The distribution of phytoplankton size and major influencing factors in the surface waters near the northern end of the Antarctic Peninsula

The waters near the Antarctic Peninsula have always been a study hot spot because of their variable and unique oceanographic conditions. To determine the distribution and possible influencing factors on phytoplankton size and abundance near the Antarctic Peninsula, a large-scale survey was conducted during the austral summer of 2018. Samples were collected in 27 stations located in the Drake Passage (DP), South Shetland Islands (SSI), and South Orkney Islands (SOI). Phytoplankton communities were described using chlorophyll a (Chl a), flow cytometry and light microscopy to cover a size range from pico- to microphytoplankton. Nanophytoplankton, especially small nanophytoplankton (2–6 µm) with abundance ranging from 0.66 × 103 cells/mL to 8.46 × 103 cells/mL, was predominant throughout the study area. Among different regions, there was an obvious size shift. The proportion of picophytoplankton near the Elephant Island (EI) and DP was higher than other regions, and larger cells were found mainly in east of SOI. The distribution of phytoplankton abundance detected by flow cytometry was not completely consistent with Chl a concentrations due to the contribution of larger cells to Chl a. Possible influencing factors on the phytoplankton size distribution were discussed. The properties of water masses such as temperature and salinity can influence the phytoplankton size distribution. Correlation analysis revealed that only picophytoplankton is significantly correlated with salinity. Light and Fe availability might affect phytoplankton abundance and size distribution especially near the waters of SSI and EI in this study. It was also speculated that the abundance of cryptophytes is possibly related to ice melting.