Short timeframe changes of environmental impacts on summer phytoplankton in the Chukchi Sea and surrounding areas in a regional scaling

Abstract

Recent changes observed in the Arctic associated with climate change call for further studies of its habitats and ecosystems, including those of the microbial communities that are the base of the food web. Limited information is available on short timeframe changes of summer phytoplankton and its driving factors. In the present study, space and time resolution in the phytoplankton community (>10 μm) and chlorophyll a (Chl a) were analyzed in Chukchi shelf and northern deep-sea areas (66°N ~ 80°N) during the Fourth Chinese National Arctic Research Expedition in the summer of 2010. Potential environmental drivers were linked to assess effects of Pacific inflow, light intensity and seasonal ice retreat on summer phytoplankton community. Results indicate a strong temporal variability in species composition, abundance and Chl a within a short interval (July 20 to August 30), as well as significant difference in regional distribution (Chukchi shelf, coast Alaska and deep-sea). A total of 159 taxa belonging to 69 genera from 9 phyla were identified by Utermöhl method. The Chukchi shelf in early summer (July 20 to 25) had a depth-integrated Chl a of 97.24 mg/m2 with a depth-averaged abundance of 101.37 × 103 cells/L. The community was dominated by Navicula pelagica, Thalassiosira nordenskiöeldii, Pseudonitzschia seriata, Leptocylindrus danicus, Nitzschia longissimi and Navicula vanhoeffenii. An ongoing ice-edge diatom bloom was observed associated to Anadyr Water. In contrast, from the deep-sea to Chukchi shelf in late summer (August 23 to 30), depth-integrated Chl a and depth-averaged abundance were lower than the previous weeks with only 25.31 mg/m2 plus 2.10 × 103 cells/L in deep-sea and 71.37 mg/m2 plus 27.00 × 103 cells/L in Chukchi shelf, respectively. At this stage, dominant species now include Thalassionema nitzschioides, Chaetoceros spp., Cylindrotheca closterium, Nitzchia longissima in deep-sea and Chaetoceros diadema, Letocylindrus danicus, N. longissima, Navicula pelagica, C. closterium, T. nitzschioides in Chukchi shelf, respectively. Generally, both Chl a and abundance were higher in the Chukchi shelf than those in the deep-sea, but was also higher in July than in August. Deep chlorophyll maxima was exclusively fixed at around 50 m in deep-sea, while a peak in Chl a appeared randomly at each layer in Chukchi shelf. A succession from pennate to centric diatoms was observed in northern Chukchi shelf closely related to ice retreat, while the opposite trend was common in the ice-free southern Chukchi shelf. Revisited stations in Chukchi shelf demonstrated the decrease in light intensity from July to August chiefly resulted in the reduction in Chl a and abundance during the return leg, while nutrients from Pacific inflows played a main role during the inbound leg.