Spatiotemporal variations in phytoplankton biomass and community structure in a meridional transect of the East/Japan Sea

Abstract:

To better understand the variations in phytoplankton biomass and community composition associated with water-column structure and environmental conditions in the East/Japan Sea (EJS), three cruses were carried out along a meridional transect in May 2007 (spring), July 2009 (summer), and October 2012 (fall). The subpolar front (SPF) was formed between warm and cold water mass (37–40 °N). Chlorophyll a concentration and phytoplankton community composition were studied using HPLC pigment and CHEMTAX analysis and the results showed no significant differences between warm and cold water masses. These results reject our initial hypothesis that different water masses between southern and northern parts of the EJS may lead to different phytoplankton community structure. During the study periods, isotherm layers (≤ 12 °C) fluctuated over 50 m depth between warm and cold water masses on the basis of the SPF. In contrast, the nitracline (i.e. 2.5 μM nitrate isopleths) depth was recorded within the limited depths ranged 20–40 m, 30–50 m, and 40–60 m in spring, summer, and fall, respectively. The chlorophyll a concentrations at the subsurface chlorophyll maxima (SCM) in spring and summer (356 ± 233 and 270 ± 182 ng L^–1, respectively) were significantly higher than those in fall (117 ± 89 ng L^–1). The relative contributions of phytoplankton groups to total chlorophyll a concentration reflected phytoplankton community composition in the SCM layer with showing a dominance of diatoms (58 ± 19, 48 ± 11, and 30 ± 20 % in spring, summer, and fall, respectively). High contribution of diatoms to total biomass may enhance the efficiency of biological pump in the EJS. In addition, canonical correspondence analysis revealed a clear distribution of phytoplankton groups associated with temperature and nutrient concentration which mean prevalence of vertical variation. Finally, our findings suggested that phytoplankton biomass and groups are regulated by surface mixed layer depth and nutricline depth.