak) during the summers of 2007 and 2013, early fall 2010, and late fall 2009. Our results show that water temperature combined with the availability of nutrients and organic substrates are the main abiotic factors controlling the abundance of heterotrophic bacteria in Labrador fjords. Bacterivory also played a crucial role, with heterotrophic bacteria exerting a significant bottom-up control on the abundance of heterotrophic nanoflagellates (r = 0.35, p < 0.05) and ciliates (r = 0.70, p < 0.01). During summer 2013, the intrinsic phytoplankton growth rate varied between <0 and 0.64 d-1, with a mean value of 0.36 d-1. The herbivory rate was highly variable, ranging from 0.01 to 0.86 d-1, with a mean value of 0.31 d-1. Grazing mortality was 6-fold higher than phytoplankton growth rate. Mean phytoplankton growth and herbivory rates in Labrador fjords were comparable to the Barents and Bering seas. The intrinsic growth rate of total heterotrophic bacteria ranged between <0 and 0.68 d-1, with a mean value of 0.30 d-1. Bacterivory varied from 0.01 to 0.95 d-1, with a mean of 0.30 d-1. Mortality due to grazing was up to 2.3 times higher than total bacterial growth rate. This study improves our understanding of the factors influencing the dynamics of heterotrophic bacteria and indicates that herbivory and bacterivory exert substantial control on microbial communities in Labrador fjords.