Phytoplankton communities and size-fractioned chlorophyll a in newly opened summer waters of the central Arctic Ocean

摘要

The rapid decline of sea ice extent during the Arctic summer season has profound implications for the pelagic ecosystem. During the 4th Chinese National Arctic Research Expedition (4th CHINARE) in August 2010, a snapshot of the spatial distribution of phytoplankton communities (10 mu m) and size-fractionated chlorophyll a (chl a) in newly opened waters in the central area of the western Arctic Ocean (80-86 degrees N) was studied. Potential environmental drivers were linked with community profiles to assess the influences of water masses and sea ice shrinking. Results indicate that the central Arctic Ocean exhibits a typical low-nutrient low-chl a environment accompanied with low phytoplankton diversity and abundance. A total of 49 taxa belonging to 25 genera from 5 classes were identified by the Utermohl method, with diatoms (Chaetoceros spp., Nitzschia spp., Thalassiosira nordenskioldii and Nitzschia longissima) numerically dominating the community. Based on chl a contributions, picophytoplankton (2 mu m) were most abundant, followed by nanophytoplankton (2-20 mu m) and microphytoplankton (20 mu m). Phytoplankton abundance and size-fractionated chl a differed significantly between the early ice-melting northern Canada Basin, influenced by summer Bering shelf water, and near the Alpha Ridge, where ice coverage is thicker. Redundancy analysis and Spearman's correlation suggest that salinity, temperature and nitrate concentration are the major factors influencing phytoplankton abundance and size-fractionated chl a. In addition, irradiance plays an important regulatory role in the vertical variation of size-fractionated chl a, where low-light adapted picophytoplankton dominate at the bottom of the euphotic zone. These results support previous studies showing the advantage of picophytoplankton in oligotrophic and light-limited environments such as the central Arctic Ocean.