Diatom and dinoflagellate cyst distribution in surface sediments of the Chukchi Sea in relation to the upper water masses

Diatom and dinoflagellate cyst analyses were performed on 22 surface sediment samples from the Chukchi Sea to document their geographical distributions in one of the most understudied sections of the Arctic Ocean and to examine the influence of upper water masses on these two major groups of phytoplankton. Total concentrations vary from 0.9 to 5.9 x 10(6) valves g(-1) for diatoms and from 0.8 to 12.5 x 10(3) cysts g(-1) for organic-walled dinoflagellate cysts, with the highest values for both groups observed in the southern part of the Chukchi Sea and away from the Bering Strait. Well-preserved microfossils were recovered, with a total of 35 and 88 taxa of dinoflagellate cysts and diatoms, respectively. The most abundant diatoms are Paralia sulcata, Thalassiosira antarctica, Thalassiosira nordenskioeldii, and resting spores of Chaetoceros spp., whereas cysts of phototrophic Alexandrium spp., Operculodinium centrocarpum sensu Wall and Dale (1966), and heterotrophic Islandinium minutum and Brigantedinium spp. were most common in the dinoflagellate cyst assemblages. Cysts of HAB-causing Alexandrium spp. were found in most of the samples, with the highest abundances in Herald Canyon where they contribute similar to 56.6% to the cyst assemblage. As expected, cysts produced by heterotrophic dinoflagellates were more abundant where sedimentary diatom concentrations were the highest. Statistical analysis identified three major diatom and dinoflagellate cyst clusters: 1. Sites influenced by the Alaska Coastal Current in the eastern part of the Chukchi Sea are characterized by high abundances of P. sulcata and O. centrocarpum sensu Wall and Dale (1966); 2. The western part and Herald Canyon in the northern part of the Chukchi Sea are distinguished by diatoms Chaetoceros spp., T. antarctica and dinoflagellate cysts of Alexandrium spp. and affected by the Siberian Coastal Current and Bering Shelf Water; and 3. Assemblages in the southern part of the Chukchi Sea are recognized by noticeable abundances of T. nordenskioeldii and cryophilic diatom taxa, and dinoflagellate cysts I. minutum, as well as by overall lower percentages of cysts of Pentapharsodinium dalei and Brigantedinium spp. This work revealed the potential applicability of the combined use of diatoms and dinoflagellate cysts for reconstructions of past dynamic water mass changes in the Chukchi Sea.